fix(engine): retract a loop's or worker's residual gauges where it parks (celeris#711) by FumingPower3925 · Pull Request #766 · goceleris/celeris

FumingPower3925 · 2026-09-27T20:50:02Z

Summary

A draining epoll loop or io_uring worker can lose its last connection after sweep(), in the same iteration that then takes the DRAINING→SUSPENDED park. The park is indefinite, and sweep(), whose empty-set retraction is the only thing that clears a loop's share of the TransplantResidual* gauges while it runs, does not run again until the loop wakes. So the residue the loop published last stood in the engine-wide gauge with nothing behind it (TransplantResidualBusy = 1, ActiveConnections = 0, every loop parked, sweep passes frozen) until the next ResumeAccept or shutdown. StopTransplant did not clear it either. Nightly 36247560882 carried it for the last 70 s of an adaptive cell, and it is the signal probatorium#416 plans to gate on.

This is telemetry only: the connection really is closed, and nothing inside celeris reads the gauges.

Fixes #711

Failing-first on main

The failing-first head is 770aaeb: origin/main 698bed6 plus the four tests, no engine change. Docker linux/arm64 (kernel 7.0.12-linuxkit), 4 CPUs, seccomp unconfined, go test -count=10 -v, in two shapes: m8 is CI's (8 MiB memlock, one io_uring worker) and unl has unlimited memlock (two workers).

test	what places the last close	m8	unl
`TestParkedLoopRetractsResidueOfAPostSweepClose` (epoll)	the header deadline at the tick-gate `checkTimeouts`, after `sweep()` (the loops' timerfds are disarmed, so no pre-sweep `checkTimeouts` is left)	FAIL 10/10	FAIL 10/10
`TestParkedLoopRetractsResidueOfAPreSweepClose` (epoll, control)	the same connection, closed by its client before the deadline: an event, before `sweep()`	PASS 10/10	PASS 10/10
`TestParkedWorkerRetractsResidueOfAPostSweepClose` (io_uring)	`ReadTimeout` 300 ms in `checkTimeouts`, which on io_uring always runs after `sweep()`; no manipulation at all	FAIL 10/10	FAIL 10/10
`TestParkedWorkerRetractsResidueOfAPreSweepClose` (io_uring, control)	the header timer's CQE, dispatched before `sweep()`	PASS 10/10	PASS 10/10

Every failure reads busy_parked=1 busy_hold=1 active=0 all_parked=true closes=1 disconnects=1 (io_uring: res_parked=1 res_hold=1). Every control reads 0 with the same close and the same park.

Each test drives the real run() of an unmodified engine: one slowloris connection (GET /slow HTTP/1.1\r\nHost: x\r\n, no blank line), StartTransplant, then PauseAccept.
One change from the issue's reproduction: since celeris#662, PauseAccept lingers 1.5 s behind TCP_DEFER_ACCEPT, so on main the 400 ms deadline fired while the listener was still open and the close never landed in a parking iteration. The tests set DisableDeferAccept, which closes the listeners at once, and they check that the connection is still open when PauseAccept returns.
The logs are logs/ff-{m8,unl}/711-ff.log in evidence/lanes-20260927/EP-2/, from ff.sh.

The fix

Both park branches now retract before they take wakeMu, when the live set is empty. This is the issue's M1:

if len(l.liveConns) == 0 {
	l.sweepRetract()
}

A parked loop holds nothing, and this is where the sweep stops, so the retraction covers every departure after sweep() in the parking iteration: the tick-gate checkTimeouts, the detach queue, the dirty flush, the H2 queue on epoll, and every Read/Idle/Write timeout close on io_uring.
sweepRetract already existed for shutdown. It returns at once when nothing is published. It runs on the loop thread, where every other gauge write happens. It takes no lock and adds none.
On epoll, connCount == 0 already implies an empty live set: the two change together on the loop thread (an off-thread hijack keeps both until drainDetachQueue). The len(liveConns) == 0 test says what the retraction relies on.
If the re-check under wakeMu then declines the park, the loop has retracted nothing it holds. Its next sweep() publishes anything a new arrival brings.
The two sweep.go comments that called the retraction "always reached" are corrected, and sweepRetract's doc names both callers.
The four tests join the two named-test interlocks in ci.yml: the io_uring step that forbids skipping and requires workers=1, and the epoll sweep step that requires loops=2. The epoll package otherwise runs only in the root step, without -v.

