autumn-web 0.8.0

//! Per-tenant custom-domain issuance and renewal (issue #1635).
//!
//! [`crate::custom_domain`] owns the registry, the verification gate and the
//! issuance budget. This module is the half that needs a CA: the
//! [`AcmeDomainIssuer`] that orders one certificate per tenant hostname, and
//! the [`CustomDomainTask`] loop that drives every registered domain through
//! verify → issue → renew and offboards the ones an app removes.
//!
//! # Composition, not a second ACME client
//!
//! Orders reuse #1608's account, store and HTTP-01 token map through the
//! helpers extracted in [`crate::acme::renewal`]. One ACME account covers the
//! deployment's own certificate and every tenant's, so the CA sees one
//! registration however many domains connect.
//!
//! # Why HTTP-01 only
//!
//! A tenant's domain lives in the *tenant's* zone, so autumn holds no
//! credential that could write a DNS-01 `_acme-challenge` record there. The
//! verification gate already proves the hostname points at this deployment
//! (and, with the TXT token, that the tenant controls it), which is exactly
//! the condition that makes HTTP-01 work — so the challenge
//! the operator can actually answer is the one used, even when the
//! deployment's own certificate is issued over DNS-01.
//!
//! # Abuse posture
//!
//! Every order passes three gates first: the hostname is registered by the
//! app, DNS independently points here and carries the registration's TXT
//! token, and the
//! [budget](crate::custom_domain::IssuanceLimiter) has headroom. An SNI
//! hostname failing the first never reaches this module at all —
//! [`SniCertResolver`](crate::custom_domain::SniCertResolver) refuses the
//! handshake.

use std::sync::Arc;

use crate::acme::challenge::Http01Tokens;
use crate::acme::store::{AcmeStore, CertId, StoredCert};
use crate::config::AcmeConfig;
use crate::custom_domain::{
    CustomDomainCertCache, CustomDomainRegistry, DomainIssuer, DomainVerifier, ExpectedIngress,
    IssuanceLimiter, IssuedCertificate, apply_verification, grade_dns_verification,
    grade_ownership,
};
use crate::scheduler::SchedulerCoordinator;
use crate::task::TaskCoordination;
use rustls::crypto::CryptoProvider;

/// The scheduled-task name custom-domain leases and alerts are keyed on.
pub const CUSTOM_DOMAIN_TASK: &str = "custom_domain_certificates";

/// How many pending domains one tick verifies at the same time.
const VERIFY_CONCURRENCY: usize = 16;

/// Callback that dispatches a custom-domain failure to the operator (#1610).
pub type ReporterFn = Arc<dyn Fn(String) + Send + Sync>;

/// Callback that clears an outstanding custom-domain alert once every domain
/// is healthy again.
pub type RecoveryFn = Arc<dyn Fn() + Send + Sync>;

/// The [`CertId`] a tenant hostname's certificate is stored under.
///
/// One hostname per certificate, so offboarding one tenant deletes exactly one
/// pair and a renewal for one domain cannot invalidate another's.
#[must_use]
pub fn cert_id_for(hostname: &str) -> CertId {
    CertId::from_domains(&[hostname.to_owned()])
}

/// `base_unix` plus the monotonic time since `started`.
///
/// Used instead of a fresh wall-clock read so a caller-injected `now_unix`
/// (tests, simulation) stays the single source of time while a long pass still
/// advances past the instant it began.
fn elapsed_since(base_unix: i64, started: tokio::time::Instant) -> i64 {
    base_unix.saturating_add(i64::try_from(started.elapsed().as_secs()).unwrap_or(i64::MAX))
}

// ── The ACME issuer ──────────────────────────────────────────────────────

/// Orders one certificate per tenant hostname over HTTP-01.
pub struct AcmeDomainIssuer {
    config: AcmeConfig,
    store: Arc<dyn AcmeStore>,
    tokens: Http01Tokens,
}

impl std::fmt::Debug for AcmeDomainIssuer {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("AcmeDomainIssuer").finish_non_exhaustive()
    }
}

impl AcmeDomainIssuer {
    /// An issuer sharing `store` (hence the ACME account) and the `:80`
    /// challenge listener's `tokens` with the deployment's own renewal loop.
    #[must_use]
    pub const fn new(config: AcmeConfig, store: Arc<dyn AcmeStore>, tokens: Http01Tokens) -> Self {
        Self {
            config,
            store,
            tokens,
        }
    }

    async fn order(&self, hostname: &str) -> Result<IssuedCertificate, String> {
        use instant_acme::{Identifier, NewOrder};

        let account =
            crate::acme::renewal::load_or_register_account(self.store.as_ref(), &self.config)
                .await?;
        let identifiers = [Identifier::Dns(hostname.to_owned())];
        let mut order = account
            .new_order(&NewOrder::new(&identifiers))
            .await
            .map_err(|e| format!("failed to create ACME order for {hostname}: {e}"))?;

        // The token stays published until the order settles: `set_ready` only
        // queues validation, so tearing it down earlier removes the answer
        // while the CA is still looking at it.
        let published = crate::acme::renewal::answer_http01(&self.tokens, &mut order).await?;
        let ready = crate::acme::renewal::await_order_ready(&mut order, None).await;
        drop(published);
        ready?;

        let (csr_der, key_pem) = crate::acme::renewal::generate_csr(&[hostname.to_owned()])?;
        order
            .finalize_csr(&csr_der)
            .await
            .map_err(|e| format!("failed to finalize order for {hostname}: {e}"))?;
        let chain_pem = order
            .poll_certificate(&instant_acme::RetryPolicy::default())
            .await
            .map_err(|e| format!("failed to download certificate for {hostname}: {e}"))?;
        Ok(IssuedCertificate { chain_pem, key_pem })
    }
}

impl DomainIssuer for AcmeDomainIssuer {
    fn issue<'a>(
        &'a self,
        hostname: &'a str,
    ) -> futures::future::BoxFuture<'a, Result<IssuedCertificate, String>> {
        Box::pin(self.order(hostname))
    }
}

