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distinguish no-value from not computed at all

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In the case of no-value, we track the revision where
the value was last changed, which will be useful for
maybe-changed-since.
This commit is contained in:
Niko Matsakis 2021-10-30 04:27:25 -04:00
parent e7a8abc648
commit 91ed93a4d4
1 changed files with 28 additions and 6 deletions
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34
src/derived/slot.rs
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@ -94,10 +94,27 @@ pub(super) enum MemoInputs {
/// Return value of `probe` helper.
enum ProbeState<V, G> {
UpToDate(Result<V, CycleError>),
/// Another thread was active but has completed.
/// Try again!
Retry,
/// No entry for this key at all.
NotComputed(G),
/// There is an entry, but its contents have not been
/// verified in this revision.
Stale(G),
Absent(G),
/// There is an entry, and it has been verified
/// in this revision, but it has no cached
/// value. The `Revision` is the revision where the
/// value last changed (if we were to recompute it).
NoValue(G, Revision),
/// There is an entry which has been verified,
/// and it has the following value-- or, we blocked
/// on another thread, and that resulted in a cycle.
UpToDate(Result<V, CycleError>),
}
impl<Q, MP> Slot<Q, MP>
@ -138,7 +155,9 @@ where
loop {
match self.probe(db, self.state.read(), runtime, revision_now) {
ProbeState::UpToDate(v) => return v,
ProbeState::Stale(_) | ProbeState::Absent(_) => break,
ProbeState::Stale(..) | ProbeState::NoValue(..) | ProbeState::NotComputed(..) => {
break
}
ProbeState::Retry => continue,
}
}
@ -165,7 +184,9 @@ where
let old_memo = loop {
match self.probe(db, self.state.upgradable_read(), runtime, revision_now) {
ProbeState::UpToDate(v) => return v,
ProbeState::Stale(state) | ProbeState::Absent(state) => {
ProbeState::Stale(state)
| ProbeState::NotComputed(state)
| ProbeState::NoValue(state, _) => {
type RwLockUpgradableReadGuard<'a, T> =
lock_api::RwLockUpgradableReadGuard<'a, RawRwLock, T>;
@ -318,7 +339,7 @@ where
StateGuard: Deref<Target = QueryState<Q>>,
{
match &*state {
QueryState::NotComputed => ProbeState::Absent(state),
QueryState::NotComputed => ProbeState::NotComputed(state),
QueryState::InProgress { id, anyone_waiting } => {
let other_id = *id;
@ -376,7 +397,8 @@ where
ProbeState::UpToDate(Ok(value))
} else {
ProbeState::Absent(state)
let changed_at = memo.revisions.changed_at;
ProbeState::NoValue(state, changed_at)
}
}
}