gunnar_ssh/

admission.rs

//! **Admission control for git transfers**: an explicit, countable
//! ceiling in place of the accidental one.
//!
//! # What the accidental ceiling was
//!
//! Every git transfer occupies one tokio **blocking-pool** thread for its whole
//! duration (`gunnar-cli`'s `WireTransport::serve` bridges an async duplex into
//! `gunnar-wire`'s blocking `BufRead`/`Write` through `SyncIoBridge`). Nothing
//! bounded how many of those could exist at once, so the real limit was
//! `tokio`'s default of 512 blocking threads, a number nobody chose. Past it,
//! transfers queued **silently**: no log, no counter, no error. The failure an
//! operator saw was "git is slow", with nothing anywhere to look at.
//!
//! # Why this refuses instead of queueing
//!
//! This gate **refuses** over the limit rather than queueing behind it, and the
//! reasoning is not "refusing is simpler":
//!
//! * **A git client retries; a git client cannot see a queue.** `git` reports
//!   whatever the server says and the user (or CI) runs it again. A refusal that
//!   arrives in milliseconds with a sentence in it is strictly better
//!   information than a socket that goes quiet for a minute.
//! * **Queueing keeps the expensive things alive.** A queued transfer still
//!   holds a TCP socket, a russh session, its channel and its key material. That
//!   is the resource a fork-bomb of clones is trying to exhaust, and a queue
//!   hands it to them for free.
//! * **The two existing timeouts fight a queue.** `inactivity_timeout` (600 s)
//!   and `exec_grace` (60 s) both measure a session that is not making progress.
//!   A transfer parked in a queue looks exactly like one of those, so a long
//!   wait ends in a disconnect anyway, after having occupied the slot for the
//!   whole wait.
//! * **A queue makes saturation invisible again.** The whole point of the ceiling is
//!   that the ceiling must be *visible*. A refusal is an event with a number
//!   attached; a wait is the thing that was already happening.
//!
//! So: [`Admission::try_admit`] never blocks and never waits. It either returns
//! a guard or a [`Refusal`], and both outcomes move a counter.
//!
//! # Two limits, because one is not enough
//!
//! [`SshConfig::max_concurrent_transfers`](crate::SshConfig::max_concurrent_transfers)
//! is the server-wide ceiling. On its own it is still exhaustible by a *single*
//! misbehaving client, which would then deny service to everybody else, so
//! [`SshConfig::max_transfers_per_principal`](crate::SshConfig::max_transfers_per_principal)
//! caps what any one identity can hold at once. Identity is the principal id,
//! not the peer address: gunnar authenticates keys, and a per-IP limit would
//! punish everyone behind one CI runner's NAT while a botnet walked past it.
//!
//! # The per-principal limit is a SHARE, not a wall
//!
//! The second limit used to be an unconditional ceiling, and that was wrong in
//! a way nobody could see from the code. **Measured on a 32-core desktop with
//! the shipped defaults, 2026-08-03:**
//!
//! | load | served | refused |
//! |---|---|---|
//! | 1 principal, 64 clients  |  32 | 32 (per-principal) |
//! | 32 principals, 320 clients | 256 | the rest |
//!
//! A client holding **one** key could never reach the advertised 256; that took
//! eight identities. Every single-team appliance — which is what gunnar is —
//! silently got one eighth of the capacity written on the box, and the gate that
//! did it was binding on an **empty server**, where there was nobody to be fair
//! to.
//!
//! The fairness mechanism itself is right: one identity must not be able to take
//! a shared host away from everybody else. So the ceiling is now **max-min
//! fair** — it applies only under contention, and what it applies is a share:
//!
//! ```text
//! contended  = occupied (this candidate included) ≥ CONTENTION_PERCENT % of the server-wide limit
//! ceiling(p) = if !contended { the server-wide limit }
//!              else          { max(configured, server_wide / active principals) }
//! admit      = server-wide gate has room  AND  p holds < ceiling(p)
//! ```
//!
//! * **Idle server, one identity** → not contended, so the only ceiling is the
//!   server-wide one. 64 clients on one key now get 64 slots, and 256 clients
//!   get 256. That is the half the old gate got wrong.
//! * **Contended server, many identities** → each gets `server_wide / active`,
//!   with the configured number as a **floor** nobody drops below however many
//!   identities show up. A greedy principal is held to its share exactly as
//!   before.
//! * **Work-conserving.** Enforcing the share only above the contention
//!   threshold is what stops "two identities, one of them idle" from costing the
//!   busy one half the server for no benefit. Below the threshold there are
//!   free slots, so refusing anything would refuse work the box could do.
//!
//! `active principals` is the number of identities holding at least one
//! transfer *right now* (the candidate included), which is the only demand
//! signal an admission gate has. It is exact, not sampled: it is the length of
//! the same map the counts live in, read under the same lock.
//!
//! # The ANONYMOUS ceiling: the one caller the share above cannot hold
//!
//! Everything above is keyed on a principal id, and that is deliberate: gunnar
//! authenticates keys, so the fair share divides the server between *identities*
//! and a greedy one is held to its slice. Anonymous read then opened a door with no key
//! behind it. Every unauthenticated reader of a public repository arrives as the
//! single principal [`gunnar_core::ANONYMOUS_PRINCIPAL_ID`] — the id is refused
//! at registration ([`gunnar_core::Principal::new`]) so nothing else can ever
//! claim it — which *looks* like the whole internet already sharing one row of
//! the map above, and therefore already bounded.
//!
//! It is not, and the reason is the paragraph before it: **the share is
//! work-conserving on purpose.** Below [`CONTENTION_PERCENT`] of the governed
//! limit the share does not bind at all and every principal's ceiling is the
//! whole server. That rule was measured and is right for a keyed identity —
//! refusing a lone team its own idle appliance is the exact mistake the 2026-08-03
//! table above records. Applied to a stranger it says: *an idle box may be taken
//! entirely by anonymous traffic.* An operator arriving into that does not even
//! get a clean refusal; they get [`Admission::admit_within`]'s wait, because
//! nothing evicts a transfer already in flight.
//!
//! So anonymous gets a **second, different kind of ceiling**, and the ways it
//! differs from the per-principal share are the whole design:
//!
//! * **Hard, not a share.** It binds on an empty server. There is no
//!   contention threshold, because the thing being rationed is not fairness
//!   between identities — it is the blast radius of a caller gunnar cannot name,
//!   bill, revoke or ring up. Work-conserving is the right instinct for a tenant
//!   and the wrong one for a stranger.
//! * **Small in absolute terms**, not a fraction. Five
//!   ([`DEFAULT_MAX_ANONYMOUS_TRANSFERS`]) against a keyed floor of 32 and a
//!   server-wide 256. Anonymous can therefore never be more than ~2 % of the
//!   box, which is what makes "an operator is never held up by an anonymous
//!   flood" a *structural* statement rather than a hope about timing.
//! * **Refused instantly, never parked.** [`Admission::admit_within`]'s wait
//!   exists because a refused keyed client reconnects, and one parked future is
//!   cheaper than that reconnect. For anonymous the arithmetic inverts: parking
//!   holds the TCP socket, the russh session and its key material, which is
//!   precisely the resource a clone fork-bomb is trying to exhaust. So an
//!   `AnonymousAtCapacity` refusal is final at the first ask.
//! * **Counted separately** ([`AdmissionStats::refused_anonymous`]). Folded into
//!   `refused_per_principal` it would be indistinguishable from a CI runner
//!   hitting its share, and the two demand opposite operator actions: one says
//!   *raise the limit*, the other says *you are being scraped*.
//!
//! ## The keyed reserve: the other half of the same door (D23b)
//!
//! The ceiling above is **absolute**, and the box it is a share of is **not**.
//! [`Governor`]'s CoDel controller sheds the governed limit down its band
//! whenever the machine is queueing, and the shed was measured reaching a
//! limit of 19. Five out of 19 is a quarter of the machine, held by a caller
//! that cannot be named — and held at exactly the moment there is least to go
//! round. The `~2 %` in the bullet above is a statement about an *unshed* box.
//! The same arithmetic undoes an operator who raised the ceiling on purpose for
//! a browse replica: the number they raised bears no relation to the number the
//! controller is enforcing an hour later.
//!
//! So the top of the box is **keyed-only**. An anonymous read is admitted only
//! while occupancy is *below* [`CONTENTION_PERCENT`] of the governed limit;
//! from there up it is refused with [`Refusal::KeyedReserve`], instantly, and
//! counted as [`AdmissionStats::refused_keyed_reserve`]. What that reserves is
//! [`Admission::keyed_reserve`] slots — 65 of the shipped 256, 2 of a shed 4 —
//! and a keyed caller can therefore never be refused *by a stranger's
//! acquisition*.
//!
//! Three things about it are decisions rather than defaults:
//!
//! * **The threshold is `CONTENTION_PERCENT`, not a fourth number.** That
//!   constant is *defined* as the occupancy at which gunnar starts asking keyed
//!   identities to share with each other. The moment it does that is exactly the
//!   moment it should stop asking them to share with strangers. A second
//!   threshold would be a second opinion about when the box is busy, and the
//!   order between the two would be an accident of arithmetic.
//! * **It refuses an acquisition; it does not evict.** Taking a slot back from
//!   a transfer already running throws away every byte of pack already built,
//!   hands a legitimate visitor a torn stream that `git` reports as a network
//!   error (indistinguishable from a broken repository), and does not even
//!   reliably give the slot to the keyed caller that triggered it — the
//!   `slot_freed` wakeup goes to whoever is parked. It would make a
//!   stranger cost *more* than serving it, because the evicted client comes back
//!   and redoes the work. Refusing before the first byte costs one
//!   compare-and-swap.
//! * **It is enforced inside the claim's compare-and-swap**
//!   ([`Governor::claim_reserving`](crate::governor::Governor::claim_reserving)),
//!   not checked around it. Both orderings of "read occupancy, then claim" were
//!   measured and both leak — the failures are quoted in
//!   `tests/keyed_reserve.rs`. A limit checked outside the atomic that enforces
//!   it is not a limit.
//!
//! ### What that does and does not promise
//!
//! Stated exactly, because the exact statement is weaker than the slogan and the
//! difference is where somebody would otherwise be surprised. A stranger
//! admitted onto a *quiet* box is still holding its slot if the box fills a
//! moment later, and this gate will not take it back. So "a keyed caller is
//! never refused while a stranger holds anything" is **not** true, and the only
//! way to make it true is the eviction ruled out above.
//!
//! What is true is a **ratchet**. From the moment occupancy reaches
//! `CONTENTION_PERCENT`, no stranger is admitted again, so anonymous occupancy
//! from there on only ever falls: every anonymous transfer that finishes hands
//! its slot to the keyed side and cannot take it back. A flood therefore cannot
//! hold a busy box at its anonymous maximum — it decays to zero at the rate
//! transfers complete, and `SshConfig::inactivity_timeout` bounds that rate from
//! above. The residue is bounded too, at `min(anonymous ceiling, limit −
//! reserve)`, which on the shipped appliance is 5 of 256. That is the number a
//! keyed caller could in the worst case be queued behind, and it is the number
//! the ceiling was chosen to make small.
//!
//! What anonymous keeps is therefore the *whole* allowance on a box that is not
//! busy, which is the ordinary state of an appliance and the only state a public
//! repository is interesting in. If keyed demand is permanently above
//! `CONTENTION_PERCENT` the public door is permanently shut — and that is
//! accepted, deliberately: a box that is three-quarters full of paid-for work
//! has nothing to spend on a stranger, the refusal is instant and names its own
//! rule so the operator can see it happening, and the two answers (a bigger
//! limit, or a browse replica) are both things an operator does rather than
//! things gunnar should guess. It is not a livelock: nothing is parked, nothing
//! is retried on gunnar's side, and a stranger costs a string compare.
//!
//! ## Why there is no priority WAKE order
//!
//! The obvious third mechanism — when a slot frees, wake keyed waiters before
//! anonymous ones — is not implemented because it would be a no-op.
//! [`Admission::admit_within`]'s queue **has no anonymous waiters in it**: every
//! refusal that names anonymity short-circuits the wait, so the only callers
//! parked on the gate's own `slot_freed` notify are keyed ones already. Adding
//! a priority queue would mean adding a data structure and a lock in order to
//! sort a queue with one class in it.
//!
//! ## Why not queue the overflow instead
//!
//! Same answer as the module's opening section, only more so. A bounded queue
//! of anonymous readers is a bounded pile of held sockets, and an unbounded one
//! is a slower outage with worse diagnostics. Refusing in milliseconds with a
//! sentence in it is the strictly better outcome for a `git` that will retry
//! anyway, and it is the only outcome that keeps the cost of an anonymous
//! request proportional to what gunnar is willing to spend on it.
//!
//! ## Fairness *among* anonymous readers
//!
//! There is none, and there cannot be: telling two strangers apart requires an
//! identity, and not having one is the definition of this caller. What bounds a
//! single greedy anonymous client is therefore not a share but the **time** any
//! one permit can be held — which is already bounded by
//! `SshConfig::inactivity_timeout` and by the transfer finishing. The ceiling's
//! job is to cap the total, and the existing timeouts recycle it.
//!
//! An operator who wants the old behaviour — a hard per-identity cap that binds
//! whether or not anyone else is there, e.g. a multi-tenant host that bills by
//! identity — sets
//! [`SshConfig::per_principal_limit_is_hard`](crate::SshConfig::per_principal_limit_is_hard).
//! The configured numbers keep their meaning either way.
//!
//! # Saturation is observable, by construction
//!
//! [`AdmissionStats`] is a snapshot of five monotonic-or-live counters, reachable
//! from [`BoundServer::admission`](crate::BoundServer::admission). Every refusal
//! is also logged at `warn` with the limit that was hit and what was in flight.
//! `tests/admission.rs` drives a server past its cap and asserts the counter
//! moved, not merely that nothing crashed.
//!
//! # The server-wide limit is a GOVERNOR now, not a constant
//!
//! `max_concurrent_transfers` used to be the number of permits in a semaphore,
//! and a fixed number is wrong in both directions at once: too high and the box
//! is already thrashing before anything is refused, too low and it refuses work
//! it could have done. It is also unknowable in advance — the right number
//! depends on the repositories, the disks and the other tenants.
//!
//! So the constant is **replaced** — not joined by a second mechanism — with
//! [`crate::governor::Governor`], and the operator's number becomes the band's
//! ceiling rather than the limit itself. Two gates, in this order, both of them
//! *at admission* and before anything has been allocated:
//!
//! 1. **time** — a CoDel controller that measures how long operations *waited*,
//!    per class, and moves one shared limit;
//! 2. **space** — a memory reservation the server owns, rather than a
//!    measurement of the machine it is taken from.
//!
//! `crate::governor` sets out why sojourn rather than queue length, why the
//! minimum rather than the average, why the target is per class, and what the
//! memory estimate does *not* yet bound. Everything below — the per-principal
//! max-min share, the counters, the refusal messages — is unchanged and now
//! divides the *governed* limit instead of the configured one, which is the
//! right divisor: a share of capacity the server does not have is not a share.
//!
//! # The usage share (`admit-share`): a class from `edda_admit`, a share from here
//!
//! With the `admit-share` feature the gate asks
//! [`edda_admit::Admit::plan`] for every **keyed** transfer, with the
//! namespace's rolling-window bytes out of the flow ledger
//! ([`gunnar_core::FlowLedger::window_bytes`]) as the usage. The plan's class
//! maps onto a share this gate already has:
//!
//! * `Full` → the per-principal max-min share, exactly as above;
//! * `Reduced` / `Floor` → **the anonymous ceiling**: a hard cap that binds on
//!   an idle box, plus the keyed reserve. An over-budget namespace is still
//!   served — at a stranger's width — and refused FAST, with
//!   [`Refusal::ReducedShare`], never parked in [`Admission::admit_within`]'s
//!   wait: the same reasoning that refuses anonymous instead of queueing it
//!   applies to anybody held to the anonymous share.
//!
//! `record` is fed when the guard drops, with the transfer's wire bytes in each
//! direction, so the estimate `Usage.AdmitPlan` shows learns per kind. The
//! policy's own slot shares are not configured here — this gate is the share.
//!
//! **Without the feature there is no policy field, no call and no branch on
//! the path**: the admission decision is today's, and `tests/admission.rs`,
//! `anonymous_ceiling.rs` and `keyed_reserve.rs` run unchanged to prove it.
//!
//! # What this is NOT
//!
//! It is not intra-request parallelism, and it is deliberately the *opposite*
//! decision. gatling has no shared pool, so fanning a clone out inside the
//! request at forge concurrency spawns tens of thousands of OS threads, and
//! the measured deflate curve is already
//! *slower* at 64 threads than at 32. Throughput at the load target comes from
//! serving many requests at once, each costing about one core, which is exactly
//! what this gate meters.