// ── The orchestrator ─────────────────────────────────────────────────────

/// Drives every registered custom domain through its lifecycle.
///
/// One tick does three passes, in order: verify the domains still awaiting
/// DNS, order for the ones now verified, and renew the active ones inside
/// their renew-before window. Each domain is handled independently — a failure
/// records a reason and a backoff on that domain only, so nothing one tenant
/// does can stop another being issued, renewed or served.
pub struct CustomDomainTask {
    /// The hostname → tenant registry, and the lifecycle state it holds.
    pub registry: Arc<CustomDomainRegistry>,
    /// The bounded cache the SNI resolver reads.
    pub cache: Arc<CustomDomainCertCache>,
    /// Certificate persistence — the same store the deployment's own
    /// certificate and the ACME account live in.
    pub certs: Arc<dyn AcmeStore>,
    /// The crypto provider shared with the TLS listener.
    pub provider: Arc<CryptoProvider>,
    /// Where a hostname currently points.
    pub verifier: Arc<dyn DomainVerifier>,
    /// How a certificate is obtained.
    pub issuer: Arc<dyn DomainIssuer>,
    /// Per-domain and deployment-wide order budgets.
    pub limiter: Arc<IssuanceLimiter>,
    /// What tenants are told to point DNS at.
    pub ingress: ExpectedIngress,
    /// Renew once a certificate has fewer than this many days left.
    pub renew_before_days: u32,
    /// Where a failure is reported (#1610's failed-scheduled-operation alert).
    pub reporter: ReporterFn,
    /// Invoked once no domain carries a failure any more, so the operator
    /// alert the reporter raised is cleared rather than left standing.
    pub recovery: Option<RecoveryFn>,
    /// Leader election, so only one replica orders per domain.
    ///
    /// The same coordinator the deployment's own renewal uses. Without it every
    /// replica orders every tenant's certificate against the one shared ACME
    /// account.
    pub coordinator: Arc<dyn SchedulerCoordinator>,
    /// Set when a distributed scheduler backend was configured but this process
    /// could not build its coordinator. Mirrors `AcmeRenewalTask`: ordering
    /// under a per-process lease would race every other replica.
    pub leadership_degraded: bool,
    /// The certificate store as a filesystem store, when it is one, so the
    /// retention prune can enumerate stored pairs. `None` disables orphan
    /// pruning rather than guessing at another store's layout.
    pub cert_store_paths: Option<Arc<crate::acme::store::FsAcmeStore>>,
    /// Certificate ids the prune must never delete — the deployment's own
    /// certificate, which shares this store but has no registry record.
    pub retained_cert_ids: std::collections::HashSet<String>,
}

impl std::fmt::Debug for CustomDomainTask {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("CustomDomainTask")
            .field("domains", &self.registry.len())
            .field("renew_before_days", &self.renew_before_days)
            .finish_non_exhaustive()
    }
}

// ── The prune-only half ────────────────────────────────────────────────────

/// The retention-prune half of [`CustomDomainTask`]: everything the
/// [`CustomDomainPruner`](crate::custom_domain::CustomDomainPruner) trait
/// needs, and none of the issuance/renewal orchestration.
///
/// The server installs this — not the task — as the `dyn CustomDomainPruner`
/// state extension, and the one-shot `autumn db retention` path installs the
/// same type. Pruning only touches the registry and the certificate store, so
/// the CLI never builds an ACME issuer: no order is placed and no CA is
/// contacted (#2652).
pub struct PruneOnlyCustomDomainPruner {
    /// The hostname → tenant registry, and the lifecycle state it holds.
    pub registry: Arc<CustomDomainRegistry>,
    /// The bounded cache the SNI resolver reads; offboarding evicts from it.
    pub cache: Arc<CustomDomainCertCache>,
    /// Certificate persistence, for deleting an offboarded pair.
    pub certs: Arc<dyn AcmeStore>,
    /// Per-domain issuance budgets; offboarding forgets the hostname.
    pub limiter: Arc<IssuanceLimiter>,
    /// The certificate store as a filesystem store, when it is one, so the
    /// retention prune can enumerate stored pairs. `None` disables orphan
    /// pruning rather than guessing at another store's layout.
    pub cert_store_paths: Option<Arc<crate::acme::store::FsAcmeStore>>,
    /// Certificate ids the prune must never delete — the deployment's own
    /// certificate, which shares this store but has no registry record.
    pub retained_cert_ids: std::collections::HashSet<String>,
    /// Where a failed certificate delete is reported (#1610's
    /// failed-scheduled-operation alert).
    pub reporter: ReporterFn,
    /// Invoked once no domain carries a failure any more, so the operator
    /// alert the reporter raised is cleared rather than left standing.
    pub recovery: Option<RecoveryFn>,
}

impl PruneOnlyCustomDomainPruner {
    /// Offboard `hostname`: stop routing, stop serving, halt renewal, and
    /// delete the stored certificate so it is not orphaned (AC7, #1605).
    ///
    /// # Errors
    ///
    /// Propagates a registry-store error. A certificate that cannot be deleted
    /// is logged rather than failing the offboarding: the domain is already
    /// unroutable and unservable by then, so leaving it registered would be
    /// worse.
    pub async fn offboard(&self, hostname: &str) -> std::io::Result<bool> {
        // Normalise once, up front: the registry normalises internally, but the
        // cache key, the budget history and the certificate id all derive from
        // the raw string, so `offboard("APP.ClientCo.com.")` would drop the
        // record and leave the private key on disk.
        let Ok(host) = crate::custom_domain::normalize_hostname(hostname) else {
            return Ok(false);
        };
        self.offboard_guarded(&host, |_| true).await
    }

    /// Remove one hostname while `guard` holds for the stored record, then drop
    /// what this process holds for it.
    ///
    /// Every offboarding path goes through here. The guard runs inside the
    /// registry's per-hostname write gate, so a caller that chose this hostname
    /// before an `.await` — a retention sweep, a tenant-wide teardown — re-
    /// asserts that choice against the record as it is now rather than
    /// disconnecting whoever holds the hostname by the time its turn comes.
    async fn offboard_guarded(
        &self,
        host: &str,
        guard: impl FnOnce(&crate::custom_domain::CustomDomain) -> bool,
    ) -> std::io::Result<bool> {
        let removed = self.registry.remove_if(host, guard).await?;
        // Purge only while the hostname is still nobody's. A successor that
        // registered in the gap owns whatever sits at this hostname's cache
        // slot and certificate id now, and deleting those would leave THEIR
        // domain recorded as active with nothing to serve.
        let purged = if self.registry.get(host).is_none() {
            self.purge_local(host).await
        } else {
            true
        };
        // A cleanup that failed has just raised its own alert, naming a private
        // key still on disk. Retracting it here — because the registry is
        // empty and therefore "healthy" — would leave that key with nothing
        // pointing at it, and no retention sweep runs by default.
        if removed && purged {
            self.clear_alert_if_healthy(crate::custom_domain::now_unix());
        }
        Ok(removed)
    }