Controls

All counts are anchored --- PASS/FAIL lines of go test -v; there was no SKIP line.

Round 1 (logs/verify-m8/) ran at the fix commit 80b6e40.
Round 2 ran at the head dabc1a6, whose only change is the test engines' ENOMEM-retried start: m8 in logs/verify2-m8/ (io_uring; the epoll files did not change), unl in logs/unl-all/.

run	post-sweep tests: epoll / io_uring	pre-sweep controls: epoll / io_uring
fixed	m8: PASS 10/10 / PASS 10/10 (r1), io_uring PASS 10/10 (r2). unl: PASS 10/10 / PASS 10/10 (r2)	PASS 10/10 / PASS 10/10 in every run
negative control: the fix's `loop.go` and `worker.go` cp'd back from main	m8: FAIL 10/10 / FAIL 10/10 (r1), io_uring FAIL 10/10 (r2). unl: FAIL 10/10 / FAIL 10/10 (r2)	PASS 10/10 / PASS 10/10
second control `MWAKE`: the retraction moved to after the wake (a gauge that heals at the next wake, as main's heals at `ResumeAccept`)	m8: FAIL 5/5 / FAIL 5/5 (r1), io_uring FAIL 5/5 (r2). unl: FAIL 5/5 / FAIL 5/5 (r2)	PASS 5/5 / PASS 5/5

On the fixed head the result lines read busy_parked=0 busy_hold=0 (epoll) and res_parked=0 res_hold=0 (io_uring). On the negative control they read =1.
MWAKE shows that the tests check the gauge while the loop is parked, not that it heals eventually.
The mutant sources are in 711/mkmutants.sh and 711/mutants/.

Suites

go test -race -count=1 -v, one package per go test, in Docker linux/arm64. Main (698bed6) ran in the same shapes.

package	shape	main	this branch
`engine/epoll`	m8	PASS 152, FAIL 0, SKIP 3	PASS 154 (+2 new), FAIL 0, SKIP 3 (r1; the epoll files did not change in r2)
`engine/epoll`	unl	PASS 152, FAIL 0, SKIP 3	PASS 154, FAIL 0, SKIP 3 (r2)
`engine/iouring`	m8	PASS 318, FAIL 0, SKIP 5	PASS 320 (+2 new), FAIL 0, SKIP 5 (r1 and r2)
`engine/iouring`	unl	PASS 321, FAIL 0, SKIP 2	PASS 323 (+2 new), FAIL 0, SKIP 2 (r2)