use std::collections::HashMap;
use std::sync::atomic::{AtomicU64, Ordering};
use std::sync::{Arc, Mutex};

use std::time::Duration;

use crate::governor::{
    Claim, Governor, Growth, Lane, LimitBand, OpClass, ServiceMark, Shed, DEFAULT_ADMISSION_WAIT,
    DEFAULT_MEMORY_BUDGET_BYTES, WIRE_MEMORY_DENOMINATOR, WIRE_MEMORY_NUMERATOR, WIRE_RESERVE_STEP,
};

/// Why a transfer was not admitted.
///
/// Every variant carries the number that was hit, because a refusal an operator
/// cannot tie to a number is a mystery rather than a signal.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Refusal {
    /// The server-wide concurrent-transfer limit is full.
    AtCapacity {
        /// The server-wide limit **in force at the moment of the refusal**.
        ///
        /// The *governed* limit, not the configured one: the configured number
        /// is the ceiling of
        /// [`LimitBand`](crate::governor::LimitBand), and what actually refuses
        /// is wherever the CoDel controller has moved to inside it. Reported
        /// rather than assumed, for the same reason the per-principal share is:
        /// an operator who read one number out of a config file and a different
        /// one out of a log would conclude the gate was broken.
        limit: usize,
    },
    /// This principal already holds its share of the server-wide limit.
    PrincipalAtCapacity {
        /// The per-principal limit **in force at the moment of the refusal**.
        ///
        /// Under [`SshConfig::per_principal_limit_is_hard`] that is the
        /// configured number. Otherwise it is the max-min share —
        /// `max(configured, server_wide / active)` — which is why it is
        /// reported rather than assumed: an operator who read the configured
        /// number out of a file and a different number out of a log would
        /// conclude the gate was broken.
        ///
        /// [`SshConfig::per_principal_limit_is_hard`]: crate::SshConfig::per_principal_limit_is_hard
        limit: usize,
        /// How many identities held a transfer when this one was refused.
        ///
        /// `1` means the refusal came from a hard configured cap: a share is
        /// never smaller than the whole server when there is only one claimant.
        /// Without this an operator cannot tell "you are being throttled
        /// because the box is shared" from "you are being throttled because
        /// somebody pinned a number".
        active_principals: usize,
    },
    /// This operation's **lane** is full while the rest of the server is held
    /// by the other lanes' floors (gate 1b, [`Lane::floor`]).
    ///
    /// Its own variant because the operator action is different: "the box is
    /// full" says raise the budget, and this one says a *write* storm is being
    /// held to five eighths of the box on purpose so that reads still land.
    LaneAtCapacity {
        /// The lane that was full.
        lane: Lane,
        /// What that lane may hold right now.
        ceiling: usize,
        /// The governed server-wide limit it is a share of.
        limit: usize,
    },
    /// The **anonymous** caller is already holding its whole ceiling.
    ///
    /// Its own variant, and not [`Refusal::PrincipalAtCapacity`] with the
    /// anonymous id in it, because the operator action is different in kind. A
    /// per-principal refusal names a client that can be found, telephoned or
    /// given a bigger share; this one says *strangers are at their cap*, which
    /// is either normal (a popular public repository) or the first sign of a
    /// scrape, and in neither case is the answer "raise the concurrency limit".
    /// It also carries no `active_principals`, because there is exactly one
    /// anonymous principal and dividing anything by it would be theatre.
    AnonymousAtCapacity {
        /// The anonymous ceiling in force, i.e.
        /// [`Admission::max_anonymous`]. Unlike every other limit reported
        /// here this one is *not* governed or shared — it is the operator's
        /// number, unmodified — which is exactly why it is worth printing:
        /// an operator who sees a refusal against 5 on a box with 200 free
        /// slots must be able to see immediately that a different knob did it.
        limit: usize,
    },
    /// The stranger asked for a slot inside the part of the box that only a
    /// **keyed** identity may hold (D23b).
    ///
    /// Its own variant, and specifically **not**
    /// [`Refusal::AnonymousAtCapacity`], because the two say opposite things
    /// about the machine and are fixed by opposite knobs. `AnonymousAtCapacity`
    /// means *strangers are at their cap* — it can fire on an idle box, and the
    /// only number that changes it is `--max-anonymous-transfers`. This one
    /// means *the box is too busy to spend any of itself on a stranger*: the
    /// anonymous ceiling is nowhere near reached, and lowering it would do
    /// nothing at all. The knob here is the server-wide limit, or the size of
    /// the box, or a browse replica.
    ///
    /// It also carries no principal and no share, because it is not a statement
    /// about this caller at all: it is a statement about occupancy, and the only
    /// thing about the caller that entered into it was that it has no key.
    KeyedReserve {
        /// Slots occupied as the gate's own compare-and-swap observed them —
        /// the reserve rides inside that atomic
        /// ([`Governor::claim_reserving`](crate::governor::Governor::claim_reserving)),
        /// so this is the number that actually made the decision and not a
        /// second reading taken nearby. Reported so an operator can see the
        /// decision rather than reconstruct it: `occupied + reserve >= limit`
        /// is the whole rule, visible in one line of log.
        occupied: usize,
        /// The **governed** server-wide limit those slots are out of, i.e.
        /// wherever the CoDel controller has moved to inside its band. The
        /// configured number would be actively misleading here: the whole
        /// reason this gate exists is that the governed limit moves and the
        /// anonymous ceiling does not.
        limit: usize,
        /// How many slots at the top of that limit are keyed-only, i.e.
        /// [`Admission::keyed_reserve`]. Printed because it is derived
        /// ([`CONTENTION_PERCENT`]) and therefore appears in no config file an
        /// operator could look it up in.
        reserve: usize,
    },
    /// The server's **memory budget** is already spoken for (gate 2).
    ///
    /// A third reason, kept separate from [`Refusal::AtCapacity`] on purpose: an
    /// operator who reads "at capacity" raises the concurrency limit, and doing
    /// that here makes the problem worse. The numbers travel so the log says
    /// which knob to turn.
    MemoryBudget {
        /// The class whose estimate did not fit.
        class: OpClass,
        /// What [`OpClass::estimated_bytes`] asked for.
        wanted: u64,
        /// What was reserved by operations already in flight.
        reserved: u64,
        /// The configured budget.
        budget: u64,
    },
    /// **The gate is PAUSED** — every transfer is refused, not for want of a
    /// slot but because the server asked for a quiet moment: the runtime
    /// data-set growth drains the box before it unmounts `/data`
    /// (`DATA-SET-GROWTH-FLOW.md` §3 step 5, T1/T12). Carries no number
    /// because no number was hit; carries the reason, whose words the client
    /// reads. Set and cleared by [`Admission::pause`] / [`Admission::unpause`]
    /// — one gate, so all three git fronts pause together.
    Paused {
        /// Why, and in which words.
        pause: Pause,
    },
    /// **This namespace is over its usage budget** (`admit-share`), so the
    /// transfer was held to the anonymous share and that share was full.
    ///
    /// Its own variant, and neither [`Refusal::AnonymousAtCapacity`] nor
    /// [`Refusal::KeyedReserve`], because the caller HAS a key and the operator
    /// action is different again: the tenant's budget (`--admit-limit`), or the
    /// tenant waiting for the window to move. The plan's sentence ("100 GB/30d
    /// used 112 GB; this clone ~1.2 GB; Reduced") is in the `warn` row beside it.
    #[cfg(feature = "admit-share")]
    ReducedShare {
        /// The anonymous ceiling the namespace is held to.
        ceiling: usize,
        /// `true` when the box was past `CONTENTION_PERCENT` and the keyed
        /// reserve refused it; `false` when its own ceiling was full.
        busy: bool,
    },
}

/// **Why the admission gate is paused** — the reason, and the sentence the
/// client is handed for it. An enum rather than a `String` so [`Refusal`]
/// stays `Copy` and the words are fixed in one place, where a test can hold
/// them.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Pause {
    /// The data set is being grown at runtime: drained, unmounted, grown,
    /// mounted again — minutes, and the box stays up throughout.
    Growing,
}

impl Pause {
    /// The client's sentence. **The word `growing` is load-bearing** the way
    /// `starting` is in [`crate::STARTING_MESSAGE`]: it is what tells a
    /// person, a CI script and the bench that this is a planned pause with an
    /// end, not a broken repository and not a full box.
    pub const fn words(self) -> &'static str {
        match self {
            Pause::Growing => GROWING_MESSAGE,
        }
    }
    /// The log's `reason` field.
    pub const fn reason(self) -> &'static str {
        match self {
            Pause::Growing => "paused-growing",
        }
    }
}

/// What a client is told while the data set grows. The contract's words
/// (`DATA-SET-GROWTH-FLOW.md` §0, ruling 1; the drain design note in
/// `.nornir/data-set-grow-drain.md`), verbatim.
///
/// It names no estate: the same build serves gunnar.rs, test2 and every sold estate, and a
/// test2 push told "gunnar.rs is growing" was told about somebody else's server (2026-10-09).
pub const GROWING_MESSAGE: &str = "The server is growing its storage to make room for new \
                                   members. Pushes are paused — please try again in a few \
                                   minutes.";

impl Refusal {
    /// What the client is told, in its own protocol.
    ///
    /// Says *try again* explicitly. "Server busy" with no instruction reads as a
    /// permanent failure to a human and to a CI log alike, and this one is
    /// transient by definition.
    pub fn client_message(&self) -> String {
        match self {
            Refusal::AtCapacity { limit } => format!(
                "the server is already serving its limit of {limit} concurrent git transfers; \
                 try again in a moment"
            ),
            // Both arms must keep the words "per-principal limit", and neither
            // may contain "concurrent git transfers": `installtest`'s
            // `refusals_in_log` tells the two refusals apart by exactly those
            // substrings, because the log records the refusal by its `Display`
            // — the same sentence the client is handed. Two witnesses of one
            // event only agree if they are reading the same string.
            Refusal::PrincipalAtCapacity {
                limit,
                active_principals,
            } if *active_principals > 1 => format!(
                "you already have {limit} git transfers in flight, which is your fair share of \
                 this server while {active_principals} identities are using it; that is the \
                 per-principal limit in force right now, and it rises again as the server \
                 empties; try again in a moment"
            ),
            Refusal::PrincipalAtCapacity { limit, .. } => format!(
                "you already have {limit} git transfers in flight on this server, which is the \
                 per-principal limit; try again in a moment"
            ),
            // Shares neither "concurrent git transfers" nor "per-principal
            // limit": `installtest`'s `refusals_in_log` tells refusals apart by
            // exactly those, and a fourth reason that collided with either
            // would be counted as the wrong one.
            Refusal::LaneAtCapacity {
                lane,
                ceiling,
                limit,
            } => format!(
                "this server is holding {lane} operations to {ceiling} of its {limit} slots so \
                 that the other kinds of request still land; try again in a moment"
            ),
            // Shares neither "concurrent git transfers" nor "per-principal
            // limit", for the reason given twice above. It says *unauthenticated*
            // in the first clause so a human reading it in a `git` output knows
            // instantly that the fix is a key, not patience: this is the one
            // refusal on the box that a caller can make go away by logging in.
            Refusal::AnonymousAtCapacity { limit } => format!(
                "this server is already serving {limit} unauthenticated readers, which is all it \
                 offers without a key; try again in a moment, or push a key and clone as \
                 yourself"
            ),
            // Shares neither "concurrent git transfers" nor "per-principal
            // limit", for the reason given twice above — and it must not be the
            // same sentence as `AnonymousAtCapacity` either, because the two
            // are fixed by different knobs. It says *with a key* so the human
            // holding the `git` prompt learns the same thing the operator does:
            // this refusal is about not having one.
            Refusal::KeyedReserve {
                occupied,
                limit,
                reserve,
            } => format!(
                "this server is {occupied} of {limit} slots busy and keeps its last {reserve} for \
                 callers with a key; try again in a moment, or push a key and clone as yourself"
            ),
            // Deliberately shares neither of the two substrings above:
            // `installtest`'s `refusals_in_log` tells refusals apart by
            // "concurrent git transfers" and "per-principal limit", and a third
            // reason that collided with either would be counted as the wrong
            // one. It says *memory* so the operator raises the budget rather
            // than the concurrency limit.
            Refusal::MemoryBudget {
                class,
                wanted,
                reserved,
                budget,
            } => format!(
                "this server holds a memory budget of {} MiB for git operations in flight, {} MiB \
                 of it is reserved, and a {class} needs {} MiB more than is left; try again in a \
                 moment",
                budget / (1024 * 1024),
                reserved / (1024 * 1024),
                wanted / (1024 * 1024),
            ),
            // Its own words entirely (see `Pause::words`): none of the three
            // substrings the other refusals are told apart by, and a
            // sentence that says WHY and for how long.
            Refusal::Paused { pause } => pause.words().to_owned(),
            // Shares neither "concurrent git transfers" nor "per-principal
            // limit" (`installtest`'s `refusals_in_log`), and says *budget* so
            // the human at the prompt learns it is about their usage, not the box.
            #[cfg(feature = "admit-share")]
            Refusal::ReducedShare { ceiling, busy } => {
                format!(
                "this namespace is over its transfer budget for the window, so it is served at \
                 the reduced share of {ceiling} git transfers at once{}; try again in a moment, \
                 or ask for a larger budget",
                if *busy { ", and none while the server is this busy" } else { "" }
            )
            }
        }
    }

    /// The **machine-readable** reason, for a log's `reason` field.
    ///
    /// The sentence above is for the person holding the `git` prompt; this is
    /// for the operator filtering a log, and they need different things. Until
    /// the anonymous ceiling existed every refusal here was fairly described by
    /// one string — the box was out of *something* server-wide — and both
    /// fronts hard-coded `admission-gate-full` accordingly. That string is now
    /// actively misleading on the one refusal that can fire with the box nearly
    /// empty: an operator who greps `admission-gate-full`, sees a hit, and
    /// looks at 59 free slots concludes the gate is broken, which is exactly
    /// the "the limit exists but produces no signal an operator can act on"
    /// failure the unbounded listener was about, wearing a different hat.
    ///
    /// Only the fronts that can actually *see* an anonymous caller use this.
    /// The gRPC control plane keeps its literal, because
    /// [`gunnar_grpc::auth`] refuses the anonymous principal outright: a
    /// `reason` there that could report anonymity would be describing a state
    /// that crate makes unreachable.
    pub const fn reason(&self) -> &'static str {
        match self {
            Refusal::AtCapacity { .. } => "admission-gate-full",
            Refusal::LaneAtCapacity { .. } => "admission-lane-full",
            Refusal::PrincipalAtCapacity { .. } => "admission-principal-share",
            Refusal::AnonymousAtCapacity { .. } => "admission-anonymous-ceiling",
            Refusal::KeyedReserve { .. } => "admission-keyed-reserve",
            Refusal::MemoryBudget { .. } => "admission-memory-budget",
            Refusal::Paused { pause } => pause.reason(),
            #[cfg(feature = "admit-share")]
            Refusal::ReducedShare { .. } => "admission-reduced-share",
        }
    }
}

impl std::fmt::Display for Refusal {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.write_str(&self.client_message())
    }
}