    /// Offboard `hostname` only while it is still the record `expected`
    /// describes — same tenant, same status, same registration time.
    ///
    /// The retention sweep's candidates come from a snapshot taken before a
    /// series of awaits; this re-asserts each one inside the registry's
    /// per-hostname write gate, so a domain that finished setup in the
    /// meantime is left alone. Nothing local is purged when the guard rejects:
    /// the certificate belongs to a domain that is now live.
    async fn offboard_if_unchanged(
        &self,
        expected: &crate::custom_domain::CustomDomain,
    ) -> std::io::Result<bool> {
        let Ok(host) = crate::custom_domain::normalize_hostname(&expected.hostname) else {
            return Ok(false);
        };
        let removed = self
            .offboard_guarded(&host, |current| {
                current.tenant == expected.tenant
                    && current.status == expected.status
                    && current.registered_at_unix == expected.registered_at_unix
            })
            .await?;
        if !removed {
            tracing::debug!(
                hostname = %host,
                "skipping a retention offboard: the domain changed while the sweep was running"
            );
        }
        Ok(removed)
    }

    /// Drop everything this process holds for an offboarded hostname.
    ///
    /// A certificate that cannot be deleted does not fail the offboarding: the
    /// domain is already unroutable and unservable, and reporting failure would
    /// suggest none of it happened. It is ALERTED instead — an offboarded
    /// tenant's private key left on disk is an operator's problem, and nothing
    /// else retries it: the orphan prune only runs when a `[retention]
    /// custom_domains` window is configured, and it is unset by default.
    /// Returns whether everything local actually went.
    async fn purge_local(&self, host: &str) -> bool {
        self.cache.remove(host);
        self.limiter.forget(host);
        if let Err(e) = self.certs.delete_cert(&cert_id_for(host)).await {
            let message = format!(
                "offboarded custom domain {host} but could not delete its certificate: {e}. The \
                 certificate and private key are still on disk"
            );
            tracing::warn!("{message}");
            (self.reporter)(message);
            return false;
        }
        true
    }

    /// Retract the operator alert once NOTHING is failing any more.
    ///
    /// Offboarding the one domain that was failing removes the failure from the
    /// registry but is not itself a success, so without this the
    /// `scheduled_task_failure` alert it raised would stand for a domain that
    /// no longer exists.
    fn clear_alert_if_healthy(&self, now_unix: i64) {
        if let Some(recovery) = &self.recovery
            && self.registry.health_report(now_unix).is_empty()
        {
            recovery();
        }
    }

    /// Offboard every domain a tenant owns. Returns how many were removed.
    ///
    /// The hostname list is a snapshot and each offboard awaits, so every
    /// removal re-asserts the tenant: a hostname freed early in the sweep can
    /// be re-registered by ANOTHER tenant before a later one runs, and removing
    /// it unconditionally would disconnect a tenant who was never part of this
    /// teardown.
    ///
    /// # Errors
    ///
    /// Propagates the registry store's delete error.
    pub async fn offboard_tenant(&self, tenant: &str) -> std::io::Result<usize> {
        let hostnames: Vec<String> = self
            .registry
            .list_for_tenant(tenant)
            .into_iter()
            .map(|d| d.hostname)
            .collect();
        let mut removed = 0;
        for hostname in hostnames {
            let Ok(host) = crate::custom_domain::normalize_hostname(&hostname) else {
                continue;
            };
            if self
                .offboard_guarded(&host, |current| current.tenant == tenant)
                .await?
            {
                removed += 1;
            }
        }
        Ok(removed)
    }

    /// Delete stored certificate pairs no registered hostname maps to.
    ///
    /// Only certificates whose [`CertId`] matches some *removed* custom domain
    /// can be identified: an id is a hash, so the deployment's own certificate
    /// (and any other) is left alone by construction — we delete only ids that
    /// no longer appear in the registry AND are not the configured cert.
    async fn prune_orphan_certs(&self, dry_run: bool) -> Result<u64, String> {
        let Some(fs) = self.cert_store_paths.as_ref() else {
            // A non-filesystem store cannot be enumerated through this seam.
            return Ok(0);
        };
        // Enumerate the store FIRST, then snapshot the registry. The reverse
        // order deletes a certificate issued in between: its hostname was
        // registered after the registry snapshot, so it is missing from `live`,
        // while its freshly written pair is already in `stored`. Activation
        // would then complete with its durable certificate gone, and the domain
        // would fail the first handshake after a cache eviction or a restart.
        // This way, a pair written after the enumeration is simply not a
        // candidate, and one written before it has a record the later snapshot
        // sees.
        let stored = fs
            .list_certs()
            .map_err(|e| format!("failed to enumerate stored certificates: {e}"))?;
        //
        // The snapshot is the in-memory index AND a fresh read of the store.
        // The index alone is only as current as this process's own writes: a
        // one-shot `autumn db retention` loaded it at startup, and a serving
        // process sharing the store may have registered and issued a hostname
        // since. Its pair would then be in `stored` while its record is missing
        // from a stale index, and an active tenant's certificate would be
        // deleted. A record either view knows keeps its certificate.
        let fresh = self
            .registry
            .stored_records()
            .await
            .map_err(|e| format!("failed to re-read the custom-domain registry: {e}"))?;
        let live: std::collections::HashSet<String> = self
            .registry
            .list()
            .into_iter()
            .chain(fresh)
            .map(|d| cert_id_for(&d.hostname).as_str().to_owned())
            .collect();
        let mut removed = 0;
        for (id, chain, key) in stored {
            if live.contains(id.as_str()) || self.retained_cert_ids.contains(id.as_str()) {
                continue;
            }
            if dry_run {
                removed += 1;
                continue;
            }
            let mut deleted = true;
            // The KEY first. `list_certs` discovers candidates from their
            // `.chain.pem`, so a crash after removing the chain would strand a
            // private key no later sweep can ever find again. This order leaves
            // at worst a public chain behind, which the next sweep re-lists.
            for path in [&key, &chain] {
                if let Err(e) = std::fs::remove_file(path) {
                    tracing::warn!(path = %path.display(), "failed to remove an orphaned certificate: {e}");
                    deleted = false;
                }
            }
            // Count only what actually went: the retention report must not
            // claim a deletion that failed.
            if deleted {
                removed += 1;
            }
        }
        Ok(removed)
    }
}

