feat(dedup): add a bounded de-duplicating channel - #263
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Important Approval pendingCodeRabbit has no unresolved comments, but it could not review the latest commit because the review limit was reached. Follow the review guidance in this comment to continue. 📝 WalkthroughWalkthroughAdds the generic ChangesDeduplicating channel
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟡 Moderate · up to A sender may successfully add work after the channel is closed, allowing consumers to observe completion and miss that work. This creates a bounded correctness risk in the new channel primitive, so the close-and-enqueue transition should be synchronized before merging. Sequence Diagram(s)sequenceDiagram
participant Caller
participant Chan
participant PendingKeys
participant Slots
participant Items
Caller->>Chan: Send(ctx, value)
Chan->>PendingKeys: Check and reserve key
Chan->>Slots: Acquire capacity
Chan->>Items: Queue entry
Caller->>Chan: Recv(ctx)
Chan->>Items: Receive entry
Chan->>PendingKeys: Release key
Chan->>Slots: Return capacity
Poem
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches 💡 1🛠️ Fix failing CI checks 💡
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Inline comments:
In `@sync/dedup/dedup.go`:
- Line 186: The enqueue path around c.items must be serialized with Close so no
value is sent after closure; recheck c.closed while holding the same
synchronization used by Close immediately before enqueueing, and if closed,
release the acquired slot and return ErrClosed. Add a test covering Close
occurring after slot acquisition but before enqueue, verifying the item is not
delivered and the slot is restored.
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ℹ️ Review info
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📒 Files selected for processing (2)
sync/dedup/dedup.gosync/dedup/dedup_test.go
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Go channels cannot de-duplicate: there is no hook on send and no way to inspect a buffer. Chan wraps one with a pending-key map so a value whose key is already queued is dropped instead of enqueued twice, coalescing repeated notifications about the same entity into a single unit of work. A sender acquires a buffer slot before reserving its key, so a send that is cancelled while the channel is full cannot drop a duplicate and leave nothing queued in its place.
govulncheck flags GO-2026-6303 in golang.org/x/crypto v0.52.0, reached from sftp.Server.handleConnection via ssh.NewServerConn.
A sender that had already acquired a slot could enqueue after Close, by which point a receiver may have observed closure over an empty buffer and given up, leaving the value unreachable. Close now takes the same mutex as the enqueue path, so an enqueue either lands before closure is observable or returns its slot and reports ErrClosed.
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🟡 Changes recommended
Close() + drain semantics are not race-safe unless enqueue can return ErrClosed and Send/TrySend propagate that error.
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Pull request overview
Adds a new sync/dedup package that implements a bounded, key-based de-duplicating queue with blocking/non-blocking send/receive APIs, intended for coalescing repeated notifications while preserving backpressure.
Changes:
- Introduce
dedup.Chan[K, T]withSend/TrySend,Recv/TryRecv,Len, andClose. - Add comprehensive concurrency/cancellation/close-drain tests for the new channel behavior.
- Bump several
golang.org/x/*dependencies ingo.mod/go.sum.
File summaries
| File | Description |
|---|---|
| sync/dedup/dedup.go | Implements the bounded de-duplicating channel and its public API. |
| sync/dedup/dedup_test.go | Adds unit/concurrency tests covering dedup, capacity, cancellation, and close/drain behavior. |
| go.mod | Updates golang.org/x/* dependency versions. |
| go.sum | Updates checksums to match the module version bumps. |
Review details
Suppressed comments (2)
sync/dedup/dedup.go:124
TrySendcurrently discards any error fromenqueueby forcingnil(viareturn c.enqueue(k, v), nil). Ifenqueueneeds to fail withErrClosedto keepClose+ drain semantics correct under races,TrySendshould propagate the error instead of always returning nil.
select {
case <-c.slots:
default:
return false, ErrFull
}
return c.enqueue(k, v), nil
}
sync/dedup/dedup.go:188
enqueuecan still push intoc.itemsafterClose()has been called (race: sender grabs a slot, thenClosehappens, thenenqueueruns). That breaks the stated behavior thatRecvwill "drain what is left" after close, because a receiver can observec.closed+ empty queue and returnok=falsewhile a lateenqueueis still about to add an item (leaking a slot and keeping a key pending until another recv). Makeenqueuecheckc.closedunder the mutex and returnErrClosed(returning the slot) before inserting/sending.
// enqueue is called holding a slot, so neither the send to items nor the
// return of the slot can block.
func (c *Chan[K, T]) enqueue(k K, v T) bool {
c.mu.Lock()
defer c.mu.Unlock()
if _, ok := c.pending[k]; ok {
c.slots <- struct{}{}
return false
}
c.pending[k] = struct{}{}
c.items <- entry[K, T]{key: k, val: v}
return true
}
- Files reviewed: 3/4 changed files
- Comments generated: 1
- Review effort level: Lite
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Review feedback addressedApplied 1 fix across 2 files from 1 review finding, and rebased onto Enqueue after Files changed:
Commit: Also on the branch: Validation: Deferred: none. |
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✅ Action performedComments resolved and changes approved. |
Motivation
Go channels cannot de-duplicate: there is no hook on send and no way to inspect a buffer. The single-key case is already idiomatic (
chan struct{}of cap 1 plus a non-blocking send); this is the multi-key generalization, for coalescing repeated notifications about the same entity into a single unit of work.sync/dedup.Chan[K, T]is a bounded channel holding at most one queued value per key. A key is reserved from enqueue until the value is received, so a duplicate is dropped only while its predecessor is still waiting to be consumed.Design
slotschannel pre-filled withsizetokens, theitemsbuffer, and the pending-key map. A sender acquires a slot before touching the map, then double-checks the map under the mutex. That ordering matters: reserving the key first and then blocking on a full buffer lets a cancelled send drop a duplicate and enqueue nothing, silently losing an entry. Since a sender always holds a token, the buffer send and the slot return can never block.itemsis never closed.Closecloses a separate channel (idempotent viasync.Once), so pending sends unblock withErrClosedandRecvdrains what is left before reporting false.Send/TrySend(false, nil= duplicate, plusErrFullandErrClosed),Recv/TryRecv,Len,Close. Entries carry their key through the buffer soreleasedeletes the key that was inserted rather than recomputing it from a possibly mutated value.Validation
make test(lint +go test -race -tags kqueue ./...) green.go test -race -count=20 ./sync/dedup/green.TrySendfull, context cancellation on both sides (including that a cancelled send reserves nothing), close/drain/double-close, 32 goroutines racing on one key (exactly one wins, no slot leaked), and an 8-producer/4-consumer hammer asserting nothing is lost and all keys and slots are returned.Performance impact
New package, no existing code paths touched. Per send: one channel receive, one mutex-guarded map lookup, one buffered send.
Documentation
No new environment variables or configuration flags, so
README.mdis unchanged; the package carries GoDoc.🤖 Generated with Claude Code
Summary by CodeRabbit
New Features
Tests