/// A snapshot of the gate's counters.
///
/// Taken atomically field by field rather than under a lock: these are a
/// **signal**, and a signal that costs a mutex on the request path would be
/// paid for by the thing it is measuring.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
pub struct AdmissionStats {
    /// Transfers let through since the server started.
    pub admitted: u64,
    /// Transfers refused because the server-wide limit was full.
    pub refused_at_capacity: u64,
    /// Transfers refused because one principal held its whole share.
    pub refused_per_principal: u64,
    /// Transfers refused because the **memory budget** was spoken for (gate 2).
    pub refused_memory: u64,
    /// Reads refused because the **anonymous ceiling** was full.
    ///
    /// Deliberately not folded into
    /// [`refused_per_principal`](Self::refused_per_principal): that counter
    /// moving means a client gunnar knows about is being throttled, and this one
    /// moving means strangers are at their cap. A dashboard that added them
    /// together would show one number that can be entirely benign (a popular
    /// public repository, working exactly as designed) or entirely alarming (a
    /// keyed CI fleet starved of its share) with no way to tell which.
    pub refused_anonymous: u64,
    /// Reads refused because the slot they wanted was inside the **keyed
    /// reserve** (D23b).
    ///
    /// Deliberately not folded into
    /// [`refused_anonymous`](Self::refused_anonymous), which is the other half
    /// of the same door and means the opposite thing. `refused_anonymous`
    /// moving on an otherwise quiet box is *normal* — a popular public
    /// repository, the ceiling doing its job — and the operator response is
    /// none. This counter moving means the box was over
    /// [`CONTENTION_PERCENT`] full of real work at the moment a stranger
    /// asked, which is a **capacity** signal: it is the public door being shut
    /// by load, and it will keep being shut until either the load falls or the
    /// box grows. Summed together the two would show one number that means
    /// "everything is fine" and "your public repositories are effectively
    /// offline" with no way to tell which.
    pub refused_keyed_reserve: u64,
    /// Transfers refused at the **reduced share** (`admit-share`): a namespace
    /// over its usage budget, held to the anonymous width.
    #[cfg(feature = "admit-share")]
    pub refused_reduced: u64,
    /// Transfers refused because the gate was **paused** ([`Pause`]): the
    /// data set was being grown. Its own counter, because this one moving is
    /// neither load nor strangers — it is a planned window, and an operator
    /// reading it wants to know how many clients met the window, not to
    /// raise any limit.
    pub refused_paused: u64,
    /// Transfers in flight right now.
    pub in_flight: u64,
    /// The most that were ever in flight at once. This is the number that says
    /// how close to the ceiling a server has actually been; `in_flight` alone
    /// never shows it, because a sampler always misses the peak.
    pub peak_in_flight: u64,
    /// The **governed** server-wide limit right now, i.e. where the CoDel
    /// controller has moved to inside its band. `0` when there is no limit.
    ///
    /// Without this the shed is invisible: an operator seeing refusals against a
    /// configured 100 has no way to know the governor has taken it to 37, which
    /// is the same "the limit exists but produces no signal" the unbounded listener was
    /// written about.
    pub limit: usize,
    /// Bytes reserved by operations in flight, against
    /// [`memory_budget`](Admission::memory_budget).
    pub reserved_bytes: u64,
    /// Operations that were admitted only after **waiting** for a slot
    /// ([`DEFAULT_ADMISSION_WAIT`]).
    ///
    /// This is the queue made countable. It is not a failure count: every one
    /// of these was served, and every one of them would have been a refusal
    /// before 2026-08-30.
    pub admitted_after_wait: u64,
    /// Milliseconds spent waiting, summed over every operation that waited.
    pub waited_ms_total: u64,
    /// The longest single admission wait, in milliseconds. The number that says
    /// whether the bound is doing anything.
    pub peak_wait_ms: u64,
    /// Waits that ran out and became refusals.
    pub refused_after_wait: u64,
    /// Refusals because the operation's **lane** was at its ceiling while the
    /// other lanes' floors held the rest.
    pub refused_lane: u64,
    /// Transfers whose reservation grew past the memory budget while they ran.
    /// Nonzero means one transfer is now keeping new ones out — which is the
    /// budget working, not failing.
    pub overdrawn: u64,
    /// Bytes charged by running transfers over and above their admission
    /// reservations.
    pub grown_bytes: u64,
    /// Milliseconds between admission and the transfer's work actually
    /// starting, summed over every transfer that reported it.
    ///
    /// **Observed, not controlled.** Until 2026-08-30 this gap *was* the CoDel
    /// controller's input, and that was the defect: it is the tokio ready queue
    /// plus the blocking-pool handoff, which is set by how many clients are
    /// connected and doing crypto, not by how many transfers are admitted. A
    /// refused client reconnects, so shedding made this number worse and the
    /// controller shed until it hit its floor — the `limit of 19`
    /// measured. It is kept as a *counter* because it is a real and useful
    /// symptom of a box that is out of CPU; it is no longer a control input.
    pub handoff_ms_total: u64,
    /// The longest single admission-to-work gap, in milliseconds.
    pub peak_handoff_ms: u64,
    /// Transfers that finished **without ever reporting that their work
    /// started**, so they contributed no sojourn sample.
    ///
    /// A blindness counter, not a health one. The CoDel controller only sees
    /// what [`AdmissionGuard::entered_service`] files; a caller that forgets to
    /// call it leaves the controller with no samples at all, and a controller
    /// with no samples grows to the top of its band and stays there — a gate
    /// that looks like it is working and is not. This number must be 0 on any
    /// path that is actually governed.
    pub unmeasured_sojourn: u64,
}

impl AdmissionStats {
    /// Every refusal, however it was refused.
    pub fn refused(&self) -> u64 {
        self.refused_at_capacity
            + self.refused_lane
            + self.refused_per_principal
            + self.refused_memory
            + self.refused_anonymous
            + self.refused_keyed_reserve
            + self.refused_paused
            + self.refused_reduced()
    }

    #[cfg(feature = "admit-share")]
    fn refused_reduced(&self) -> u64 {
        self.refused_reduced
    }

    #[cfg(not(feature = "admit-share"))]
    fn refused_reduced(&self) -> u64 {
        0
    }

    /// The mean admission wait over the operations that waited, in
    /// milliseconds. `0` when nothing has ever waited.
    pub fn mean_wait_ms(&self) -> u64 {
        self.waited_ms_total
            .checked_div(self.admitted_after_wait.max(1))
            .unwrap_or(0)
    }
}

#[derive(Debug, Default)]
struct Counters {
    admitted: AtomicU64,
    refused_at_capacity: AtomicU64,
    refused_lane: AtomicU64,
    refused_per_principal: AtomicU64,
    refused_memory: AtomicU64,
    /// Reads refused by the anonymous ceiling.
    refused_anonymous: AtomicU64,
    /// Reads refused by the keyed reserve (D23b).
    refused_keyed_reserve: AtomicU64,
    /// Transfers refused while the gate was paused ([`Pause`]).
    refused_paused: AtomicU64,
    /// Transfers refused at the reduced share (`admit-share`).
    #[cfg(feature = "admit-share")]
    refused_reduced: AtomicU64,
    in_flight: AtomicU64,
    peak_in_flight: AtomicU64,
    unmeasured_sojourn: AtomicU64,
    /// Admissions that had to wait for a slot at all.
    admitted_after_wait: AtomicU64,
    /// Milliseconds spent waiting, summed over every operation that waited.
    waited_ms_total: AtomicU64,
    /// The longest single wait, in milliseconds.
    peak_wait_ms: AtomicU64,
    /// Waits that ran out and became refusals.
    refused_after_wait: AtomicU64,
    /// Transfers whose growing reservation went past the budget
    /// ([`Growth::Overdrawn`]).
    overdrawn: AtomicU64,
    /// Bytes charged to the ledger by running transfers, over and above their
    /// admission reservations.
    grown_bytes: AtomicU64,
    /// Milliseconds between admission and the transfer's work starting, summed.
    handoff_ms_total: AtomicU64,
    /// The longest single admission-to-work gap, in milliseconds.
    peak_handoff_ms: AtomicU64,
}

/// How full the server-wide gate must be, in percent, before the per-principal
/// share is enforced at all.
///
/// **Chosen from the measurement, not picked round.** With the shipped defaults
/// (256 server-wide, 32 per principal) 75 % is 192 in flight, which leaves 64
/// free slots — exactly **twice** the configured per-principal floor — as
/// headroom in which a newly arriving identity is never asked about fairness.
/// Two floors rather than one, so a second identity can arrive *and* be
/// throttled to its floor without the first having to give anything back.
///
/// Below the threshold there are free slots by definition, and refusing a
/// transfer there refuses work the box could actually do: that is the exact
/// mistake the old unconditional cap made, where 32 clients on one key were
/// refused against 224 idle slots. 192 in flight is also already 6× the core
/// count of the 32-core box the defaults were sized for
/// (Measurement 3 — 63 % efficiency at 32
/// threads), so the machine is deep into queueing before fairness engages, and
/// fairness is only interesting once queueing has started.
///
/// It is deliberately **not** configurable. A third number to get wrong is how
/// the first two ended up unexamined for four sessions; the two an operator
/// sizes are the limit and the floor.
pub const CONTENTION_PERCENT: usize = 75;

/// **How many git reads the server will serve without a key at all.**
///
/// Five. Chosen small and chosen *absolute*, and both halves are the decision:
///
/// * **Small.** Against the shipped 256 server-wide this is under 2 % of the
///   box. That number is not a guess at demand, it is a statement about
///   *liability*: it is the most an operator can lose to a caller who cannot be
///   named, revoked, billed or telephoned. Everything above five anonymous
///   readers is served by them coming back a moment later, which git clients do
///   by themselves; nothing above five is worth an operator's console being
///   slow.
/// * **Absolute, not a fraction of the limit.** A percentage would grow the
///   stranger's allowance every time an operator bought a bigger box, which is
///   exactly backwards — a bigger appliance is bought to serve *the team*
///   better, and the untrusted share should stay where it was. It also keeps
///   this ceiling meaningful when the CoDel controller has shed the governed
///   limit right down: a fraction of a shrinking limit shrinks with it, and the
///   moment the box is in trouble is the moment this cap most needs to hold
///   still.
///
/// It is far below [`DEFAULT_MAX_TRANSFERS_PER_PRINCIPAL`] (32), which is the
/// number *one keyed identity is guaranteed*, and that ordering is asserted by
/// `tests/anonymous_ceiling.rs` rather than left to two constants drifting
/// apart. It is well above 1, because a public repository that can only ever be
/// cloned by one person at a time is not public in any useful sense — and
/// because a ceiling of 1 turns every ordinary CI runner pair into a refusal.
///
/// Raise it on a box whose *purpose* is public serving (a browse replica, see
/// [`crate::control::AnonymousPolicy`]); lower it to 0 — or close the door with
/// [`SshConfig::without_anonymous_access`](crate::SshConfig::without_anonymous_access)
/// — on one whose purpose is not.
///
/// [`DEFAULT_MAX_TRANSFERS_PER_PRINCIPAL`]: crate::config::DEFAULT_MAX_TRANSFERS_PER_PRINCIPAL
pub const DEFAULT_MAX_ANONYMOUS_TRANSFERS: usize = 5;

/// How long a parked admission sleeps before re-asking, when nothing wakes it.
///
/// A waiter is normally woken by a guard being dropped
/// ([`Admission::slot_freed`]). This bound exists for the other way the answer
/// changes: the CoDel limit grows by one every window
/// ([`crate::governor::INTERVAL`], 100 ms) whose delay was under target, so a
/// server that is not finishing anything can still have room a moment later.
/// One window is therefore exactly the right period — sleeping longer would
/// hold clients past the moment their slot existed, shorter would poll for no
/// new information.
const RETRY_INTERVAL: Duration = crate::governor::INTERVAL;

/// How often the `git.transfer.waited` row may be written, in milliseconds.
///
/// One second. See [`Admission::wait_row_due`]: the fact that clients are
/// waiting is worth a row, and one row per waiting client is worth none because
/// it is the only thing left on the screen.
const WAIT_ROW_INTERVAL_MS: u64 = 1000;

/// Whether an admission attempt that fails should move a refusal counter.
///
/// A wait loop asks the same door many times for one operation; only the last
/// answer is a refusal. See [`Admission::attempt`].
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Count {
    /// This ask is final: a refusal is a refusal.
    Yes,
    /// This ask is a retry inside a wait: a refusal here is a poll.
    No,
}

impl Count {
    fn bump(self, counter: &AtomicU64) {
        if self == Count::Yes {
            counter.fetch_add(1, Ordering::Relaxed);
        }
    }
}

/// The gate itself.
///
/// Cheap to clone by `Arc`; it holds one semaphore and one small map.
#[derive(Debug)]
pub struct Admission {
    /// Both gates, or `None` when the operator turned the server-wide limit
    /// off. That is a legitimate configuration (an appliance behind its own
    /// front door), and it is spelled as an `Option` rather than as `usize::MAX`
    /// so a log can say "unlimited" and mean it. Unlimited means unlimited: no
    /// concurrency band and no memory budget, because a budget that binds on a
    /// gate the operator switched off is a limit they did not ask for.
    governor: Option<Governor>,
    max_concurrent: Option<usize>,
    max_per_principal: Option<usize>,
    /// When `true`, `max_per_principal` binds unconditionally — the behaviour
    /// before max-min fairness. `false` (the default) makes it the **floor** of
    /// a share that only applies under contention.
    hard_per_principal: bool,
    /// The **anonymous** ceiling: the most reads
    /// [`gunnar_core::ANONYMOUS_PRINCIPAL_ID`] may hold at once, or `None` to
    /// let anonymous take the same max-min share as any keyed identity.
    ///
    /// Kept as its own field rather than as an entry in some per-principal
    /// override map, because a map would invite a second override and the
    /// second override is a policy engine. There is exactly one caller that has
    /// no identity, so there is exactly one number here.
    anonymous_ceiling: Option<usize>,
    /// Only ever holds principals with a transfer in flight; an entry is removed
    /// when its count reaches zero, so an idle server carries an empty map
    /// however many identities have ever connected.
    ///
    /// Its **length** is therefore the live count of active identities, which is
    /// what the max-min share divides by. Keeping the map free of zero entries
    /// is no longer only hygiene: a leaked entry would shrink everybody's share.
    per_principal: Mutex<HashMap<String, usize>>,
    counters: Arc<Counters>,
    /// Woken every time a guard is dropped, so a waiting admission learns of a
    /// free slot the instant there is one rather than by polling.
    ///
    /// `tokio::sync::Notify` and not a semaphore: the thing being waited for is
    /// not one permit but *the three gates agreeing*, and only
    /// [`Admission::try_admit`] knows whether they do. A waiter is woken, it
    /// re-asks, and if the answer is still no it waits again — which is also
    /// why a lost wakeup is impossible here: the `Notified` future is created
    /// **before** the try, so a release that lands in between is still seen.
    slot_freed: tokio::sync::Notify,
    /// How long an operation may wait for a slot before it is refused. Zero
    /// restores the instant refusal exactly.
    wait: Duration,
    /// How long a **keyed** (authenticated) caller may wait, when longer than
    /// [`Admission::wait`]. `None`: the same wait as everybody. See
    /// [`crate::governor::DEFAULT_KEYED_ADMISSION_WAIT`].
    keyed_wait: Option<Duration>,
    /// When this gate was built, so [`Admission::wait_row_due`] can hold a
    /// deadline in one atomic instead of a lock.
    epoch: std::time::Instant,
    /// Milliseconds since [`Admission::epoch`] at which the next `waited` row
    /// may be written.
    next_wait_row_ms: AtomicU64,
    /// **The pause** ([`Pause`]), `0` for none, else `1 + Pause as u8`. One
    /// atomic read on every admission and nothing else on the hot path; set
    /// by [`Self::pause`] from the runtime growth's drain, cleared by
    /// [`Self::unpause`] when `/data` is back — or when the growth aborted.
    pause: std::sync::atomic::AtomicU8,
    /// **The per-namespace flow ledger, when the server wired one.**
    ///
    /// `None` in every test in this crate and in any build with no control
    /// store behind it, which is why it is an `Option` rather than something
    /// this crate constructs: the gate must work on a bare `Admission::new`,
    /// and a ledger it invented would count into a store nobody reads.
    ///
    /// It is here rather than at the three doors that call [`Self::try_admit`]
    /// — the SSH exec line, the smart-HTTP handler and the gRPC transfer gate —
    /// because those three converge on [`Self::count_refusal`] and a refusal
    /// counted at each of them would be the same event counted three ways, one
    /// of which would go stale the first time a fourth door was added.
    flow: Option<Arc<gunnar_core::FlowLedger>>,
    /// **The admission policy** (`admit-share`): asked for a keyed transfer's
    /// class. [`edda_admit::PassThrough`] until the server hands it the
    /// configured one ([`Admission::with_policy`]), so a gate built by a test
    /// that never heard of budgets decides exactly as before.
    #[cfg(feature = "admit-share")]
    policy: Policy,
}

/// `Arc<dyn Admit>` with a `Debug` that names the policy, so [`Admission`]
/// keeps its derive.
#[cfg(feature = "admit-share")]
#[derive(Clone)]
struct Policy(Arc<dyn edda_admit::Admit>);

#[cfg(feature = "admit-share")]
impl std::fmt::Debug for Policy {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.write_str(self.0.policy())
    }
}