impl crate::custom_domain::CustomDomainPruner for PruneOnlyCustomDomainPruner {
    fn prune(
        &self,
        cutoff_unix: i64,
        dry_run: bool,
    ) -> futures::future::BoxFuture<'_, Result<u64, String>> {
        Box::pin(async move {
            // An index that failed to hydrate knows nothing, so EVERY tenant
            // certificate would read as an orphan and be deleted. One transient
            // read error at boot must not cost the deployment every private key
            // it holds.
            if !self.registry.is_hydrated() {
                return Err(
                    "refusing to prune: the custom-domain registry did not load at boot, so \
                     every certificate would look orphaned"
                        .to_owned(),
                );
            }
            let mut removed = 0_u64;
            // What the store holds NOW, not only what this process's index
            // saw at load: another process sharing the store (a serving app
            // while a one-shot `autumn db retention` runs) may have verified
            // or re-registered a hostname since. A candidate must still be the
            // same abandoned registration in the store, or it is left alone.
            let fresh: std::collections::HashMap<String, crate::custom_domain::CustomDomain> = self
                .registry
                .stored_records()
                .await
                .map_err(|e| format!("failed to re-read the custom-domain registry: {e}"))?
                .into_iter()
                .map(|d| (d.hostname.clone(), d))
                .collect();
            // Abandoned connections: a tenant was handed DNS instructions and
            // never published the record. Nothing else ever deletes these.
            for domain in self.registry.list() {
                let still_abandoned_in_store = fresh.get(&domain.hostname).is_some_and(|stored| {
                    stored.status == domain.status
                        && stored.tenant == domain.tenant
                        && stored.registered_at_unix == domain.registered_at_unix
                });
                if domain.status == crate::custom_domain::DomainStatus::PendingDns
                    && domain.registered_at_unix < cutoff_unix
                    && still_abandoned_in_store
                {
                    if dry_run {
                        removed += 1;
                        continue;
                    }
                    // These candidates came from a `list()` snapshot and each
                    // offboard below awaits, so by the time this one runs the
                    // domain may have verified, issued and gone `Active` — the
                    // orchestrator ticks concurrently. Deleting it then would
                    // disconnect a tenant seconds after their domain came up,
                    // and take the certificate with it, so the record is
                    // re-asserted inside the registry's per-hostname gate.
                    if self
                        .offboard_if_unchanged(&domain)
                        .await
                        .map_err(|e| format!("failed to offboard {}: {e}", domain.hostname))?
                    {
                        removed += 1;
                    }
                }
            }
            // Orphaned certificates: a pair whose hostname is no longer
            // registered at all. Pruned regardless of the cutoff — there is no
            // record left to age.
            removed += self.prune_orphan_certs(dry_run).await?;
            Ok(removed)
        })
    }

    fn offboard_domain<'a>(
        &'a self,
        hostname: &'a str,
    ) -> futures::future::BoxFuture<'a, std::io::Result<bool>> {
        Box::pin(self.offboard(hostname))
    }

    fn offboard_tenant_domains<'a>(
        &'a self,
        tenant: &'a str,
    ) -> futures::future::BoxFuture<'a, std::io::Result<usize>> {
        Box::pin(self.offboard_tenant(tenant))
    }
}

impl CustomDomainTask {
    /// Run until `shutdown`, ticking every `interval`.
    ///
    /// The loop never returns an error: a tick that fails leaves the affected
    /// domain's reason recorded and every other domain serving.
    pub async fn run(
        &self,
        interval: std::time::Duration,
        shutdown: tokio_util::sync::CancellationToken,
    ) {
        // Warm the cache before the first tick so a restart serves stored
        // certificates on the first handshake instead of after an order.
        self.warm_all().await;
        self.hydrate_limiter(crate::custom_domain::now_unix());
        loop {
            // The tick is inside the `select!`: a pass over a thousand domains
            // is long, and shutdown must not wait for it.
            tokio::select! {
                () = self.tick(crate::custom_domain::now_unix()) => {}
                () = shutdown.cancelled() => break,
            }
            tokio::select! {
                () = tokio::time::sleep(interval) => {}
                () = shutdown.cancelled() => break,
            }
        }
    }

    /// One pass over every registered domain.
    pub async fn tick(&self, now_unix: i64) {
        // `now_unix` is when the pass began. A pass over many domains, each a
        // CA round trip, runs for minutes, so an order late in it must not be
        // stamped with the start time: its budget entry would age early and a
        // failure's `next_attempt_unix` could already be in the past, sending
        // the next tick straight back to the CA. Elapsed monotonic time keeps
        // the stamps honest and the caller's clock injectable.
        let started = tokio::time::Instant::now();
        let at = move || elapsed_since(now_unix, started);
        // Each check can wait out a DNS timeout, so checks run
        // `VERIFY_CONCURRENCY` at a time instead of adding up. Each write goes
        // through the registry's per-hostname gate.
        futures::StreamExt::for_each_concurrent(
            futures::stream::iter(self.registry.pending_verification(now_unix)),
            VERIFY_CONCURRENCY,
            |domain| async move { self.verify_one(&domain.hostname, now_unix).await },
        )
        .await;
        for domain in self.registry.due_for_issuance(now_unix) {
            self.issue_one(
                &domain.hostname,
                &domain.tenant,
                domain.verification_token.as_deref(),
                at(),
            )
            .await;
        }
        // Renewal re-verifies first. A tenant who repointed or gave up their
        // domain would otherwise be renewed forever: every cycle spends an
        // order plus a failed validation against the shared ACME account, for a
        // certificate that reaches nobody.
        for domain in self
            .registry
            .due_for_renewal(now_unix, self.renew_before_days)
        {
            if !self.still_points_here(&domain, now_unix).await {
                continue;
            }
            self.issue_one(
                &domain.hostname,
                &domain.tenant,
                domain.verification_token.as_deref(),
                at(),
            )
            .await;
        }
        // An active domain whose stored certificate has gone (a torn write, a
        // partial restore, a manual delete) is refused at the handshake but is
        // NOT due for renewal — its recorded `notAfter` may be months away — so
        // without this it stays hard down until that window opens.
        for domain in self.registry.list() {
            if domain.is_servable()
                && domain.is_due(now_unix)
                && !self.certificate_present(&domain.hostname)
            {
                tracing::warn!(
                    hostname = %domain.hostname,
                    "custom domain is active but its certificate is missing; re-ordering"
                );
                self.issue_one(
                    &domain.hostname,
                    &domain.tenant,
                    domain.verification_token.as_deref(),
                    at(),
                )
                .await;
            }
        }
    }

