Enum ProbeHoldOutcome 

pub enum ProbeHoldOutcome {
    Held,
    Unavailable,
    BudgetExhausted,
    HoldsDisabled,
}

Outcome of crate::CacheEngine::try_probe_hold, carrying two independent facts (ADR 005 §Probe-triggered eviction hold):

• whether to advertise the blob — ProbeHoldOutcome::advertises;
• whether a hold was actually placed — ProbeHoldOutcome::hold_placed.

Holds are best-effort, so these are two independent bits: Self::BudgetExhausted advertises without holding. Use the two predicates rather than matching on variants, so the mapping stays in one place; the variants themselves exist so the probe handler can emit the right metric without a second cache lookup.

Variants

Held

Blob present (and not operator-evicted) and an eviction hold was placed, so the blob stays resident for the follow-up pull — sign has_blob: true.

Unavailable

Blob absent or operator-evicted — sign has_blob: false. Not a degradation: the node simply does not have the content.

Deliberately the one outcome with no metric. The probe_hold_unavailable counter records hold refusals for content the node has; a true negative is not a refusal, and counting it would swamp the signal every alert on that metric depends on. Despite the shared word, this is not ProbeHoldUnavailableReason — that enum’s values all mean “we have it but would not hold it”.

BudgetExhausted

Blob present but all hold slots are in use (max_probe_holds > 0 and the live-hold count has reached it). The caller still advertises has_blob: true but places no hold, so the blob may be LRU-evicted before the pull — costing one wasted round trip, never a slash. Counted as probe_hold_unavailable{reason="exhausted"} (ADR 005 §Hold budget); the operator-actionable remedy for the lost holds is to raise max_probe_holds.

HoldsDisabled

Blob present but the eviction-hold path is disabled by config (max_probe_holds == 0) — the operator opt-out from advertising store-backed content, so the caller signs has_blob: false. Operationally distinct from Self::BudgetExhausted (#739): a deliberate choice, not load. Counted as probe_hold_unavailable{reason="disabled"}, never as {reason="exhausted"} (whose alert remedy is “increase max_probe_holds”, nonsensical when holds are deliberately off).

Scope note: this silences store-backed advertisements only. Content servable from a configured origin takes no hold and is advertised regardless — see the origin-held fallback in the probe handler.

Implementations

impl ProbeHoldOutcome

pub const fn advertises(self) -> bool

Whether this outcome authorises signing has_blob: true for the store-backed answer (the origin-held fallback may still promote a false afterwards).

Presence governs the answer, not the hold: Self::BudgetExhausted advertises despite placing no hold, because refusing there would let a probe flood that fills the hold cache suppress truthful answers.

pub const fn hold_placed(self) -> bool

Whether an eviction hold was actually placed, making the blob LRU-exempt for PROBE_HOLD_DURATION.

Distinct from Self::advertises: holds are best-effort, and only Self::Held consumes a slot.

Trait Implementations

impl Clone for ProbeHoldOutcome

fn clone(&self) -> Self

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for ProbeHoldOutcome

impl Debug for ProbeHoldOutcome

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Eq for ProbeHoldOutcome

impl PartialEq for ProbeHoldOutcome

fn eq(&self, other: &Self) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for ProbeHoldOutcome