/// **The `edda_admit` kind of work an [`OpClass`] is.** Estimates learn per kind.
pub fn work_kind(class: OpClass) -> edda_admit::WorkKind {
    match class {
        OpClass::Clone => edda_admit::WorkKind::Clone,
        OpClass::Fetch => edda_admit::WorkKind::Fetch,
        OpClass::Push => edda_admit::WorkKind::Push,
        OpClass::LsRemote | OpClass::Gc => edda_admit::WorkKind::Other,
    }
}

impl Admission {
    /// Build a gate from the two configured limits, with the per-principal one
    /// as a max-min **share** (see the module docs), the server-wide one as the
    /// ceiling of the governor's band, and the default memory budget.
    pub fn new(max_concurrent: Option<usize>, max_per_principal: Option<usize>) -> Admission {
        Admission::with_memory_budget(
            max_concurrent,
            max_per_principal,
            DEFAULT_MEMORY_BUDGET_BYTES,
        )
    }

    /// **Give the gate a flow ledger**, so a refusal is charged to the
    /// namespace that met it and not only to the box's own counters.
    ///
    /// Builder-style and optional for [`Admission::flow`]'s reason. A gate
    /// without one refuses exactly as it always did and simply reports the
    /// refusal nowhere but its own `AdmissionStats`, which is what every build
    /// before the flow ledger existed did.
    pub fn with_flow(mut self, flow: Arc<gunnar_core::FlowLedger>) -> Admission {
        self.flow = Some(flow);
        self
    }

    /// **The flow ledger this gate was given**, or `None`.
    ///
    /// Handed out because this crate's other tenant-facing door — the exec line
    /// that denies a principal who holds no grant on the repository it named —
    /// needs to charge that tenant and has nowhere else in this crate to reach
    /// a ledger. One handle, wired once by the server, rather than a second
    /// field on `Shared` that a future constructor could forget to fill.
    pub fn flow(&self) -> Option<&Arc<gunnar_core::FlowLedger>> {
        self.flow.as_ref()
    }

    /// The same gate with the memory budget (gate 2) chosen explicitly.
    ///
    /// The budget has to be an operator's number: by the OS-less law this code
    /// cannot read the machine's RAM, so
    /// [`DEFAULT_MEMORY_BUDGET_BYTES`](crate::governor::DEFAULT_MEMORY_BUDGET_BYTES)
    /// is a constant with a measurement behind it rather than a fit to the box.
    pub fn with_memory_budget(
        max_concurrent: Option<usize>,
        max_per_principal: Option<usize>,
        memory_budget: u64,
    ) -> Admission {
        Admission {
            governor: max_concurrent
                .map(|budget| Governor::new(LimitBand::for_budget(budget), memory_budget)),
            max_concurrent,
            max_per_principal,
            hard_per_principal: false,
            // On by default, because the anonymous door is open by default: a limit
            // that has to be switched on is not protecting the configuration
            // anybody actually runs.
            anonymous_ceiling: Some(DEFAULT_MAX_ANONYMOUS_TRANSFERS),
            per_principal: Mutex::new(HashMap::new()),
            counters: Arc::new(Counters::default()),
            slot_freed: tokio::sync::Notify::new(),
            wait: DEFAULT_ADMISSION_WAIT,
            keyed_wait: None,
            epoch: std::time::Instant::now(),
            next_wait_row_ms: AtomicU64::new(0),
            pause: std::sync::atomic::AtomicU8::new(0),
            flow: None,
            #[cfg(feature = "admit-share")]
            policy: Policy(Arc::new(edda_admit::PassThrough::new())),
        }
    }

    /// **Give the gate its admission policy** (`admit-share`) — the one built
    /// from `--admit-limit` ([`edda_admit::Settings::build`]; `none` is
    /// PassThrough).
    #[cfg(feature = "admit-share")]
    pub fn with_policy(mut self, policy: Arc<dyn edda_admit::Admit>) -> Admission {
        self.policy = Policy(policy);
        self
    }

    /// **The admission policy in force**, for `Usage.AdmitPlan`.
    ///
    /// Without `admit-share` the gate has none, and this is a fresh
    /// [`edda_admit::PassThrough`] — every kind `Full`, no budget, nothing
    /// learnt — which is the truth about that build.
    pub fn admit_policy(&self) -> Arc<dyn edda_admit::Admit> {
        #[cfg(feature = "admit-share")]
        {
            Arc::clone(&self.policy.0)
        }
        #[cfg(not(feature = "admit-share"))]
        {
            Arc::new(edda_admit::PassThrough::new())
        }
    }

    /// **The usage `Usage.AdmitPlan` reports for `namespace`** — a READ for
    /// the admin view, never on the admission path.
    ///
    /// A policy with a budget answers from its own table (the same figure the
    /// gate plans with). Without one — PassThrough, or a build without
    /// `admit-share` — the flow ledger's window is summed directly, so the
    /// view still shows what moved; a gate with no ledger counted nothing and
    /// says zero.
    pub fn admit_usage(
        &self,
        policy: &dyn edda_admit::Admit,
        namespace: &str,
    ) -> edda_admit::Usage {
        if policy.limit().is_some() {
            return policy.usage(&edda_admit::Consumer::Namespace(namespace.to_owned()));
        }
        let window = edda_admit::DEFAULT_WINDOW_DAYS;
        let bytes = match &self.flow {
            Some(flow) if !namespace.is_empty() => {
                flow.window_bytes(namespace, gunnar_core::now_ms(), window)
            }
            _ => 0,
        };
        edda_admit::Usage::new(bytes, window)
    }

    /// **The flow ledger as a seed** for a policy's table: a namespace's
    /// bytes per UTC day over the window, read ONCE per namespace on first
    /// sight. See [`gunnar_core::FlowLedger::moved`].
    pub fn flow_seed(flow: Arc<gunnar_core::FlowLedger>) -> Arc<dyn edda_admit::Seed> {
        Arc::new(
            move |who: &edda_admit::Consumer, first_day: u64, today: u64| {
                let edda_admit::Consumer::Namespace(namespace) = who else {
                    return Vec::new();
                };
                let days = u32::try_from(today.saturating_sub(first_day) + 1).unwrap_or(u32::MAX);
                let now = gunnar_core::now_ms();
                flow.moved(namespace, now, days)
                    .into_iter()
                    .filter_map(|(day, bytes)| u64::try_from(day).ok().map(|d| (d, bytes)))
                    .collect()
            },
        )
    }

    /// **The plan for a keyed transfer** (`admit-share`); `None` for the
    /// anonymous principal, which has its own ceiling and no namespace budget.
    #[cfg(feature = "admit-share")]
    fn plan(&self, principal: &str, namespace: &str, class: OpClass) -> Option<edda_admit::Plan> {
        if principal == gunnar_core::ANONYMOUS_PRINCIPAL_ID {
            return None;
        }
        let policy = &*self.policy.0;
        let who = edda_admit::Consumer::Namespace(namespace.to_owned());
        // The policy's own table: lock-free loads for a namespace already
        // seen, seeded ONCE from the flow ledger on first sight
        // (`GunnarSsh::with_flow`). PassThrough answers without a table.
        let usage = policy.usage(&who);
        Some(policy.plan(&edda_admit::Work::new(work_kind(class), None), &who, &usage))
    }

    /// The same gate with `max_per_principal` as an **unconditional** ceiling.
    ///
    /// The escape hatch for a host where an identity is a tenant and the cap is
    /// what is being sold, rather than an appliance where the identities are all
    /// the same team. Selected by
    /// [`SshConfig::per_principal_limit_is_hard`](crate::SshConfig::per_principal_limit_is_hard).
    pub fn hard_capped(
        max_concurrent: Option<usize>,
        max_per_principal: Option<usize>,
    ) -> Admission {
        Admission {
            hard_per_principal: true,
            ..Admission::new(max_concurrent, max_per_principal)
        }
    }

    /// The same gate with a different memory budget (gate 2).
    ///
    /// A builder rather than a fourth constructor argument, so the two
    /// concurrency numbers keep the shape every existing caller passes them in.
    /// Only meaningful before the gate has admitted anything: it replaces the
    /// governor, and a governor holds live occupancy.
    pub fn with_budget(self, memory_budget: u64) -> Admission {
        Admission {
            governor: self
                .max_concurrent
                .map(|budget| Governor::new(LimitBand::for_budget(budget), memory_budget)),
            ..self
        }
    }

    /// A gate with no limits at all; every request is admitted.
    ///
    /// **Including the anonymous ceiling.** "Unlimited" has to mean unlimited or
    /// it is a trap: every test that reaches for this gate does so to take the
    /// admission gate *out* of the picture, and one surviving ceiling would make
    /// those tests fail for a reason that has nothing to do with what they are
    /// about. An operator who wants only the anonymous ceiling builds a real
    /// gate and passes `None` for the other two.
    pub fn unlimited() -> Admission {
        Admission {
            anonymous_ceiling: None,
            ..Admission::new(None, None)
        }
    }

    /// The same gate with the **anonymous ceiling** set explicitly;
    /// `None` switches it off and lets anonymous take a keyed identity's share.
    ///
    /// A builder and not a fourth constructor argument for the same reason
    /// [`Admission::with_budget`] is one: the two concurrency numbers keep the
    /// shape every existing caller already passes them in, and a gate built by
    /// a test that has never heard of the anonymous door still gets the default ceiling rather
    /// than silently getting none.
    pub fn with_anonymous_ceiling(self, anonymous_ceiling: Option<usize>) -> Admission {
        Admission {
            anonymous_ceiling,
            ..self
        }
    }

    /// The anonymous ceiling in force, or `None` when anonymous is held
    /// only by the ordinary per-principal share.
    pub fn max_anonymous(&self) -> Option<usize> {
        self.anonymous_ceiling
    }

    /// **How many slots at the top of the box are keyed-only** right now
    /// (D23b), or `0` on a gate with no server-wide limit to reserve out of.
    ///
    /// Derived, never configured. It is whatever is left of the **governed**
    /// limit above [`CONTENTION_PERCENT`], so it moves with the CoDel
    /// controller — which is the entire point: the anonymous ceiling is an
    /// absolute number and the box it is a share of is not, and a stranger
    /// holding five of a shed nineteen is holding a quarter of the machine at
    /// the moment the machine has least to give.
    ///
    /// Reusing `CONTENTION_PERCENT` rather than inventing a fourth number is
    /// the decision, not a shortcut. That constant is *defined* as the
    /// occupancy at which gunnar starts asking keyed identities to share with
    /// each other; the moment it does that is exactly the moment it should stop
    /// asking them to share with strangers. Two thresholds would be two
    /// opinions about when the box is busy, and the order between them would be
    /// an accident of arithmetic rather than a decision.
    ///
    /// At least `1` whenever there is a limit at all: a reserve that rounded to
    /// zero on a small box would be an empty guarantee at exactly the size where
    /// a single stranger is the whole problem.
    pub fn keyed_reserve(&self) -> usize {
        let Some(governor) = &self.governor else {
            return 0;
        };
        let limit = governor.limit();
        // The largest occupancy still BELOW contention is
        // `ceil(limit * P / 100) - 1`; everything from there up is reserved.
        // Integer throughout — a float threshold would put the boundary
        // somewhere an operator cannot reproduce with a calculator.
        limit - limit.saturating_mul(CONTENTION_PERCENT).div_ceil(100) + 1
    }

    /// Whether the per-principal limit binds unconditionally.
    pub fn per_principal_is_hard(&self) -> bool {
        self.hard_per_principal
    }

    /// The server-wide limit, if there is one.
    pub fn max_concurrent(&self) -> Option<usize> {
        self.max_concurrent
    }

    /// The per-principal limit, if there is one.
    pub fn max_per_principal(&self) -> Option<usize> {
        self.max_per_principal
    }

    /// The **governed** server-wide limit right now. `None` when unlimited.
    pub fn governed_limit(&self) -> Option<usize> {
        self.governor.as_ref().map(Governor::limit)
    }

    /// The memory budget (gate 2). `None` when unlimited.
    pub fn memory_budget(&self) -> Option<u64> {
        self.governor.as_ref().map(|g| g.memory().budget())
    }

    /// The governor, for a caller that wants the controller's own counters.
    pub fn governor(&self) -> Option<&Governor> {
        self.governor.as_ref()
    }

    /// Take a slot for `principal` running an operation of `class`, or say why
    /// not. **Never blocks.**
    ///
    /// This is the ONE door. Every user-level git operation — clone, fetch,
    /// push, ls-remote, gc — passes through here and through nothing else, so
    /// there is a single place where "the server cannot keep up" is decided and
    /// a single place a counter has to be trusted.
    ///
    /// Three gates, in this order, all of them **before any of the operation's
    /// work is allocated**:
    ///
    /// 1. **time** — the governed concurrency limit, moved by the per-class
    ///    CoDel controller;
    /// 2. **space** — the memory reservation for `class`;
    /// 3. **fairness** — the per-principal max-min share.
    ///
    /// Time and space come first because they are facts about the *server*, and
    /// a server that cannot serve anybody should not spend a lock working out
    /// whose turn it was. The returned guard releases everything when it is
    /// dropped — slot, reservation and principal count — so no error path, and
    /// no panic, can skip it. That is how a leaked permit turns a working limit
    /// into a server that refuses everything an hour after start-up.
    ///
    /// The caller **must** call [`AdmissionGuard::entered_service`] when the
    /// operation's work actually begins, or the controller gets no sojourn
    /// sample from it and [`AdmissionStats::unmeasured_sojourn`] counts it.
    /// `namespace` is the repository's namespace, for the flow ledger; `""`
    /// where the door does not know one, which charges the refusal to the box
    /// alone rather than inventing a tenant for it.
    pub fn try_admit(
        self: &Arc<Self>,
        principal: &str,
        namespace: &str,
        class: OpClass,
    ) -> Result<AdmissionGuard, Refusal> {
        self.attempt(principal, namespace, class, Count::Yes)
    }

    /// [`Admission::try_admit`], told whether this ask is the **final** one.
    ///
    /// [`Admission::admit_within`] asks many times for one operation while it
    /// waits, and a refusal counter that moved on every ask would say 21 where
    /// one client was refused once. So the wait loop asks with [`Count::No`]
    /// and counts once, at the end, when the refusal is final.
    fn attempt(
        self: &Arc<Self>,
        principal: &str,
        namespace: &str,
        class: OpClass,
        counting: Count,
    ) -> Result<AdmissionGuard, Refusal> {
        let outcome = self.attempt_inner(principal, namespace, class, counting);
        // **The per-namespace half of a FINAL refusal**, and the second of the
        // two places it is charged from — see [`Self::charge_flow`] for why
        // there are two and not one.
        //
        // RED when this was missing: `attempt` counts the box's own refusal
        // counters INLINE (`counting.bump`, six sites) and never calls
        // [`Self::count_refusal`], which only [`Self::admit_within`] reaches.
        // So the SSH exec line charged the tenant and the two doors that call
        // [`Self::try_admit`] — smart HTTP and the gRPC transfer gate — charged
        // nobody. The compiler said so out loud (`unused variable: namespace`)
        // and the claim that all three doors converged on one counting point
        // was simply false.
        if counting == Count::Yes && outcome.is_err() {
            self.charge_flow(namespace);
        }
        outcome
    }

    /// **Charge one admission refusal to the namespace that met it.**
    ///
    /// The one writer, called from the two places a refusal becomes FINAL:
    /// [`Self::attempt`] for the doors that do not wait
    /// ([`Self::try_admit`] — smart HTTP and the gRPC transfer gate), and
    /// [`Self::count_refusal`] for the door that does
    /// ([`Self::admit_within`] — the SSH exec line, which asks many times for
    /// one operation and must charge exactly once, at the end).
    ///
    /// Two call sites and not one because "final" means two different moments
    /// on those two paths, and that is the same distinction [`Count`] already
    /// exists to draw. What must not be two is the ARITHMETIC, and it is not.
    fn charge_flow(&self, namespace: &str) {
        if let Some(flow) = &self.flow {
            flow.refuse(
                namespace,
                gunnar_core::now_ms(),
                gunnar_core::Refusal::Admission,
            );
        }
    }

    fn attempt_inner(
        self: &Arc<Self>,
        principal: &str,
        namespace: &str,
        class: OpClass,
        counting: Count,
    ) -> Result<AdmissionGuard, Refusal> {
        // ── D23b: is this caller allowed the last slots? ───────────────────
        //
        // `true` only for the one principal that has no key behind it, so on the
        // path that carries essentially all traffic this is a string compare
        // against a `&'static str` and then nothing at all. Computed here rather
        // than after the claim because it has to ride INSIDE the claim's
        // compare-and-swap; see `Governor::claim_reserving` for the race that
        // taught us so, and `Admission::keyed_reserve` for where the number
        // comes from.
        //
        // Skipped when the ceiling is off (the operator opted out of anonymous read
        // entirely, and `Admission::unlimited` must mean unlimited) and when it
        // is 0 (the door is shut at every occupancy, and `AnonymousAtCapacity`
        // is the truer sentence for that).
        //
        // ── the PAUSE, before any slot is looked at ─────────────────────
        // A paused gate refuses everybody with the pause's words; nothing is
        // claimed, so nothing has to be released, and the in-flight count
        // keeps meaning "transfers that were admitted before the pause" —
        // which is exactly what the drain waits on.
        if let Some(pause) = self.paused() {
            counting.bump(&self.counters.refused_paused);
            return Err(Refusal::Paused { pause });
        }
        // ── admit-share: a keyed namespace over its budget takes the anonymous
        // share (module docs). Without the feature this block does not exist
        // and `reduced` is the constant `false`.
        #[cfg(feature = "admit-share")]
        let plan = self.plan(principal, namespace, class);
        #[cfg(feature = "admit-share")]
        let reduced = plan.as_ref().is_some_and(|p| {
            matches!(
                p.class,
                edda_admit::Class::Reduced | edda_admit::Class::Floor
            )
        });
        #[cfg(not(feature = "admit-share"))]
        let (reduced, _) = (false, namespace);
        let anonymous_ceiling = self
            .anonymous_ceiling
            .filter(|_| principal == gunnar_core::ANONYMOUS_PRINCIPAL_ID || reduced);
        let reserve = match anonymous_ceiling {
            Some(cap) if cap > 0 => self.keyed_reserve(),
            _ => 0,
        };