    /// Is `hostname`'s certificate available AND usable?
    ///
    /// Present-on-disk is not enough: a corrupt or mismatched pair passes a
    /// `stat` but is rejected by both `warm` and the handshake source, so
    /// answering `true` for it would suppress the repair below and leave the
    /// domain hard down until its renew window opened — months, potentially.
    /// The pair is therefore parsed, through the same loader the handshake
    /// uses, and a pair that will not load counts as absent.
    ///
    /// Only for a store this task can enumerate; a store it cannot answers
    /// `true`, so a non-filesystem store is never wrongly re-ordered.
    fn certificate_present(&self, hostname: &str) -> bool {
        let Some(fs) = self.cert_store_paths.as_ref() else {
            return true;
        };
        let Some((chain_path, key_path)) = fs.find_cert_for_domains(&[hostname.to_owned()]) else {
            return false;
        };
        // A cache hit is not evidence the DURABLE pair is still there: a
        // certificate deleted or corrupted after it was cached keeps serving
        // from memory, and the repair pass — the only thing that would re-order
        // it — used to skip it for exactly as long as it stayed resident. The
        // first eviction or restart then failed every handshake for that
        // domain. Enumerating the pair is a `stat`; the parse below is the part
        // worth skipping while the cache holds a usable copy of it.
        if self.cache.get(hostname).is_some() {
            return true;
        }
        let (Ok(chain), Ok(key)) = (std::fs::read(&chain_path), std::fs::read(&key_path)) else {
            return false;
        };
        match crate::tls::certified_key_from_pem(&chain, &key, &self.provider) {
            Ok(_) => true,
            Err(e) => {
                tracing::warn!(
                    hostname,
                    "stored custom-domain certificate is present but unusable ({e}); \
                     treating it as missing so the repair pass re-orders it"
                );
                false
            }
        }
    }

    /// Does `domain` still resolve to this deployment?
    ///
    /// A lookup that returns nothing answers `true`: a resolver blip must not
    /// stop a healthy renewal, and the CA's own validation is the real gate.
    ///
    /// A domain that has genuinely moved away is RECORDED, not merely logged.
    /// It is inside its renewal window and will expire — the tenant repointed
    /// or gave the hostname up — so an operator has to act. Skipping silently
    /// left `/actuator/health` `Up` and raised no alert until the certificate
    /// expired, at which point the domain was already down.
    async fn still_points_here(
        &self,
        domain: &crate::custom_domain::CustomDomain,
        now_unix: i64,
    ) -> bool {
        let hostname = domain.hostname.as_str();
        let observed = self.verifier.observe(hostname).await;
        if matches!(observed, crate::custom_domain::ObservedTarget::None) {
            return true;
        }
        let outcome = grade_dns_verification(&observed, &self.effective_ingress().await);
        if outcome.is_verified() {
            return true;
        }
        tracing::warn!(
            hostname,
            "skipping renewal: the domain no longer points at this deployment"
        );
        let reason = outcome
            .reason()
            .unwrap_or_else(|| "the domain no longer points at this deployment".to_owned());
        self.record_failure(
            hostname,
            &domain.tenant,
            domain.verification_token.as_deref(),
            now_unix,
            format!(
                "renewal skipped: {reason}. The certificate will expire unless the record is \
                 restored or the domain is offboarded"
            ),
            true,
        )
        .await;
        false
    }

    /// Check where one hostname points, and that its owner published this
    /// registration's token, and record the result.
    ///
    /// Pointing here is not enough on its own: a record another tenant left
    /// behind also points here, and HTTP-01 would then issue to whoever
    /// registered the hostname next (#2642).
    async fn verify_one(&self, hostname: &str, now_unix: i64) {
        let Some(mut domain) = self.registry.get(hostname) else {
            return;
        };
        // `load` gives a pre-token record its token; this retries a load whose
        // write failed.
        if domain.needs_verification_token() {
            if let Err(e) = self
                .registry
                .adopt_verification_token(hostname, now_unix)
                .await
            {
                tracing::warn!(
                    hostname,
                    "could not give a custom domain its verification token: {e}"
                );
                return;
            }
            match self.registry.get(hostname) {
                Some(adopted) if adopted.verification_token.is_some() => domain = adopted,
                _ => return,
            }
        }
        let observed = self.verifier.observe(hostname).await;
        let mut outcome = grade_dns_verification(&observed, &self.effective_ingress().await);
        if outcome.is_verified() {
            let txt = self
                .verifier
                .observe_txt(&domain.verification_record_name())
                .await;
            outcome = grade_ownership(&domain, &txt);
        }
        let backoff = i64::try_from(
            self.limiter
                .backoff_for(domain.consecutive_failures.saturating_add(1)),
        )
        .unwrap_or(i64::MAX);
        if let Err(e) =
            apply_verification(&self.registry, &domain, &outcome, now_unix, backoff).await
        {
            tracing::warn!(
                hostname,
                "failed to persist custom-domain verification: {e}"
            );
        }
    }

    /// The ingress to compare a tenant's DNS against.
    ///
    /// A resolver reports the ADDRESSES a name ends at, following any CNAME
    /// silently — so an operator who configured only `ingress_hostname` has
    /// nothing to compare an observed address against. The hostname's addresses
    /// are therefore resolved and **added to** whatever was configured
    /// explicitly, rather than replacing them or being skipped.
    ///
    /// The union matters for the setup the guide documents: subdomains CNAME to
    /// a load balancer while apex domains use static A/AAAA records. Those are
    /// two DIFFERENT address sets, and a tenant subdomain resolves to the load
    /// balancer's. Returning only the configured apex addresses would leave
    /// every subdomain stuck at `pending_dns` forever.
    ///
    /// Resolved per tick, not cached: an ingress behind a load balancer whose
    /// address changes must not leave every tenant domain failing verification
    /// until the process restarts.
    async fn effective_ingress(&self) -> ExpectedIngress {
        let mut ingress = self.ingress.clone();
        let Some(host) = ingress.hostname.clone() else {
            return ingress;
        };
        if let crate::custom_domain::ObservedTarget::Addresses(addrs) =
            self.verifier.observe(&host).await
        {
            for addr in addrs {
                match addr {
                    std::net::IpAddr::V4(v4) if !ingress.ipv4.contains(&v4) => {
                        ingress.ipv4.push(v4);
                    }
                    std::net::IpAddr::V6(v6) if !ingress.ipv6.contains(&v6) => {
                        ingress.ipv6.push(v6);
                    }
                    _ => {}
                }
            }
        }
        ingress
    }

