watch cache: Deflake TestCacheLaggingWatcher

[zhijun42]

Close issue #20852

I was able to reproduce the issue with Docker using the same resource constraint as in the CI Kubernetes specs.

docker run --rm -v "$(pwd)":/etcd -w /etcd \
  --cpus=4 --memory=4g etcd-linux-amd64:latest \
  bash -c "go test -covermode=set -run TestCacheLaggingWatcher/pipeline_overflow ./tests/integration/... -count=500"

Statistics (500 runs each)

Configuration	Failures	Rate
4 CPU, 4GB memory, with coverage	25	5.0%
2 CPU, 4GB memory, with coverage	28	5.6%
1 CPU, 4GB memory, with coverage	29	5.8%
4 CPU, 2GB memory, with coverage	15	3.0%
4 CPU, 1GB memory, with coverage	39	7.8%

Looks like memory constraint has the biggest impact here.

There are actually two types of errors (example CI failure), and they have different root causes, although they both come from the same test case.

Error Type 1: gotEvents=X, wantEvents<1

What the test expects:

• 12 events generated with buffer=0 (unbuffered channel) and window=10
• Watcher should overflow immediately and receive at most 1 event
• Watcher should eventually be closed (compacted)

What races:

1. Goroutine A (Event Generator): generateEvents() writes 12 events to etcd
2. Goroutine B (Store Watcher): Receives events from etcd server and enqueues to storeW.respCh
3. Goroutine C (Demux Broadcaster): broadcastEventsLocked() sends events to active watchers
4. Goroutine D (Resync Loop): resyncLaggingWatchers() runs every 10ms
5. Goroutine E (Client Reader): Test's collectAndAssertAtomicEvents() reads from watch channel

The Race Sequence:

Timeline:
────────────────────────────────────────────────────────────────────────
t0: Events 1-12 arrive at cache
t1: Client watcher receives event 1 (buffer=0, so must be consumed immediately)
t2: Events 2-12 try to enqueue but buffer full → watcher becomes "lagging"
t3: Resync timer fires (every 10ms)
t4: resyncLaggingWatchers() replays events from history to lagging watcher
t5: If client reader is ready at t4, events get delivered successfully
t6: Test counts received events → gotEvents=8 (or more) > wantEvents<1

Why non-deterministic:

• If resync fires BEFORE client reader times out, events get delivered
• If resync fires AFTER client reader times out, no extra events
• Linux/Docker scheduling makes resync fire at unpredictable times

Fixed in tests/integration/cache_test.go

• Added skipCloseCheck flag since close status is non-deterministic with resync
• Skipped the event count because is also non-deterministic
• The test now verifies no errors occur, not specific deterministic behavior
• See the comment there for more details.

Error Type 2: cache: stale event batch (rev 32 < latest 42)

What races:

1. Goroutine A (Store Watcher): Applies events to store via store.Apply()
2. Goroutine B (Progress Notifications): etcd server sends periodic progress notifications
3. Goroutine C (Watch Channel): Watch channel can close and restart

The Race Sequence:

Timeline:
────────────────────────────────────────────────────────────────────────
t0: Store watcher receives events 1-12, applies them → store.latestRev = 12
t1: Progress notification arrives with rev=42 → store.latestRev = 42
t2: Store watcher buffer overflows → watcher becomes lagging
t3: Resync tries to replay events from history (starting at rev 11)
t4: store.Apply() called with events at rev 11-12
t5: Validation fails: rev 11 < latestRev 42 → ERROR!

Another scenario (demux-level):

t0: demux.maxRev = 42 (from events or progress)
t1: Watch channel closes and restarts from older revision
t2: Events arrive at demux with rev < 42
t3: demux.Broadcast() rejects: "stale event batch (rev 32 < latest 42)"

Fixed in cache/store.go and cache/demux.go

• Filter stale events instead of returning errors

Results

Both error types are fully eliminated. Would like to verify this on CI as well.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @zhijun42. Thanks for your PR.

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