        // The server-wide gates first: they are the resources that actually run
        // out, and taking them first means the per-principal map is only ever
        // touched by a request that could have run.
        let counts = counting == Count::Yes;
        let claim = match &self.governor {
            Some(governor) => match governor.claim_reserving(class, reserve, counts) {
                Ok(claim) => Some(claim),
                Err(Shed::Reserved {
                    occupied,
                    limit,
                    reserve,
                }) => {
                    #[cfg(feature = "admit-share")]
                    if reduced {
                        counting.bump(&self.counters.refused_reduced);
                        self.warn_reduced(plan.as_ref(), namespace);
                        return Err(Refusal::ReducedShare {
                            ceiling: anonymous_ceiling.unwrap_or(0),
                            busy: true,
                        });
                    }
                    counting.bump(&self.counters.refused_keyed_reserve);
                    return Err(Refusal::KeyedReserve {
                        occupied,
                        limit,
                        reserve,
                    });
                }
                Err(Shed::Concurrency { limit, .. }) => {
                    counting.bump(&self.counters.refused_at_capacity);
                    return Err(Refusal::AtCapacity { limit });
                }
                Err(Shed::LaneFull {
                    lane,
                    ceiling,
                    limit,
                }) => {
                    counting.bump(&self.counters.refused_lane);
                    return Err(Refusal::LaneAtCapacity {
                        lane,
                        ceiling,
                        limit,
                    });
                }
                Err(Shed::Memory {
                    class,
                    wanted,
                    reserved,
                    budget,
                }) => {
                    counting.bump(&self.counters.refused_memory);
                    return Err(Refusal::MemoryBudget {
                        class,
                        wanted,
                        reserved,
                        budget,
                    });
                }
            },
            None => None,
        };

        // The absolute anonymous ceiling, which is a different question
        // from the reserve above — that one asked *may this caller take the
        // slots that are left*, and this one asks *how many may this caller ever
        // hold at once*. Consulted here rather than before the claim because it
        // needs the per-principal map, and the map is only ever touched by a
        // request that could have run.
        if self.max_per_principal.is_some() || anonymous_ceiling.is_some() {
            let mut held = self.per_principal.lock().unwrap_or_else(|e| e.into_inner());

            // Everything the decision needs, read once under the one lock the
            // counts live under. `active` includes this candidate whether or
            // not it already has an entry, because a share divided by the
            // claimants that are already in is not this claimant's share.
            let already_active = held.contains_key(principal);
            let active = held.len() + usize::from(!already_active);
            let count = held.entry(principal.to_string()).or_insert(0);

            // The anonymous ceiling REPLACES the share for the anonymous
            // principal; it does not combine with it. Not `min` of the two,
            // because the two numbers answer different questions and a `min`
            // would let the max-min rule's contention threshold leak in: on an
            // idle server the share evaluates to the whole box, so `min` would
            // read 5 — correct by accident — while under contention it could
            // evaluate lower and the refusal would then be reported as a
            // *share*, sending the operator to the wrong knob. One caller, one
            // rule, one number in the log.
            let (ceiling, anonymous) = match anonymous_ceiling {
                Some(cap) => (cap, true),
                // Unreachable with `None`: the `if` above is exactly the
                // disjunction "there is a per-principal limit OR an anonymous
                // ceiling", and this arm is the half where the second is off.
                None => (
                    self.ceiling_for(self.max_per_principal.unwrap_or(usize::MAX), active),
                    false,
                ),
            };
            if *count >= ceiling {
                // Do not leave a zero entry behind for a principal that was
                // refused: `or_insert` may have just created one. This matters
                // more than it used to — a stale entry inflates `active` and
                // shrinks everyone's share.
                if *count == 0 {
                    held.remove(principal);
                }
                drop(held);
                // Hand the slot AND the memory reservation straight back. The
                // guard is what normally does this, and there is no guard yet.
                if let (Some(governor), Some(claim)) = (&self.governor, &claim) {
                    governor.release(claim, 0);
                }
                #[cfg(feature = "admit-share")]
                if anonymous && reduced {
                    counting.bump(&self.counters.refused_reduced);
                    self.warn_reduced(plan.as_ref(), namespace);
                    return Err(Refusal::ReducedShare {
                        ceiling,
                        busy: false,
                    });
                }
                if anonymous {
                    counting.bump(&self.counters.refused_anonymous);
                    return Err(Refusal::AnonymousAtCapacity { limit: ceiling });
                }
                counting.bump(&self.counters.refused_per_principal);
                return Err(Refusal::PrincipalAtCapacity {
                    limit: ceiling,
                    active_principals: active,
                });
            }
            *count += 1;
        }

        self.counters.admitted.fetch_add(1, Ordering::Relaxed);
        let in_flight = self.counters.in_flight.fetch_add(1, Ordering::Relaxed) + 1;
        self.counters
            .peak_in_flight
            .fetch_max(in_flight, Ordering::Relaxed);

        // admit-share: the policy's own slot shares are not configured here
        // (this gate IS the share), so its `admit` cannot refuse a namespace;
        // should a policy ever do so, the gate's decision above stands and the
        // transfer simply teaches the estimate nothing.
        #[cfg(feature = "admit-share")]
        let admitted = plan.and_then(|plan| self.policy.0.admit(&plan).ok());
        Ok(AdmissionGuard {
            #[cfg(feature = "admit-share")]
            admitted,
            gate: Arc::clone(self),
            // Some **exactly when** the block above incremented a count, so the
            // guard's `Drop` decrements precisely what was incremented. The two
            // conditions have to be the same expression: an anonymous
            // admission counted under a gate with no per-principal limit and
            // released by `self.max_per_principal.map(..)` would never be
            // released, and the ceiling would fall by one per anonymous clone
            // until the door was shut for good.
            principal: (self.max_per_principal.is_some() || anonymous_ceiling.is_some())
                .then(|| principal.to_string()),
            claim,
            marked: ServiceMark::default(),
            wire: AtomicU64::new(0),
            grown: AtomicU64::new(0),
        })
    }

    /// The `warn` row for a reduced-share refusal, carrying the plan's sentence
    /// — the one thing a `Copy` [`Refusal`] cannot.
    #[cfg(feature = "admit-share")]
    fn warn_reduced(&self, plan: Option<&edda_admit::Plan>, namespace: &str) {
        tracing::warn!(
            target: "gunnar::git",
            event = "git.admission.reduced",
            namespace,
            why = plan.map_or("", |p| p.why.as_str()),
            "a namespace over its transfer budget met a full reduced share"
        );
    }

    /// The same gate with a different **admission wait**.
    ///
    /// `Duration::ZERO` is the pre-2026-08-30 gate exactly: refuse on the spot.
    /// See [`DEFAULT_ADMISSION_WAIT`] for why waiting is cheaper than refusing
    /// once the client's reconnect is counted.
    pub fn with_wait(self, wait: Duration) -> Admission {
        Admission { wait, ..self }
    }

    /// How long this gate lets an operation wait for a slot.
    pub fn wait(&self) -> Duration {
        self.wait
    }

    /// The same gate with a **keyed wait**: how long an authenticated caller
    /// may be queued for a slot ([`crate::governor::DEFAULT_KEYED_ADMISSION_WAIT`]).
    /// `None` gives keyed callers the ordinary [`Admission::wait`].
    pub fn with_keyed_wait(self, keyed_wait: Option<Duration>) -> Admission {
        Admission { keyed_wait, ..self }
    }

    /// **How long `principal` may wait for a slot.** Anonymous: the ordinary
    /// wait. A keyed identity: the keyed wait when it is longer — a refused
    /// key reconnects (git-lfs's pure-SSH transfer cannot even do that: it
    /// falls back to `git-lfs-authenticate` and fails the push), while the
    /// parked future costs one session the key's own share already bounds. A
    /// zero wait (`--admission-wait-ms 0`) is "refuse on the spot" for all.
    pub fn wait_for(&self, principal: &str) -> Duration {
        if self.wait.is_zero() || principal == gunnar_core::ANONYMOUS_PRINCIPAL_ID {
            return self.wait;
        }
        self.keyed_wait.map_or(self.wait, |k| k.max(self.wait))
    }

    /// **Take a slot, waiting up to [`Admission::wait`] for one.**
    ///
    /// The door every transport goes through. It is
    /// [`Admission::try_admit`] in a bounded loop, and everything that decides
    /// admission still lives there — this adds only the bound and the counters
    /// that make the bound visible.
    ///
    /// Returns the guard and **how long it waited**, which is the number the
    /// caller logs and the number the CoDel controller is fed
    /// ([`Governor::record_sojourn`](crate::governor::Governor::record_sojourn)):
    /// the wait in *this* queue is the one signal the gate has authority over,
    /// because shedding is exactly what shortens it.
    ///
    /// A refusal after the wait is the same [`Refusal`] the instant path
    /// returns, with the same sentence — a client must not be able to tell "the
    /// server refused at once" from "the server tried for two seconds first"
    /// by the shape of the failure, only by the log.
    ///
    /// # Cancellation
    ///
    /// Safe to drop at any await point: nothing is held across one. The slot is
    /// taken by the *last* `try_admit`, and either it returns a guard (which
    /// owns the release) or it took nothing at all.
    pub async fn admit_within(
        self: &Arc<Self>,
        principal: &str,
        namespace: &str,
        class: OpClass,
    ) -> Result<(AdmissionGuard, Duration), Refusal> {
        let started = std::time::Instant::now();
        let wait = self.wait_for(principal);
        let mut refusal = match self.attempt(principal, namespace, class, Count::No) {
            Ok(guard) => {
                // A wait of zero is a sample too, and it is the one that grows
                // the limit back on a server that is coping.
                self.record_wait(class, Duration::ZERO);
                return Ok((guard, Duration::ZERO));
            }
            Err(refusal) => refusal,
        };
        // No wait configured — or the caller is anonymous and was refused for
        // being anonymous, which are the refusals this door never parks.
        //
        // The wait exists because a refused *keyed* client reconnects, and one
        // parked future is cheaper than the reconnect. For anonymous that trade
        // inverts: the parked future is holding the TCP socket, the russh
        // session and its key material for up to `self.wait`, and those are
        // exactly the resources a clone fork-bomb is spending gunnar's box to
        // acquire. Parking N anonymous readers to protect them from a reconnect
        // hands an attacker N held sessions for free — the queue becomes the
        // attack. It is also pointless on its own terms: the ceiling is small
        // and hard, so unlike a governed limit it will not have grown by the
        // time the waiter re-asks; only a transfer finishing can free it, and
        // a stranger has no claim on being first in line for that.
        //
        // `KeyedReserve` (D23b) is parked no more than `AnonymousAtCapacity` is,
        // for the same reason twice over. The socket-and-session argument is
        // unchanged. And what a parked stranger would be waiting for here is the
        // box falling back below `CONTENTION_PERCENT` — which is a KEYED caller
        // finishing. Parking a stranger to wait for that is queueing it for a
        // slot the whole rule exists to deny it, and every millisecond it spends
        // in that queue it is holding the resources the rule was protecting.
        //
        // `Paused` is never parked either: the pause lasts minutes, not the
        // milliseconds a wait covers, and a client parked into a growth is a
        // client holding a session across an unmount for nothing.
        if wait.is_zero()
            || matches!(
                refusal,
                Refusal::AnonymousAtCapacity { .. }
                    | Refusal::KeyedReserve { .. }
                    | Refusal::Paused { .. }
            )
            || is_reduced(&refusal)
        {
            self.count_refusal(namespace, &refusal);
            return Err(refusal);
        }
        // Parked, and COUNTED as parked: the count is the demand signal
        // `Governor::lane_ceiling` reserves other lanes' floors from, and the
        // guard pairs the decrement onto every exit including a cancelled
        // future.
        let _parked = Parked::new(self.governor.as_ref(), class.lane());
        loop {
            let remaining = match wait.checked_sub(started.elapsed()) {
                Some(remaining) if !remaining.is_zero() => remaining,
                _ => {
                    self.counters
                        .refused_after_wait
                        .fetch_add(1, Ordering::Relaxed);
                    // The refusal is final now, so it is counted now — once,
                    // not once per poll.
                    self.count_refusal(namespace, &refusal);
                    // A wait that ran out is the strongest evidence of a
                    // standing queue there is; file it before refusing.
                    self.record_wait(class, wait);
                    return Err(refusal);
                }
            };
            // Registered BEFORE the retry, so a slot freed between the retry
            // and the wait is not a lost wakeup.
            let freed = self.slot_freed.notified();
            match self.attempt(principal, namespace, class, Count::No) {
                Ok(guard) => {
                    let waited = started.elapsed();
                    let ms = u64::try_from(waited.as_millis()).unwrap_or(u64::MAX);
                    self.counters
                        .admitted_after_wait
                        .fetch_add(1, Ordering::Relaxed);
                    self.counters
                        .waited_ms_total
                        .fetch_add(ms, Ordering::Relaxed);
                    self.counters.peak_wait_ms.fetch_max(ms, Ordering::Relaxed);
                    self.record_wait(class, waited);
                    return Ok((guard, waited));
                }
                // The same rule inside the loop, for the caller whose FIRST
                // refusal was a full server and whose next one is the anonymous
                // ceiling: a parked stranger must stop being parked the moment
                // the reason becomes "you are a stranger". Without this arm the
                // short-circuit above would be defeated by ordering alone.
                Err(
                    again @ (Refusal::AnonymousAtCapacity { .. } | Refusal::KeyedReserve { .. }),
                ) => {
                    self.count_refusal(namespace, &again);
                    self.record_wait(class, started.elapsed());
                    return Err(again);
                }
                // The reduced share is never parked either (module docs).
                Err(again) if is_reduced(&again) => {
                    self.count_refusal(namespace, &again);
                    self.record_wait(class, started.elapsed());
                    return Err(again);
                }
                Err(again) => refusal = again,
            }
            // A timeout as well as a notification, because the thing being
            // waited for is not only a released slot: the CoDel limit grows on
            // its own, and a server whose limit came back while nothing was
            // finishing would otherwise keep everybody parked until it did.
            let _ = tokio::time::timeout(remaining.min(RETRY_INTERVAL), freed).await;
        }
    }

    /// Move the counter this refusal belongs to. Called exactly once per
    /// refused operation, however many times the wait loop asked.
    fn count_refusal(&self, namespace: &str, refusal: &Refusal) {
        // The tenant's number and the box's number are taken off the one event,
        // so they cannot disagree about how many times the door was shut.
        self.charge_flow(namespace);
        let counter = match refusal {
            Refusal::AtCapacity { .. } => &self.counters.refused_at_capacity,
            Refusal::LaneAtCapacity { .. } => &self.counters.refused_lane,
            Refusal::PrincipalAtCapacity { .. } => &self.counters.refused_per_principal,
            Refusal::AnonymousAtCapacity { .. } => &self.counters.refused_anonymous,
            Refusal::KeyedReserve { .. } => &self.counters.refused_keyed_reserve,
            Refusal::MemoryBudget { .. } => &self.counters.refused_memory,
            Refusal::Paused { .. } => &self.counters.refused_paused,
            #[cfg(feature = "admit-share")]
            Refusal::ReducedShare { .. } => &self.counters.refused_reduced,
        };
        counter.fetch_add(1, Ordering::Relaxed);
    }