    /// Order (or renew) one hostname's certificate, budget permitting.
    async fn issue_one(&self, hostname: &str, tenant: &str, token: Option<&str>, now_unix: i64) {
        // A distributed scheduler backend was configured but this process fell
        // back to a per-process coordinator. Ordering now would give every
        // replica its own lease, so all of them would order the SAME
        // certificate against the one shared account. Refuse, and say why.
        if self.leadership_degraded {
            self.record_failure(
                hostname,
                tenant,
                token,
                now_unix,
                "refusing to order: a distributed scheduler backend is configured but its \
                 coordinator is unavailable in this process, so a lease would not exclude the \
                 other replicas",
                true,
            )
            .await;
            return;
        }

        // The budget is checked BEFORE any network call, so a spent budget
        // costs the CA nothing. The failure backoff is already applied by
        // `is_due` on the record, so the budget is the only thing left to ask.
        if !self.within_budget(hostname, tenant, token, now_unix).await {
            return;
        }

        // One replica per hostname orders. `Fleet` grants unconditionally on
        // the in-process backend (correct for a single replica) and to exactly
        // one replica on a distributed one.
        let tick_key = format!("custom-domain:{hostname}");
        let lease = match self
            .coordinator
            .try_acquire(CUSTOM_DOMAIN_TASK, &tick_key, TaskCoordination::Fleet)
            .await
        {
            Ok(Some(lease)) => lease,
            // Another replica leads for this domain. Adopt whatever it has
            // already written rather than ordering a second certificate.
            Ok(None) => {
                self.warm(hostname).await;
                return;
            }
            Err(e) => {
                self.record_failure(
                    hostname,
                    tenant,
                    token,
                    now_unix,
                    format!("leader election failed: {e}"),
                    true,
                )
                .await;
                return;
            }
        };

        // The lease wait is an await, and the registry can move underneath it.
        // Re-assert ownership as part of the transition: ordering for a
        // hostname that was offboarded meanwhile spends a slot of the budget
        // and the CA's rate limit on a certificate `install` would only
        // discard, and ordering for one re-registered by another tenant would
        // move THEIR record to `Issuing` for an order that was never theirs.
        match self.registry.record_issuing_for(hostname, tenant).await {
            Ok(true) => {}
            Ok(false) => {
                tracing::debug!(
                    hostname,
                    tenant,
                    "abandoning a custom-domain order: the hostname is no longer this tenant's to \
                     order for"
                );
                if let Err(e) = lease.release().await {
                    tracing::warn!(hostname, error = %e, "failed to release the custom-domain lease");
                }
                return;
            }
            Err(e) => {
                // No durable in-flight state means even a successful order
                // could not be recorded: the record stays `Verified`, the
                // certificate would fail to activate the same way, and the
                // next tick would spend another CA attempt on it. Stop before
                // contacting the CA.
                self.record_failure(
                    hostname,
                    tenant,
                    token,
                    now_unix,
                    format!("refusing to order: the issuing state could not be persisted ({e})"),
                    true,
                )
                .await;
                if let Err(e) = lease.release().await {
                    tracing::warn!(hostname, error = %e, "failed to release the custom-domain lease");
                }
                return;
            }
        }
        self.limiter.record_attempt(hostname, now_unix);
        self.persist_attempt(hostname, tenant, token, now_unix)
            .await;

        let order_started = tokio::time::Instant::now();
        let outcome = self.issuer.issue(hostname).await;
        // An order can take minutes; what follows it is stamped with when it
        // actually finished, not when it began.
        let now_unix = elapsed_since(now_unix, order_started);
        // Always release the lease, whatever the order did.
        if let Err(e) = lease.release().await {
            tracing::warn!(hostname, error = %e, "failed to release the custom-domain lease");
        }

        let issued = match outcome {
            Ok(issued) => issued,
            Err(e) => {
                self.record_failure(hostname, tenant, token, now_unix, e, true)
                    .await;
                return;
            }
        };

        if let Err(e) = self.install(hostname, tenant, &issued, now_unix).await {
            self.record_failure(hostname, tenant, token, now_unix, e, true)
                .await;
        }
    }

    /// Persist, parse and start serving a freshly issued certificate.
    ///
    /// Persisting first means a crash between the two still leaves the
    /// certificate on disk for the next boot to warm.
    async fn install(
        &self,
        hostname: &str,
        tenant: &str,
        issued: &IssuedCertificate,
        now_unix: i64,
    ) -> Result<(), String> {
        // The order took a network round trip. If the app offboarded the domain
        // — or another tenant registered it — meanwhile, installing would
        // resurrect a deleted certificate and promote a record straight to
        // active that was never verified, past the gate this module exists to
        // enforce. Discard instead.
        match self.registry.get(hostname) {
            Some(record) if record.tenant == tenant => {}
            Some(record) => {
                return Err(format!(
                    "discarding the certificate: {hostname} now belongs to tenant {}",
                    record.tenant
                ));
            }
            None => {
                return Err(format!(
                    "discarding the certificate: {hostname} was offboarded while the order ran"
                ));
            }
        }