    /// **May a `waited` row be written now?** At most one per
    /// [`WAIT_ROW_INTERVAL`].
    ///
    /// A row per waiting transfer is a row per *client* under load: the
    /// 256-client shape produced 5411 of them in one run, which on the
    /// appliance's twenty-row console is the whole screen and nothing else.
    /// The fact still has to be visible, so what is emitted is one row per
    /// second carrying the AGGREGATE (`admitted_after_wait`, `peak_wait_ms`),
    /// which is the shape an operator can actually read. The per-transfer
    /// detail stays at `debug`.
    ///
    /// One compare-and-swap on a deadline, so a hundred racing waiters produce
    /// one row and not a hundred.
    pub fn wait_row_due(&self) -> bool {
        let now = self.epoch.elapsed().as_millis() as u64;
        self.next_wait_row_ms
            .fetch_update(Ordering::Relaxed, Ordering::Relaxed, |due| {
                (now >= due).then_some(now + WAIT_ROW_INTERVAL_MS)
            })
            .is_ok()
    }

    /// File one admission wait against the controller.
    fn record_wait(&self, class: OpClass, waited: Duration) {
        if let Some(governor) = &self.governor {
            governor.record_sojourn(class, waited);
        }
    }

    /// How many transfers this principal may hold **right now**.
    ///
    /// The whole max-min rule, in one place, so there is exactly one writer of
    /// it and a test can call it directly instead of inferring it from a
    /// refusal. Called with the per-principal lock held and the candidate's own
    /// server-wide permit already taken.
    ///
    /// `configured` is the operator's number and `active` counts the identities
    /// holding a transfer, this candidate included.
    fn ceiling_for(&self, configured: usize, active: usize) -> usize {
        // A pinned cap, or no server-wide limit to take a share OF. In the
        // second case `max_per_principal` is the only limit there is, so it has
        // to bind unconditionally or it means nothing.
        let (Some(governor), false) = (&self.governor, self.hard_per_principal) else {
            return configured;
        };

        // The share divides the **governed** limit, not the configured ceiling.
        // A share of capacity the governor has already decided the box does not
        // have is not a share; dividing the configured 100 while the controller
        // is holding 37 would hand every identity a ceiling above the whole
        // server.
        let server_wide = governor.limit();

        // Occupancy read from the governor rather than from `in_flight`: our own
        // slot is already taken, and so is the slot of every other request
        // currently between `try_claim` and its `in_flight` increment. The
        // counter would miss those and read the server as emptier than it is,
        // which is the direction that lets a burst past the share.
        let occupied = governor.occupied();

        // Integer form of `occupied >= CONTENTION_PERCENT% of server_wide`, so
        // there is no float and no rounding to argue about.
        let contended =
            occupied.saturating_mul(100) >= server_wide.saturating_mul(CONTENTION_PERCENT);
        if !contended {
            // Free slots exist; the server-wide gate is the only thing that
            // should be able to refuse. This is the case the old code got
            // wrong.
            return server_wide;
        }

        // Max-min: an equal share of the server, but never less than the
        // configured floor — an appliance that promised 32 keeps promising 32
        // however many identities turn up, and the server-wide semaphore is
        // what stops the floors from over-committing the box.
        configured.max(server_wide / active.max(1))
    }

    /// **Pause the gate**: from here until [`Self::unpause`] every transfer
    /// on every front door is refused with `pause.words()`, and nothing new
    /// takes a slot. Idempotent; a second pause replaces the first's reason.
    pub fn pause(&self, pause: Pause) {
        self.pause.store(1 + pause as u8, Ordering::SeqCst);
        tracing::warn!(
            target: "gunnar::limits",
            event = "admission.paused",
            reason = pause.reason(),
            in_flight = self.counters.in_flight.load(Ordering::Relaxed),
            "the admission gate is PAUSED: every new git transfer is refused with `{}` until the \
             gate is unpaused; transfers already admitted run to completion",
            pause.words()
        );
    }

    /// **Unpause the gate.** Idempotent; an unpaused gate stays unpaused.
    pub fn unpause(&self) {
        if self.pause.swap(0, Ordering::SeqCst) != 0 {
            tracing::info!(
                target: "gunnar::limits",
                event = "admission.unpaused",
                refused_while_paused = self.counters.refused_paused.load(Ordering::Relaxed),
                "the admission gate is open again"
            );
        }
    }

    /// Count a refusal the session gave at the paused door BEFORE reaching
    /// [`Self::admit_within`] (the door answers before the command is
    /// resolved, so the gate never sees the request).
    pub fn note_refused_paused(&self) {
        self.counters.refused_paused.fetch_add(1, Ordering::Relaxed);
    }

    /// The pause in force, if any.
    pub fn paused(&self) -> Option<Pause> {
        match self.pause.load(Ordering::SeqCst) {
            0 => None,
            n if n == 1 + Pause::Growing as u8 => Some(Pause::Growing),
            _ => None,
        }
    }

    /// Transfers in flight right now — the number the drain waits to see
    /// reach zero. A snapshot of one atomic.
    pub fn in_flight(&self) -> u64 {
        self.counters.in_flight.load(Ordering::Relaxed)
    }

    /// Everything the gate knows, as of now.
    pub fn stats(&self) -> AdmissionStats {
        AdmissionStats {
            admitted: self.counters.admitted.load(Ordering::Relaxed),
            refused_at_capacity: self.counters.refused_at_capacity.load(Ordering::Relaxed),
            refused_per_principal: self.counters.refused_per_principal.load(Ordering::Relaxed),
            refused_memory: self.counters.refused_memory.load(Ordering::Relaxed),
            refused_anonymous: self.counters.refused_anonymous.load(Ordering::Relaxed),
            refused_keyed_reserve: self.counters.refused_keyed_reserve.load(Ordering::Relaxed),
            refused_paused: self.counters.refused_paused.load(Ordering::Relaxed),
            #[cfg(feature = "admit-share")]
            refused_reduced: self.counters.refused_reduced.load(Ordering::Relaxed),
            in_flight: self.counters.in_flight.load(Ordering::Relaxed),
            peak_in_flight: self.counters.peak_in_flight.load(Ordering::Relaxed),
            limit: self.governor.as_ref().map_or(0, Governor::limit),
            reserved_bytes: self.governor.as_ref().map_or(0, |g| g.memory().reserved()),
            unmeasured_sojourn: self.counters.unmeasured_sojourn.load(Ordering::Relaxed),
            admitted_after_wait: self.counters.admitted_after_wait.load(Ordering::Relaxed),
            waited_ms_total: self.counters.waited_ms_total.load(Ordering::Relaxed),
            peak_wait_ms: self.counters.peak_wait_ms.load(Ordering::Relaxed),
            refused_after_wait: self.counters.refused_after_wait.load(Ordering::Relaxed),
            refused_lane: self.counters.refused_lane.load(Ordering::Relaxed),
            overdrawn: self.counters.overdrawn.load(Ordering::Relaxed),
            grown_bytes: self.counters.grown_bytes.load(Ordering::Relaxed),
            handoff_ms_total: self.counters.handoff_ms_total.load(Ordering::Relaxed),
            peak_handoff_ms: self.counters.peak_handoff_ms.load(Ordering::Relaxed),
        }
    }

    fn release(&self, principal: Option<&str>) {
        if let Some(principal) = principal {
            let mut held = self.per_principal.lock().unwrap_or_else(|e| e.into_inner());
            if let Some(count) = held.get_mut(principal) {
                *count = count.saturating_sub(1);
                if *count == 0 {
                    held.remove(principal);
                }
            }
        }
        self.counters.in_flight.fetch_sub(1, Ordering::Relaxed);
    }
}

/// Whether `refusal` is the reduced share — the one `admit_within` must not
/// park. The constant `false` without `admit-share`.
fn is_reduced(refusal: &Refusal) -> bool {
    #[cfg(feature = "admit-share")]
    {
        matches!(refusal, Refusal::ReducedShare { .. })
    }
    #[cfg(not(feature = "admit-share"))]
    {
        let _ = refusal;
        false
    }
}

/// A held transfer slot **and** its memory reservation. Releasing both is
/// [`Drop`], not a method, so no error path — and no panic — can skip it.
///
/// That is the whole answer to "an operation that ERRORS must still release":
/// there is no release call to forget, because unwinding runs `Drop`.
#[derive(Debug)]
pub struct AdmissionGuard {
    gate: Arc<Admission>,
    /// `None` when there is no per-principal limit, so the map is never touched
    /// and never grown by a configuration that does not use it.
    principal: Option<String>,
    /// `None` on an unlimited gate, where there is neither a slot nor a
    /// reservation to give back.
    claim: Option<Claim>,
    marked: ServiceMark,
    /// Bytes seen on this transfer's wire, in both directions.
    wire: AtomicU64,
    /// What has been charged to the ledger on top of [`Claim::bytes`]. The
    /// guard owns this so [`Drop`] releases exactly what was taken.
    grown: AtomicU64,
    /// The policy's guard (`admit-share`), handed to `record` on drop with
    /// [`AdmissionGuard::wire`] as the cost — the counter the memory gate
    /// already keeps, so recording adds nothing per byte.
    #[cfg(feature = "admit-share")]
    admitted: Option<edda_admit::Guard>,
}

impl AdmissionGuard {
    /// **The operation's work has actually started.** Files this item's sojourn
    /// against its class's window.
    ///
    /// Called by the caller and not by the gate, because only the caller knows
    /// when the waiting stopped. In `crate::server` that is the first line of
    /// the spawned transfer task: everything between admission and there is the
    /// runtime's ready queue and the blocking-pool handoff, which is exactly the
    /// invisible queue this whole module exists to make visible.
    ///
    /// One-shot: a second call is ignored rather than filing a second, longer
    /// sample for the same item.
    pub fn entered_service(&self) {
        let (Some(_governor), Some(claim)) = (&self.gate.governor, &self.claim) else {
            return;
        };
        if self.marked.claim() {
            let handoff = claim.enqueued.elapsed();
            let ms = u64::try_from(handoff.as_millis()).unwrap_or(u64::MAX);
            self.gate
                .counters
                .handoff_ms_total
                .fetch_add(ms, Ordering::Relaxed);
            self.gate
                .counters
                .peak_handoff_ms
                .fetch_max(ms, Ordering::Relaxed);
        }
    }

    /// The class this operation was admitted as.
    pub fn class(&self) -> Option<OpClass> {
        self.claim.as_ref().map(|claim| claim.class)
    }

    /// **Charge this transfer for `bytes` more on the wire.**
    ///
    /// Called by the transport as bytes actually move, in either direction, and
    /// this is what makes the memory gate cost-based rather than flat: the
    /// admission reservation ([`OpClass::admission_bytes`]) is the entry cost,
    /// and everything proportional to the transfer arrives here.
    ///
    /// Charges [`WIRE_MEMORY_NUMERATOR`]/[`WIRE_MEMORY_DENOMINATOR`] of the
    /// bytes, in steps of [`WIRE_RESERVE_STEP`], so a per-byte quantity never
    /// becomes a per-byte decision. Returns `Some(Growth)` on the calls that
    /// actually charged and `None` in between, so a caller can log the
    /// overdraw without testing for it.
    ///
    /// It cannot refuse — see [`Growth`]. What an overdraw does is stop *new*
    /// admissions until this transfer finishes.
    pub fn reserve_more(&self, bytes: u64) -> Option<Growth> {
        let (Some(governor), Some(_)) = (&self.gate.governor, &self.claim) else {
            return None;
        };
        let wire = self.wire.fetch_add(bytes, Ordering::Relaxed) + bytes;
        let want = wire * WIRE_MEMORY_NUMERATOR / WIRE_MEMORY_DENOMINATOR / WIRE_RESERVE_STEP
            * WIRE_RESERVE_STEP;
        let held = self.grown.load(Ordering::Relaxed);
        if want <= held {
            return None;
        }
        let extra = want - held;
        // `fetch_max`, not a store: two directions can be metering at once and
        // the ledger must be charged once per step, not once per racer.
        if self.grown.fetch_max(want, Ordering::Relaxed) >= want {
            return None;
        }
        self.gate
            .counters
            .grown_bytes
            .fetch_add(extra, Ordering::Relaxed);
        let growth = governor.charge(want, extra);
        if matches!(growth, Growth::Overdrawn { .. }) {
            self.gate.counters.overdrawn.fetch_add(1, Ordering::Relaxed);
        }
        Some(growth)
    }
}

impl Drop for AdmissionGuard {
    fn drop(&mut self) {
        // ONE record per transfer, here, off the stream: the wire total is the
        // counter `reserve_more` already keeps for the memory gate, and its
        // direction is the class's — a push's bytes came in, everything
        // else's went out. Nothing was added to the per-byte path for it.
        #[cfg(feature = "admit-share")]
        if let Some(admitted) = self.admitted.take() {
            let wire = self.wire.load(Ordering::Relaxed);
            let push = matches!(self.claim.as_ref().map(|c| c.class), Some(OpClass::Push));
            self.gate.policy.0.record(
                admitted,
                &edda_admit::Cost {
                    net_in: if push { wire } else { 0 },
                    net_out: if push { 0 } else { wire },
                    ..edda_admit::Cost::default()
                },
            );
        }
        if let (Some(governor), Some(claim)) = (&self.gate.governor, &self.claim) {
            governor.release(claim, self.grown.load(Ordering::Relaxed));
            // A transfer that never said its work started gave the controller
            // nothing. Counted rather than guessed at: filing the whole
            // lifetime as a "sojourn" here would be worse than filing nothing,
            // because service time is not wait time and every long clone would
            // read as a standing queue.
            if !self.marked.claimed() {
                self.gate
                    .counters
                    .unmeasured_sojourn
                    .fetch_add(1, Ordering::Relaxed);
            }
        }
        self.gate.release(self.principal.as_deref());
        // The slot is free now, so anybody parked in `admit_within` should
        // re-ask. `notify_waiters` wakes everyone parked *at this moment* and
        // stores nothing for a future waiter, which is right: a waiter that
        // arrives later runs `try_admit` itself before it ever parks.
        self.gate.slot_freed.notify_waiters();
    }
}

/// One operation's presence in [`Governor::waiting_in`], released on every
/// path — return, refusal, or a dropped future.
struct Parked<'a> {
    governor: Option<&'a Governor>,
    lane: Lane,
}

impl<'a> Parked<'a> {
    fn new(governor: Option<&'a Governor>, lane: Lane) -> Parked<'a> {
        if let Some(governor) = governor {
            governor.entered_wait(lane);
        }
        Parked { governor, lane }
    }
}

impl Drop for Parked<'_> {
    fn drop(&mut self) {
        if let Some(governor) = self.governor {
            governor.left_wait(self.lane);
        }
    }
}

/// **The wire meter**: an `AsyncRead + AsyncWrite` that charges its
/// [`AdmissionGuard`] for every byte that crosses it.
///
/// This is the other half of cost-based admission. [`OpClass::admission_bytes`]
/// is what an operation reserves before anybody knows how big it is; this is
/// how the ledger finds out. It wraps the same duplex the transport was already
/// given, so there is no second copy of the bytes and no second place where a
/// transfer's size is known.
///
/// Both directions are metered through one counter: a push's cost is the pack
/// coming in and a clone's is the pack going out, and the ledger does not care
/// which way a byte was travelling when it made the server hold something.
///
/// Zero cost when it is not doing anything: [`AdmissionGuard::reserve_more`]
/// returns on an integer comparison until a whole [`WIRE_RESERVE_STEP`] has
/// gone by.
pub struct Metered<S> {
    inner: S,
    guard: Arc<AdmissionGuard>,
}

impl<S> Metered<S> {
    /// Meter `inner` against `guard`.
    pub fn new(inner: S, guard: Arc<AdmissionGuard>) -> Metered<S> {
        Metered { inner, guard }
    }