        let stored = StoredCert {
            chain_pem: issued.chain_pem.clone(),
            key_pem: issued.key_pem.clone(),
        };
        let not_after = crate::tls::leaf_not_after_from_pem(stored.chain_pem.as_bytes())?;
        let certified = crate::tls::certified_key_from_pem(
            stored.chain_pem.as_bytes(),
            stored.key_pem.as_bytes(),
            &self.provider,
        )?;
        self.certs
            .save_cert(&cert_id_for(hostname), &stored)
            .await
            .map_err(|e| format!("failed to persist the certificate for {hostname}: {e}"))?;
        // Activate only while `tenant` STILL owns the hostname. The ownership
        // check above ran before `save_cert` awaited; an offboard plus a
        // re-registration in that window would otherwise stamp `Active` onto
        // the new tenant's record, carrying it from `pending_dns` to serving on
        // a certificate issued for someone else. Losing the race means the
        // certificate belongs to nobody: drop it rather than leave a private
        // key on disk for a hostname its owner never verified.
        let activated = self
            .registry
            .record_active_for(hostname, tenant, now_unix, not_after)
            .await
            .map_err(|e| format!("failed to persist the active state for {hostname}: {e}"))?;
        if !activated {
            self.cache.remove(hostname);
            // Delete only the pair THIS order wrote. The certificate id is a
            // hash of the hostname, so a successor that issued and saved its
            // own pair in the meantime holds the same id: an unconditional
            // delete here would take that valid certificate with it, leaving
            // the successor recorded as active with nothing on disk.
            match self.certs.load_cert(&cert_id_for(hostname)).await {
                Ok(Some(current)) if current.chain_pem == stored.chain_pem => {
                    if let Err(e) = self.certs.delete_cert(&cert_id_for(hostname)).await {
                        tracing::warn!(hostname, "failed to delete a superseded certificate: {e}");
                    }
                }
                Ok(Some(_)) => tracing::debug!(
                    hostname,
                    "leaving the stored certificate alone: it is the successor's, not this order's"
                ),
                Ok(None) => {}
                Err(e) => tracing::warn!(
                    hostname,
                    "cannot tell whether the stored certificate is this order's, so leaving it: {e}"
                ),
            }
            return Err(format!(
                "discarding the certificate: {hostname} changed hands while it was being issued"
            ));
        }
        self.cache.insert(hostname, certified);
        tracing::info!(hostname, not_after, "custom domain is active");
        // Backstop: a certificate already inside its renew-before window the
        // moment it is issued — a CA issuing a shorter lifetime than
        // `renew_before_days` — would be re-ordered every tick until the budget
        // stopped it, then again the next day, forever. Park it and say why.
        if crate::custom_domain::needs_renewal(not_after, self.renew_before_days, now_unix) {
            let backoff = i64::try_from(self.limiter.max_backoff()).unwrap_or(i64::MAX);
            let _ = self
                .registry
                .record_failure(
                    hostname,
                    now_unix,
                    format!(
                        "the issued certificate is already inside its renew-before window ({} \
                         days); lower [server.tls.acme] renew_before_days below the certificate \
                         lifetime",
                        self.renew_before_days
                    ),
                    backoff,
                )
                .await;
            return Ok(());
        }
        // Only once NOTHING is failing: with a thousand domains, recovering one
        // while another is still broken must not retract an alert that is
        // still true.
        self.clear_alert_if_healthy(now_unix);
        Ok(())
    }

    /// Seed the issuance budget from the attempts persisted on each domain
    /// record, so a restart does not hand the shared ACME account a fresh
    /// quota. Called once before the first pass of [`Self::run`]; safe to call
    /// again.
    pub fn hydrate_limiter(&self, now_unix: i64) {
        self.limiter.hydrate(
            self.registry.list().into_iter().flat_map(|d| {
                let hostname = d.hostname;
                d.issuance_attempts_unix
                    .into_iter()
                    .map(move |at| (hostname.clone(), at))
            }),
            now_unix,
        );
    }

    /// Is there issuance budget for `hostname`? A refusal defers the domain
    /// until the refusing window rolls; it is not a failure, because no order
    /// was placed, so it never counts toward the failure backoff.
    async fn within_budget(
        &self,
        hostname: &str,
        tenant: &str,
        token: Option<&str>,
        now_unix: i64,
    ) -> bool {
        let decision = self.limiter.check(hostname, now_unix);
        if decision.is_allowed() {
            return true;
        }
        if let (Some(reason), Some(wait)) = (decision.reason(), decision.retry_after_secs()) {
            self.defer(
                hostname,
                tenant,
                token,
                now_unix.saturating_add(wait),
                reason,
            )
            .await;
        }
        false
    }

    /// Persist that an order is about to go out, so the issuance budget
    /// survives a restart, including a crash mid-order.
    ///
    /// Fails open: the in-memory limiter already counts it, and refusing to
    /// order would turn a store hiccup into an outage for the tenant.
    async fn persist_attempt(
        &self,
        hostname: &str,
        tenant: &str,
        token: Option<&str>,
        now_unix: i64,
    ) {
        if let Err(e) = self
            .registry
            .record_attempt_for_registration(hostname, tenant, token, now_unix)
            .await
        {
            tracing::warn!(
                hostname,
                error = %e,
                "could not persist a custom-domain issuance attempt; the budget will not survive \
                 a restart"
            );
        }
    }

    /// Hold a domain back until `retry_at_unix` without recording a failure.
    async fn defer(
        &self,
        hostname: &str,
        tenant: &str,
        token: Option<&str>,
        retry_at_unix: i64,
        reason: String,
    ) {
        match self
            .registry
            .defer_for_registration(hostname, tenant, token, retry_at_unix, reason)
            .await
        {
            Ok(true) => {}
            Ok(false) => tracing::debug!(
                hostname,
                tenant,
                "discarded a custom-domain deferral: the hostname changed hands or was \
                 re-registered meanwhile"
            ),
            Err(e) => tracing::warn!(
                hostname,
                error = %e,
                "could not record a custom-domain issuance deferral"
            ),
        }
    }

    /// Record a failure on one domain, alerting the operator when it is an
    /// issuance failure rather than a deferral the budget already explains.
    ///
    /// `token` is the registration's ownership token as read when the work
    /// started: the failure is recorded only while the stored record is still
    /// that registration.
    async fn record_failure(
        &self,
        hostname: &str,
        tenant: &str,
        token: Option<&str>,
        now_unix: i64,
        reason: impl Into<String>,
        alert: bool,
    ) {
        let reason = reason.into();
        let failures = self
            .registry
            .get(hostname)
            .filter(|d| d.tenant == tenant && d.verification_token.as_deref() == token)
            .map_or(0, |d| d.consecutive_failures)
            .saturating_add(1);
        let backoff = i64::try_from(self.limiter.backoff_for(failures)).unwrap_or(i64::MAX);
        // Only while this registration still holds the hostname. An order runs
        // across several awaits: if the domain was offboarded and re-registered
        // meanwhile — by another tenant or the same one — charging the failure
        // here would put a dead order's reason and backoff on the successor.
        match self
            .registry
            .record_failure_for_registration(
                hostname,
                tenant,
                token,
                now_unix,
                reason.clone(),
                backoff,
            )
            .await
        {
            Ok(true) => {}
            Ok(false) => {
                tracing::debug!(
                    hostname,
                    tenant,
                    "discarded a custom-domain failure: the hostname changed hands or was \
                     re-registered while the order was in flight"
                );
                return;
            }
            Err(e) => {
                tracing::warn!(hostname, "failed to persist custom-domain failure: {e}");
            }
        }
        // Naming the domain AND the tenant is what lets an operator act on the
        // alert without a lookup — one tenant's broken domain among a thousand.
        let message =
            format!("custom domain {hostname} (tenant {tenant}) failed to issue: {reason}");
        tracing::warn!("{message}");
        if alert {
            (self.reporter)(message);
        }
    }