    /// Charge for `bytes`, and say so once when the charge did not fit.
    fn charge(&self, bytes: usize) {
        let Some(growth) = self.guard.reserve_more(bytes as u64) else {
            return;
        };
        if let Growth::Overdrawn {
            held,
            reserved,
            budget,
        } = growth
        {
            tracing::warn!(
                target: "gunnar::git",
                event = "git.transfer.overdrawn",
                held_bytes = held,
                reserved_bytes = reserved,
                budget_bytes = budget,
                "this transfer has now charged the memory budget past its limit; it finishes, \
                 and NEW transfers are refused by name until it does"
            );
        }
    }
}

impl<S: tokio::io::AsyncRead + Unpin> tokio::io::AsyncRead for Metered<S> {
    fn poll_read(
        mut self: std::pin::Pin<&mut Self>,
        cx: &mut std::task::Context<'_>,
        buf: &mut tokio::io::ReadBuf<'_>,
    ) -> std::task::Poll<std::io::Result<()>> {
        let before = buf.filled().len();
        let polled = std::pin::Pin::new(&mut self.inner).poll_read(cx, buf);
        if polled.is_ready() {
            let read = buf.filled().len().saturating_sub(before);
            if read > 0 {
                self.charge(read);
            }
        }
        polled
    }
}

impl<S: tokio::io::AsyncWrite + Unpin> tokio::io::AsyncWrite for Metered<S> {
    fn poll_write(
        mut self: std::pin::Pin<&mut Self>,
        cx: &mut std::task::Context<'_>,
        buf: &[u8],
    ) -> std::task::Poll<std::io::Result<usize>> {
        let polled = std::pin::Pin::new(&mut self.inner).poll_write(cx, buf);
        if let std::task::Poll::Ready(Ok(written)) = &polled {
            let written = *written;
            if written > 0 {
                self.charge(written);
            }
        }
        polled
    }

    fn poll_flush(
        mut self: std::pin::Pin<&mut Self>,
        cx: &mut std::task::Context<'_>,
    ) -> std::task::Poll<std::io::Result<()>> {
        std::pin::Pin::new(&mut self.inner).poll_flush(cx)
    }

    fn poll_shutdown(
        mut self: std::pin::Pin<&mut Self>,
        cx: &mut std::task::Context<'_>,
    ) -> std::task::Poll<std::io::Result<()>> {
        std::pin::Pin::new(&mut self.inner).poll_shutdown(cx)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    /// **A keyed caller is QUEUED past the short wait; a stranger is not.**
    ///
    /// gunnar.rs, 2026-10-25: every lane pushes and fetches as one key, and
    /// git-lfs cannot retry a refused pure-SSH `git-lfs-transfer` — it falls
    /// back to `git-lfs-authenticate` and fails the whole push ("batch
    /// request: exec request failed"). So an authenticated identity waits for
    /// a slot up to the keyed wait (30 s on the appliance, under `exec_grace`'s
    /// 60 s), while anonymous keeps the short wait and its never-park rules.
    #[tokio::test]
    async fn a_keyed_caller_waits_out_the_keyed_bound_and_a_stranger_does_not() {
        let gate = Arc::new(
            Admission::new(Some(1), None)
                .with_wait(Duration::from_millis(50))
                .with_keyed_wait(Some(Duration::from_secs(5))),
        );
        assert_eq!(gate.wait_for("admin"), Duration::from_secs(5));
        assert_eq!(
            gate.wait_for(gunnar_core::ANONYMOUS_PRINCIPAL_ID),
            Duration::from_millis(50)
        );
        let holder = gate
            .try_admit("other", "team", OpClass::Fetch)
            .expect("the one slot");

        // The slot frees after 400 ms: well past the short wait, inside the keyed one.
        let freer = tokio::spawn(async move {
            tokio::time::sleep(Duration::from_millis(400)).await;
            drop(holder);
        });
        let started = std::time::Instant::now();
        let (guard, waited) = gate
            .admit_within("admin", "team", OpClass::Fetch)
            .await
            .expect("a keyed caller is queued for the slot, not refused after 50 ms");
        assert!(
            waited >= Duration::from_millis(300),
            "it really waited: {waited:?}"
        );
        assert!(started.elapsed() < Duration::from_secs(5));
        freer.await.unwrap();

        // The slot is held again: a stranger is refused at the short bound.
        let started = std::time::Instant::now();
        gate.admit_within(gunnar_core::ANONYMOUS_PRINCIPAL_ID, "team", OpClass::Fetch)
            .await
            .expect_err("anonymous is never queued for the keyed bound");
        assert!(
            started.elapsed() < Duration::from_secs(1),
            "{:?}",
            started.elapsed()
        );
        drop(guard);

        // `--admission-wait-ms 0` still means "refuse on the spot" for everyone.
        let instant = Admission::new(Some(1), None)
            .with_wait(Duration::ZERO)
            .with_keyed_wait(Some(Duration::from_secs(30)));
        assert_eq!(instant.wait_for("admin"), Duration::ZERO);
        // No keyed bound configured: the keyed caller gets the ordinary wait.
        let plain = Admission::new(Some(1), None).with_wait(Duration::from_millis(50));
        assert_eq!(plain.wait_for("admin"), Duration::from_millis(50));
    }

    /// **The drain's door** (`DATA-SET-GROWTH-FLOW.md` §3 step 5, T1/T12):
    /// a paused gate refuses every new transfer with the growing sentence,
    /// takes no slot, counts the refusal under its own counter, never parks
    /// the caller, and leaves what was already admitted in flight — which
    /// is the number the drain waits on. Unpaused, it admits again.
    #[tokio::test]
    async fn a_paused_gate_refuses_with_the_growing_words_and_keeps_in_flight_honest() {
        let gate = Arc::new(Admission::new(Some(4), None).with_wait(Duration::from_secs(5)));
        let running = gate
            .try_admit("one", "team", OpClass::Push)
            .expect("admitted before the pause");
        assert_eq!(gate.in_flight(), 1);
        assert_eq!(gate.paused(), None);

        gate.pause(Pause::Growing);
        assert_eq!(gate.paused(), Some(Pause::Growing));
        let refused = gate
            .try_admit("two", "team", OpClass::Clone)
            .expect_err("paused: refused");
        assert_eq!(
            refused,
            Refusal::Paused {
                pause: Pause::Growing
            }
        );
        assert_eq!(refused.client_message(), GROWING_MESSAGE);
        assert!(
            refused.client_message().contains("growing"),
            "the word is load-bearing"
        );
        assert!(refused.client_message().contains("try again"));
        // ★ Every estate says it, not only gunnar.rs (the UpCloud mockup, 2026-10-09: test2's
        // pushes were told "gunnar.rs is growing its storage"): no estate's domain is spelled.
        assert!(
            !refused.client_message().contains("gunnar.rs"),
            "a test2 push was told gunnar.rs is growing: {}",
            refused.client_message()
        );
        assert!(
            refused
                .client_message()
                .starts_with("The server is growing its storage"),
            "{}",
            refused.client_message()
        );
        assert_eq!(refused.reason(), "paused-growing");
        for other in [
            "concurrent git transfers",
            "per-principal limit",
            "unauthenticated",
        ] {
            assert!(
                !refused.client_message().contains(other),
                "shares no substring with {other}"
            );
        }
        // The waiting door does not park a paused caller: it answers at once.
        let started = std::time::Instant::now();
        let parked = gate
            .admit_within("three", "team", OpClass::Fetch)
            .await
            .expect_err("paused: refused, not parked");
        assert!(matches!(parked, Refusal::Paused { .. }));
        assert!(
            started.elapsed() < Duration::from_secs(1),
            "not parked for the wait"
        );
        // Counted under its own counter, in the total, and no slot was taken.
        let stats = gate.stats();
        assert_eq!(stats.refused_paused, 2);
        assert_eq!(stats.refused(), 2);
        assert_eq!(
            stats.in_flight, 1,
            "the pre-pause transfer is still what is in flight"
        );
        gate.pause(Pause::Growing);
        assert_eq!(gate.paused(), Some(Pause::Growing), "idempotent");

        drop(running);
        assert_eq!(
            gate.in_flight(),
            0,
            "the drain sees zero once the last one finishes"
        );
        gate.unpause();
        gate.unpause();
        assert_eq!(gate.paused(), None);
        let _again = gate
            .try_admit("two", "team", OpClass::Clone)
            .expect("unpaused: admitted");
        assert_eq!(gate.in_flight(), 1);
    }

    #[test]
    fn the_server_wide_limit_refuses_and_counts() {
        let gate = Arc::new(Admission::new(Some(2), None));
        let a = gate
            .try_admit("one", "team", OpClass::LsRemote)
            .expect("the first is admitted");
        let b = gate
            .try_admit("two", "team", OpClass::LsRemote)
            .expect("the second is admitted");
        let refused = gate
            .try_admit("three", "team", OpClass::LsRemote)
            .expect_err("the third is refused");
        assert_eq!(refused, Refusal::AtCapacity { limit: 2 });

        let stats = gate.stats();
        assert_eq!(stats.admitted, 2, "two got in");
        assert_eq!(stats.refused_at_capacity, 1, "the refusal must be COUNTED");
        assert_eq!(stats.in_flight, 2);
        assert_eq!(stats.peak_in_flight, 2);

        // …and the slot comes back, so this is a limit and not a budget.
        drop(a);
        assert_eq!(gate.stats().in_flight, 1);
        let c = gate
            .try_admit("three", "team", OpClass::LsRemote)
            .expect("a freed slot is reusable");
        assert_eq!(gate.stats().in_flight, 2);
        assert_eq!(
            gate.stats().peak_in_flight,
            2,
            "the peak is a high-water mark, not the current value"
        );
        drop((b, c));
        assert_eq!(gate.stats().in_flight, 0);
    }

    /// **Direction A of the max-min bar.** A single principal must be able to
    /// reach the *server-wide* limit on an otherwise idle server.
    ///
    /// This is the bug, at the shipped ratio. Before the fix the per-principal
    /// number was a wall, so this run stopped at 32 with the rest of the server
    /// free, and the whole limit needed eight identities to reach.
    ///
    /// # What the governor changed here, and what it did not
    ///
    /// The number a lone principal can reach is now the **governed** limit —
    /// the top of [`LimitBand::for_ceiling`] of the configured ceiling — and not
    /// the configured ceiling itself. With the shipped
    /// `DEFAULT_MAX_CONCURRENT_TRANSFERS` of 256 that is
    /// [`MAX_LIMIT`](crate::governor::MAX_LIMIT), 100: 256 concurrent git
    /// transfers is the sort of number the governor exists to stop a server
    /// from believing about itself.
    ///
    /// The property under test is unchanged and is still the one that matters:
    /// **the per-principal number is a floor, not a wall**, so one identity on
    /// an idle server reaches the whole of whatever the server-wide limit
    /// currently is. It is asserted against `governed` rather than a literal so
    /// it cannot drift away from the band.
    #[test]
    fn one_principal_alone_reaches_the_server_wide_limit() {
        use crate::config::{
            DEFAULT_MAX_CONCURRENT_TRANSFERS as CONFIGURED,
            DEFAULT_MAX_TRANSFERS_PER_PRINCIPAL as FLOOR,
        };
        let gate = Arc::new(Admission::new(Some(CONFIGURED), Some(FLOOR)));
        let governed = gate.governed_limit().expect("a governed limit");
        // The top of the band is the CONFIGURED ceiling, not the design's
        // default. This assertion read `MAX_LIMIT` while the band was capped at
        // 100, which made a server told 256 serve 100 and called it correct —
        // the test agreed with the bug because both had the same number in them.
        assert_eq!(
            governed, CONFIGURED,
            "a fresh governor starts at the top of its band, and the top is what \
             the operator configured"
        );
        assert!(
            governed > FLOOR,
            "a test where the governed limit is not above the per-principal floor cannot tell a \
             floor from a wall"
        );

        let held: Vec<_> = (0..governed)
            .map(|i| {
                gate.try_admit("solo", "team", OpClass::LsRemote)
                    .unwrap_or_else(|refusal| {
                        panic!(
                            "transfer {i} of {governed} was refused on an EMPTY server holding \
                             one identity: {refusal}. The per-principal number is a floor, not a \
                             wall; a single-key client that stops at {FLOOR} is the old invisible ceiling, back"
                        )
                    })
            })
            .collect();
        assert_eq!(held.len(), governed);

        let stats = gate.stats();
        assert_eq!(
            stats.admitted, governed as u64,
            "the server-wide limit in force must be reachable by ONE identity"
        );
        assert_eq!(
            stats.refused_per_principal, 0,
            "not one of these {governed} may be refused for fairness: there was nobody to be \
             fair to"
        );
        assert_eq!(stats.peak_in_flight, governed as u64);
        assert_eq!(stats.limit, governed, "the stats must report what refuses");

        // …and the server-wide gate is still a gate. Without this the test
        // above would also pass on an Admission that had lost both limits.
        let full = gate
            .try_admit("solo", "team", OpClass::LsRemote)
            .expect_err("one past the governed limit");
        assert_eq!(full, Refusal::AtCapacity { limit: governed });
        drop(held);
    }

    /// **Direction B of the max-min bar.** Once the server IS contended, a
    /// greedy principal is held to its share and a quiet one is still served.
    ///
    /// `(4, 1)` so the contention threshold (75 % → 3 of 4) is reachable in a
    /// handful of admissions, and so the share (`4 / 2 = 2`) is strictly above
    /// the configured floor of 1 — a share equal to the floor could not tell
    /// max-min fairness from the old wall.
    #[test]
    fn a_greedy_principal_is_held_to_its_share_once_the_server_is_contended() {
        let gate = Arc::new(Admission::new(Some(4), Some(1)));

        // The quiet one arrives first, so it is *active* and has a claim.
        let _polite = gate
            .try_admit("polite", "team", OpClass::LsRemote)
            .expect("polite's first");
        // Greedy takes its share: 4 slots / 2 identities = 2.
        let _g1 = gate
            .try_admit("greedy", "team", OpClass::LsRemote)
            .expect("greedy's first");
        let _g2 = gate
            .try_admit("greedy", "team", OpClass::LsRemote)
            .expect("greedy's second is still its share");

        // Three of four are held, so the server is over the 75 % threshold and
        // fairness is in force. Greedy has its two.
        let refused = gate
            .try_admit("greedy", "team", OpClass::LsRemote)
            .expect_err("greedy's third is past its share of a contended server");
        assert_eq!(
            refused,
            Refusal::PrincipalAtCapacity {
                limit: 2,
                active_principals: 2
            },
            "the refusal must name the SHARE in force (4/2), not the configured floor (1), or an \
             operator reads a number that matches nothing they set"
        );
        assert!(
            refused.client_message().contains("per-principal limit"),
            "installtest's `refusals_in_log` tells the two refusals apart by this exact phrase; \
             losing it silently zeroes the server-side witness: {}",
            refused.client_message()
        );

        // The whole reason fairness exists: the fourth slot was NOT taken by
        // greedy, and it is there for somebody else.
        let _other = gate
            .try_admit("polite", "team", OpClass::LsRemote)
            .expect("the slot greedy was refused must be available to the quiet principal");

        let stats = gate.stats();
        assert_eq!(stats.refused_per_principal, 1);
        assert_eq!(
            stats.refused_at_capacity, 0,
            "the server had a free slot; attributing this to capacity sends an operator to raise \
             the wrong number"
        );
        assert_eq!(stats.in_flight, 4);
    }

    /// The share is *work-conserving*: two identities on a server that is
    /// nowhere near full do not each get half of it.
    ///
    /// Without the contention threshold this is what max-min fairness costs —
    /// a busy principal capped at `server_wide / active` while most of the box
    /// sits idle, which is the same "binds when there is nobody to be fair to"
    /// mistake in a smaller costume.
    #[test]
    fn fairness_does_not_bind_while_there_are_free_slots() {
        let gate = Arc::new(Admission::new(Some(100), Some(1)));
        let _quiet = gate
            .try_admit("quiet", "team", OpClass::LsRemote)
            .expect("one transfer from a quiet identity");

        // An equal share of 100 between 2 identities is 50. The server is only
        // going to 74 % full, so the share must not be enforced at all.
        let busy: Vec<_> = (0..73)
            .map(|i| {
                gate.try_admit("busy", "team", OpClass::LsRemote)
                    .unwrap_or_else(|r| panic!("transfer {i} refused at {}% full: {r}", i + 2))
            })
            .collect();
        assert_eq!(
            busy.len(),
            73,
            "74 of 100 held, so nothing may be refused yet"
        );
        assert_eq!(gate.stats().refused_per_principal, 0);

        // The very next one crosses the threshold — 75 of 100 occupied — and
        // the share engages at once: 100/2 = 50, and `busy` is already at 73.
        let refused = gate
            .try_admit("busy", "team", OpClass::LsRemote)
            .expect_err(
                "the transfer that makes the server 75% full is the first one weighed \
                         against the share",
            );
        assert_eq!(
            refused,
            Refusal::PrincipalAtCapacity {
                limit: 50,
                active_principals: 2
            }
        );
    }