    /// The prune-only half of this task: the
    /// [`CustomDomainPruner`](crate::custom_domain::CustomDomainPruner)
    /// implementation the server and `autumn db retention` both install.
    pub(crate) fn pruner(&self) -> PruneOnlyCustomDomainPruner {
        PruneOnlyCustomDomainPruner {
            registry: Arc::clone(&self.registry),
            cache: Arc::clone(&self.cache),
            certs: Arc::clone(&self.certs),
            limiter: Arc::clone(&self.limiter),
            cert_store_paths: self.cert_store_paths.clone(),
            retained_cert_ids: self.retained_cert_ids.clone(),
            reporter: Arc::clone(&self.reporter),
            recovery: self.recovery.clone(),
        }
    }

    /// Retract the operator alert once NOTHING is failing any more.
    ///
    /// Shared with the prune half; see
    /// [`PruneOnlyCustomDomainPruner::clear_alert_if_healthy`].
    fn clear_alert_if_healthy(&self, now_unix: i64) {
        self.pruner().clear_alert_if_healthy(now_unix);
    }

    /// Offboard `hostname`: stop routing, stop serving, halt renewal, and
    /// delete the stored certificate so it is not orphaned (AC7, #1605).
    ///
    /// # Errors
    ///
    /// Propagates a registry-store error; see
    /// [`PruneOnlyCustomDomainPruner::offboard`].
    pub async fn offboard(&self, hostname: &str) -> std::io::Result<bool> {
        self.pruner().offboard(hostname).await
    }

    /// Offboard every domain a tenant owns. Returns how many were removed.
    ///
    /// # Errors
    ///
    /// Propagates the registry store's delete error.
    pub async fn offboard_tenant(&self, tenant: &str) -> std::io::Result<usize> {
        self.pruner().offboard_tenant(tenant).await
    }

    /// Load one domain's stored certificate into the cache. Returns whether a
    /// usable certificate was found.
    ///
    /// This is AC6's incremental load: a handshake for a domain evicted from
    /// the bounded cache re-reads its certificate here instead of the
    /// deployment needing every certificate resident at boot.
    pub async fn warm(&self, hostname: &str) -> bool {
        let Ok(host) = crate::custom_domain::normalize_hostname(hostname) else {
            return false;
        };
        let hostname = host.as_str();
        if !self.registry.is_servable(hostname) {
            return false;
        }
        match self.certs.load_cert(&cert_id_for(hostname)).await {
            Ok(Some(stored)) => match crate::tls::certified_key_from_pem(
                stored.chain_pem.as_bytes(),
                stored.key_pem.as_bytes(),
                &self.provider,
            ) {
                Ok(certified) => {
                    self.cache.insert(hostname, certified);
                    true
                }
                Err(e) => {
                    tracing::warn!(
                        hostname,
                        "stored custom-domain certificate is unusable: {e}"
                    );
                    false
                }
            },
            Ok(None) => false,
            Err(e) => {
                tracing::warn!(hostname, "failed to read a custom-domain certificate: {e}");
                false
            }
        }
    }

    /// Warm as many active domains as the cache holds, newest activation
    /// first, so a restart serves the busiest domains without a disk read.
    ///
    /// Deliberately bounded by the cache: warming every domain would make
    /// "all certificates resident" a boot requirement, which AC6 rules out.
    async fn warm_all(&self) {
        let mut active: Vec<_> = self
            .registry
            .list()
            .into_iter()
            .filter(crate::custom_domain::CustomDomain::is_servable)
            .collect();
        active.sort_by_key(|d| std::cmp::Reverse(d.activated_at_unix.unwrap_or(0)));
        for domain in active.into_iter().take(self.cache.capacity()) {
            self.warm(&domain.hostname).await;
        }
    }
}

impl crate::custom_domain::CustomDomainPruner for CustomDomainTask {
    fn prune(
        &self,
        cutoff_unix: i64,
        dry_run: bool,
    ) -> futures::future::BoxFuture<'_, Result<u64, String>> {
        let pruner = self.pruner();
        Box::pin(async move {
            <PruneOnlyCustomDomainPruner as crate::custom_domain::CustomDomainPruner>::prune(
                &pruner,
                cutoff_unix,
                dry_run,
            )
            .await
        })
    }

    fn offboard_domain<'a>(
        &'a self,
        hostname: &'a str,
    ) -> futures::future::BoxFuture<'a, std::io::Result<bool>> {
        let pruner = self.pruner();
        Box::pin(async move {
            <PruneOnlyCustomDomainPruner as crate::custom_domain::CustomDomainPruner>::offboard_domain(
                &pruner, hostname,
            )
            .await
        })
    }

    fn offboard_tenant_domains<'a>(
        &'a self,
        tenant: &'a str,
    ) -> futures::future::BoxFuture<'a, std::io::Result<usize>> {
        let pruner = self.pruner();
        Box::pin(async move {
            <PruneOnlyCustomDomainPruner as crate::custom_domain::CustomDomainPruner>::offboard_tenant_domains(
                &pruner, tenant,
            )
            .await
        })
    }
}

/// A [`SniCertSource`](crate::custom_domain::SniCertSource) reading the
/// filesystem certificate store on the handshake path.
///
/// Two `read`s and a parse, only for a hostname the registry has already
/// confirmed is registered and active. That is what lets `cert_cache_size` be
/// much smaller than the number of connected domains without any of them
/// stopping being served.
#[derive(Debug)]
pub struct FsSniCertSource {
    store: Arc<crate::acme::store::FsAcmeStore>,
    provider: Arc<CryptoProvider>,
}

impl FsSniCertSource {
    /// A source over `store`, parsing with `provider`.
    #[must_use]
    pub const fn new(
        store: Arc<crate::acme::store::FsAcmeStore>,
        provider: Arc<CryptoProvider>,
    ) -> Self {
        Self { store, provider }
    }
}

impl crate::custom_domain::SniCertSource for FsSniCertSource {
    fn load(&self, hostname: &str) -> Option<Arc<rustls::sign::CertifiedKey>> {
        let (chain_path, key_path) = self.store.find_cert_for_domains(&[hostname.to_owned()])?;
        let chain = std::fs::read(&chain_path).ok()?;
        let key = std::fs::read(&key_path).ok()?;
        match crate::tls::certified_key_from_pem(&chain, &key, &self.provider) {
            Ok(certified) => Some(certified),
            Err(e) => {
                tracing::warn!(
                    hostname,
                    "stored custom-domain certificate is unusable: {e}"
                );
                None
            }
        }
    }
}

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

    #[test]
    fn a_hostname_gets_its_own_cert_id() {
        assert_ne!(cert_id_for("a.test"), cert_id_for("b.test"));
        assert_eq!(cert_id_for("a.test"), cert_id_for("a.test"));
    }
}