    /// The pinned mode: an operator who wants the old wall still has it.
    #[test]
    fn a_hard_cap_binds_on_an_empty_server() {
        let fair = Arc::new(Admission::new(Some(64), Some(2)));
        let _a = fair
            .try_admit("solo", "team", OpClass::LsRemote)
            .expect("first");
        let _b = fair
            .try_admit("solo", "team", OpClass::LsRemote)
            .expect("second");
        fair.try_admit("solo", "team", OpClass::LsRemote)
            .expect("a fair gate lets a lone principal past its floor on an idle server");

        let hard = Arc::new(Admission::hard_capped(Some(64), Some(2)));
        let _c = hard
            .try_admit("solo", "team", OpClass::LsRemote)
            .expect("first");
        let _d = hard
            .try_admit("solo", "team", OpClass::LsRemote)
            .expect("second");
        let refused = hard
            .try_admit("solo", "team", OpClass::LsRemote)
            .expect_err("a pinned cap binds whether or not anyone else is there");
        assert_eq!(
            refused,
            Refusal::PrincipalAtCapacity {
                limit: 2,
                active_principals: 1
            }
        );
        assert!(
            !refused.client_message().contains("fair share"),
            "a pinned cap is not a share and must not be explained as one: {}",
            refused.client_message()
        );
        assert_eq!(hard.stats().refused_per_principal, 1);
        assert_eq!(fair.stats().refused_per_principal, 0);
    }

    #[test]
    fn a_refused_transfer_does_not_leak_a_server_wide_slot() {
        // The subtle one: the server-wide permit is taken BEFORE the
        // per-principal check, so a per-principal refusal must give it back. If
        // it did not, a client hammering its own limit would drain the server.
        //
        // Driven through the *fair* path rather than a hard cap, because that
        // is what ships: `polite` first so the server is contended and greedy's
        // share (4/2 = 2) is what it hammers.
        let gate = Arc::new(Admission::new(Some(4), Some(1)));
        let _polite = gate
            .try_admit("polite", "team", OpClass::LsRemote)
            .expect("polite is active");
        let _g1 = gate
            .try_admit("greedy", "team", OpClass::LsRemote)
            .expect("greedy's share is 2");
        let _g2 = gate
            .try_admit("greedy", "team", OpClass::LsRemote)
            .expect("greedy's share is 2");
        for _ in 0..100 {
            gate.try_admit("greedy", "team", OpClass::LsRemote)
                .expect_err("refused every time");
        }
        // Three slots are held; the fourth must still be free for anybody at all.
        let _other = gate
            .try_admit("somebody-else", "team", OpClass::LsRemote)
            .expect("the fourth slot survived 100 refusals");
        assert_eq!(gate.stats().in_flight, 4);
        assert_eq!(gate.stats().refused_per_principal, 100);
    }

    /// A principal that finishes its work must give its share back to the
    /// others, or the map's length — the divisor of every share — is a
    /// high-water mark rather than a live count.
    #[test]
    fn a_departed_principal_stops_shrinking_everybody_elses_share() {
        let gate = Arc::new(Admission::new(Some(4), Some(1)));
        let leaving = gate
            .try_admit("leaving", "team", OpClass::LsRemote)
            .expect("two identities are active");
        let _g1 = gate
            .try_admit("stayer", "team", OpClass::LsRemote)
            .expect("first");
        let _g2 = gate
            .try_admit("stayer", "team", OpClass::LsRemote)
            .expect("share of 2 with 2 active");
        gate.try_admit("stayer", "team", OpClass::LsRemote)
            .expect_err("held to 2 while `leaving` is still there");

        drop(leaving);
        // Now one identity is active, so the share is the whole server again.
        let _g3 = gate
            .try_admit("stayer", "team", OpClass::LsRemote)
            .expect("with the other identity gone the share must widen back out");
        let _g4 = gate
            .try_admit("stayer", "team", OpClass::LsRemote)
            .expect("all four slots are reachable by the only identity left");
        assert_eq!(gate.stats().in_flight, 4);
    }

    #[test]
    fn the_map_does_not_grow_with_every_principal_ever_seen() {
        let gate = Arc::new(Admission::new(Some(4), Some(1)));
        for i in 0..500 {
            let guard = gate
                .try_admit(&format!("principal-{i}"), "team", OpClass::LsRemote)
                .expect("admitted");
            drop(guard);
        }
        // Read through the gate's own lock rather than a counter, because the
        // thing being asserted is that the MAP is empty; a counter could agree
        // while the map leaked.
        let held = gate.per_principal.lock().expect("the per-principal map");
        assert!(
            held.is_empty(),
            "the per-principal map kept {} entries for principals with nothing in flight",
            held.len()
        );
    }

    /// The three refusals must stay **tellable apart by their words**, because
    /// that is how the only cross-process witness of them counts.
    ///
    /// `installtest`'s `refusals_in_log` reads the server log and sorts
    /// refusals by the substrings `"concurrent git transfers"` and
    /// `"per-principal limit"` — the log records a refusal by its `Display`,
    /// which is the same sentence the client is handed. A third reason that
    /// collided with either phrase would be silently counted as the wrong one,
    /// and an operator would raise the wrong number: the concurrency limit,
    /// when the thing that ran out was memory.
    ///
    /// # Seen red
    ///
    /// By giving `Refusal::MemoryBudget` the natural wording, "the server is
    /// already serving its limit of N concurrent git transfers within its
    /// memory budget". Failure:
    ///
    /// ```text
    /// the memory refusal contains "concurrent git transfers", which
    /// installtest counts as a CAPACITY refusal; an operator reading that
    /// raises the concurrency limit and makes the memory problem worse
    /// ```
    #[test]
    fn the_three_refusals_do_not_borrow_each_others_words() {
        let capacity = Refusal::AtCapacity { limit: 37 }.client_message();
        let principal = Refusal::PrincipalAtCapacity {
            limit: 4,
            active_principals: 3,
        }
        .client_message();
        let memory = Refusal::MemoryBudget {
            class: OpClass::Clone,
            wanted: 2048 * 1024 * 1024,
            reserved: 6144 * 1024 * 1024,
            budget: 8192 * 1024 * 1024,
        }
        .client_message();

        assert!(capacity.contains("concurrent git transfers"));
        assert!(!capacity.contains("per-principal limit"));
        assert!(principal.contains("per-principal limit"));
        assert!(!principal.contains("concurrent git transfers"));

        assert!(
            !memory.contains("concurrent git transfers"),
            "the memory refusal contains \"concurrent git transfers\", which installtest counts \
             as a CAPACITY refusal; an operator reading that raises the concurrency limit and \
             makes the memory problem worse: {memory}"
        );
        assert!(
            !memory.contains("per-principal limit"),
            "the memory refusal contains \"per-principal limit\", so installtest counts a \
             server-wide memory exhaustion as one identity's fairness throttle: {memory}"
        );
        assert!(
            memory.contains("memory budget"),
            "the memory refusal must name the knob to turn: {memory}"
        );
        // All three are transient and every one of them must say so, or a CI
        // log reads a busy server as a broken one.
        for message in [&capacity, &principal, &memory] {
            assert!(
                message.contains("try again"),
                "a transient refusal that does not say `try again` reads as permanent: {message}"
            );
        }
    }

    /// The work an operation would do, if it were admitted.
    ///
    /// Deliberately something with an **observable cost**: it allocates, and it
    /// records that it allocated. "No work was performed" asserted against a
    /// bool nobody sets is a hollow guard; asserted against a
    /// byte count that a real `Vec` really pushed into is not.
    #[derive(Default)]
    struct Work {
        runs: AtomicU64,
        bytes_allocated: AtomicU64,
    }

    impl Work {
        /// One operation's buffer. A clone's real one is proportional to the
        /// pack; this stands in for it at a size a test can afford.
        fn perform(&self) -> Vec<u8> {
            let buffer = vec![0u8; 1024 * 1024];
            self.runs.fetch_add(1, Ordering::Relaxed);
            self.bytes_allocated
                .fetch_add(buffer.len() as u64, Ordering::Relaxed);
            buffer
        }

        fn runs(&self) -> u64 {
            self.runs.load(Ordering::Relaxed)
        }

        fn bytes(&self) -> u64 {
            self.bytes_allocated.load(Ordering::Relaxed)
        }
    }

    /// **A denial happens AT admission, and nothing the operation would have
    /// done gets done.**
    ///
    /// This is the entire value of the gate. A refusal that arrives after the
    /// buffer has been built is not load shedding, it is a slower way of paying
    /// the same cost — and for the clone class that cost is gigabytes
    /// (2314 MB peak RSS, measured 2026-08-13).
    ///
    /// Both gates are driven, because they refuse on different paths and each
    /// could grow a bug the other's test would not see.
    ///
    /// # Seen red
    ///
    /// By moving `Work::perform` above the `try_admit` in the loop below, which
    /// is precisely the mistake a real caller makes when it opens the pack
    /// before asking whether it may. Failure:
    ///
    /// ```text
    /// 30 operations were refused, but the work ran 40 times and allocated
    /// 41943040 bytes. A refusal after the allocation is not load shedding
    /// ```
    #[test]
    fn a_denial_is_paid_for_by_nobody() {
        // Gate 1: the concurrency limit. A pinned band of 10, so the refusals
        // are the governor's and not the memory ledger's.
        let gate = Arc::new(Admission::new(Some(10), None));
        let work = Work::default();
        let mut held = Vec::new();
        let mut refused = 0;
        for _ in 0..40 {
            match gate.try_admit("solo", "team", OpClass::LsRemote) {
                Ok(guard) => held.push((guard, work.perform())),
                Err(_) => refused += 1,
            }
        }
        assert_eq!(refused, 30, "10 of 40 fit under the limit");
        assert_eq!(
            work.runs(),
            10,
            "{refused} operations were refused, but the work ran {} times and allocated {} \
             bytes. A refusal after the allocation is not load shedding",
            work.runs(),
            work.bytes()
        );
        assert_eq!(work.bytes(), 10 * 1024 * 1024);

        // Gate 2: the memory budget. Four clones' worth, and eight arrivals.
        let clone = OpClass::Clone.admission_bytes();
        let gate = Arc::new(Admission::new(None, None).with_budget(clone * 4));
        assert!(
            gate.memory_budget().is_none(),
            "an unlimited concurrency gate has no budget to attach one to; the memory gate must \
             ride on the same governor or a configuration could get one without the other"
        );

        let gate = Arc::new(Admission::with_memory_budget(Some(100), None, clone * 4));
        let work = Work::default();
        let mut held = Vec::new();
        let mut refused = Vec::new();
        for _ in 0..8 {
            match gate.try_admit("solo", "team", OpClass::Clone) {
                Ok(guard) => held.push((guard, work.perform())),
                Err(refusal) => refused.push(refusal),
            }
        }
        assert_eq!(work.runs(), 4, "a 4-clone budget admits 4 clones");
        assert_eq!(
            work.bytes(),
            4 * 1024 * 1024,
            "the 4 refused clones allocated {} bytes between them, which is 4 more than a \
             refusal at admission can possibly cost",
            work.bytes() - 4 * 1024 * 1024
        );
        assert_eq!(refused.len(), 4);
        assert!(
            refused.iter().all(|r| matches!(
                r,
                Refusal::MemoryBudget {
                    class: OpClass::Clone,
                    ..
                }
            )),
            "the refusals must name MEMORY, or an operator raises the concurrency limit and \
             makes it worse: {refused:?}"
        );
        let stats = gate.stats();
        assert_eq!(stats.refused_memory, 4);
        assert_eq!(
            stats.refused_at_capacity, 0,
            "the limit was 100 and 4 were held"
        );
        assert_eq!(stats.reserved_bytes, clone * 4);
    }

    /// **An operation that ERRORS — or panics — still releases both its slot
    /// and its reservation.**
    ///
    /// Leak this and the server wedges after N failures: a budget that only
    /// comes back on the happy path is a budget that drains to zero the first
    /// time a client hangs up mid-clone, which is the single most ordinary
    /// thing a git client does.
    ///
    /// # Seen red
    ///
    /// By making `AdmissionGuard::drop` release only on the success path —
    /// `if !std::thread::panicking() { … }` around the governor release, which
    /// is exactly how a hand-written release-on-completion would behave.
    /// Failure:
    ///
    /// ```text
    /// 6 operations ended in an error or a panic and 6442450944 bytes are still
    /// reserved of 8589934592; the 7th would be refused, so the server has
    /// wedged
    /// ```
    ///
    /// Three panics, 2 GiB each: three quarters of the whole budget gone to
    /// operations that no longer exist.
    #[test]
    fn an_operation_that_fails_still_gives_its_reservation_back() {
        let clone = OpClass::Clone.admission_bytes();
        let gate = Arc::new(Admission::with_memory_budget(Some(100), None, clone * 4));

        for _ in 0..3 {
            // The error path: a guard that goes out of scope through `?`-style
            // early return rather than through completion.
            let outcome: Result<(), &str> = (|| {
                let _guard = gate
                    .try_admit("solo", "team", OpClass::Clone)
                    .map_err(|_| "refused")?;
                Err("the transport died mid-pack")
            })();
            assert!(outcome.is_err());
        }
        for _ in 0..3 {
            // The panic path, which no explicit release call survives.
            let panicked = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
                let _guard = gate
                    .try_admit("solo", "team", OpClass::Clone)
                    .expect("admitted before the panic");
                panic!("the pack walk blew up");
            }));
            assert!(panicked.is_err());
        }

        let stats = gate.stats();
        assert_eq!(
            stats.reserved_bytes,
            0,
            "6 operations ended in an error or a panic and {} bytes are still reserved of {}; \
             the 7th would be refused, so the server has wedged",
            stats.reserved_bytes,
            gate.memory_budget().unwrap_or(0)
        );
        assert_eq!(stats.in_flight, 0);
        assert_eq!(
            gate.governor().expect("a governor").occupied(),
            0,
            "the concurrency slots leaked even though the reservations did not"
        );

        // The applied output: the budget is genuinely reusable, four times over.
        let held: Vec<_> = (0..4)
            .map(|i| {
                gate.try_admit("solo", "team", OpClass::Clone)
                    .unwrap_or_else(|r| panic!("clone {i} after 6 failures was refused: {r}"))
            })
            .collect();
        assert_eq!(held.len(), 4);
    }

    /// A transfer that never reports entering service is COUNTED, not guessed
    /// at.
    ///
    /// The controller only sees what [`AdmissionGuard::entered_service`] files.
    /// A caller that forgets leaves it with no samples, and a CoDel controller
    /// with no samples sits at the top of its band forever — a gate that looks
    /// like it works and does not. This is the counter that makes that visible,
    /// and `tests/admission.rs` is where the real SSH path is held to zero.
    ///
    /// # Seen red
    ///
    /// By making `AdmissionGuard::drop` increment the counter unconditionally
    /// instead of only when the mark was never claimed. Failure:
    ///
    /// ```text
    /// assertion `left == right` failed: a transfer that DID report entering
    /// service was counted as unmeasured: 2
    ///   left: 2
    ///  right: 1
    /// ```
    #[test]
    fn a_transfer_that_never_enters_service_is_counted_as_unmeasured() {
        let gate = Arc::new(Admission::new(Some(100), None));

        drop(
            gate.try_admit("solo", "team", OpClass::LsRemote)
                .expect("admitted"),
        );
        assert_eq!(
            gate.stats().unmeasured_sojourn,
            1,
            "a transfer that never said its work started filed no sojourn sample and must be \
             counted; a controller starved of samples grows to its ceiling and stays there"
        );

        let guard = gate
            .try_admit("solo", "team", OpClass::LsRemote)
            .expect("admitted");
        guard.entered_service();
        drop(guard);
        assert_eq!(
            gate.stats().unmeasured_sojourn,
            1,
            "a transfer that DID report entering service was counted as unmeasured: {}",
            gate.stats().unmeasured_sojourn
        );

        // …and the sample really reached the controller's window, rather than
        // merely flipping a flag. One sample opens a window; nothing has closed
        // one yet, so the limit has not moved and the class's window is live.
        let governor = gate.governor().expect("a governor");
        assert_eq!(governor.codel().shrinks() + governor.codel().grows(), 0);
        let guard = gate
            .try_admit("solo", "team", OpClass::LsRemote)
            .expect("admitted");
        guard.entered_service();
        drop(guard);
        assert_eq!(
            gate.stats().unmeasured_sojourn,
            1,
            "the one-shot mark must not turn a second measured transfer into an unmeasured one"
        );
    }

    #[test]
    fn an_unlimited_gate_admits_everything_and_still_counts() {
        let gate = Arc::new(Admission::unlimited());
        let mut held = Vec::new();
        for _ in 0..1000 {
            held.push(
                gate.try_admit("anyone", "team", OpClass::LsRemote)
                    .expect("no limit means no refusal"),
            );
        }
        let stats = gate.stats();
        assert_eq!(stats.admitted, 1000);
        assert_eq!(stats.refused(), 0);
        assert_eq!(stats.in_flight, 1000);
        assert_eq!(stats.peak_in_flight, 1000);
    }
}