The SKIPs have the same names on main and here, and CI's steps enforce each of them elsewhere:
- epoll: TestWriteBufBackpressureClosesSlowConsumer and the two tcp_synack_retries=0 tests.
- io_uring: the two synack tests, plus, at m8 only, the three celeris#656 init-failure tests, which need two workers.
No data race in any run.
With all three lane branches together (review round 2, 2026-09-28): this head dabc1a6, test(iouring): regression arms for #712 (fixed by #793) #767 at d9307b5 and fix(iouring): read a worker's clock on every CQE batch, as epoll does, so a live connection is not timed out on a stale stamp (celeris#713) #768 at 9fb68fe, merged onto current main dfd044f (tree 421e683, the same in all six merge orders: rv2/merge_check.sh). On that tree, with -race:
- engine/iouring: m8 PASS 357, FAIL 0, SKIP 5; unl PASS 360, FAIL 0, SKIP 2. Main dfd044f gives 339/0/5 and 342/0/2, so the difference is the three PRs' 18 tests (logs/rv2-m8c/, logs/rv2-unl3/).
- engine/epoll (m8): PASS 156, FAIL 0, SKIP 3, including this PR's two epoll tests.
- The three PRs' io_uring named tests at m8, count 2: PASS 36/36.
- ./adaptive/... at unl: PASS 115, FAIL 0, SKIP 0, the same as main dfd044f (PASS 115).
- The earlier merge with main 0cf0c52 gave the same (logs/rv2-m8b/, logs/rv2-unl/), except one unl FAIL in fix(iouring): read a worker's clock on every CQE batch, as epoll does, so a live connection is not timed out on a stale stamp (celeris#713) #768's old draining-adoption arm: its premise failed (its worker parked). fix(iouring): read a worker's clock on every CQE batch, as epoll does, so a live connection is not timed out on a stale stamp (celeris#713) #768 has since replaced that rig.
amd64 (logs/amd64/):
- Host cross-builds: GOOS=linux GOARCH=amd64 go build ./..., go vet and go test -c of both engine packages are clean.
- Under Docker's linux/amd64 emulation, the epoll pair PASSES 2/2 on the head, and the post-sweep test FAILS 2/2 on 770aaeb.
- The io_uring pair SKIPs there, because io_uring is not available under emulation. A SKIP is absent, not a pass: the arm64 runs above are the io_uring evidence.
Static checks (tools/lint.sh): go build ./..., go vet ./engine/... ./adaptive/... ., go test -c of both engine packages, and golangci-lint on ./engine/... (the repo's .golangci.yml), for linux amd64 and arm64: all rc=0. actionlint on ci.yml is clean.
CI. Round 1 at 80b6e40 was 17/17 green (CI run 36349485187). At dabc1a6, CI run 36351042153 is green on attempt 2.
- Attempt 1 failed one test outside this change: TestDriverRetireClosesOutsideItsLock ("apparatus: the close did not linger"). That is a unit test of driverConn.retire with a lingering TCP socket. It involves no engine and no park, and no file it covers is changed here.
- The same apparatus line failed on another branch the same day: the Coverage job of the io_uring: UnregisterConn then Close leaks the driver socket — the fd-keyed ASYNC_CANCEL misses once the caller has closed the fd, so onClose never fires and the peer never sees EOF #691 branch, run 36328082525, job 108644612710. fix(iouring): close a driver conn's engine descriptor off the worker, and fire onClose after the close (celeris#735) #744 rewrites that test.
- Locally it passes 300/300 on main and on this branch (logs/unl-all/flake-{main,711}.log).
- The Unit job's interlock lines read: celeris#657 epoll sweep tests: want 19, ran 19, passed 19, SKIP lines 0, and witness and fd-lifetime tests: want 42, ran 42, passed 42, SKIP lines 0, engines 11 at workers=1: 11.

Cost

Not on the request path. The retraction runs once per park, and parks happen only on a paused engine that has run out of connections.

Relation to #712 and #713

This branch and #767 touch the same park block in worker.go, in different lines: #767's change sits before the A5 submit, and this one after it. After review round 2, #768 no longer touches the park block. All three heads merge cleanly onto current main dfd044f, pairwise and in all six orders, into the same tree (rv2/merge_check.sh, logs/rv2-merge.txt).

Evidence

evidence/lanes-20260927/EP-2/ (README.md, RESULTS.md from tools/results.py, ff.sh, verify.sh, verify2.sh, queue4.sh, jobs/, logs/, 711/mkmutants.sh, 711/mutants/).

…h no residue (celeris#711) Failing-first: a draining epoll loop whose last connection closes at the tick-gate checkTimeouts (after sweep()), and a draining io_uring worker whose last connection closes on ReadTimeout, park with their last published residue still in the engine-wide gauges. The pre-sweep closes (an event on epoll, the header-timer CQE on io_uring) are the controls.

…rks (celeris#711) A draining epoll loop or io_uring worker whose last connection leaves after sweep(), in the iteration that then takes the DRAINING->SUSPENDED park, never runs sweep() again until it wakes, so sweep()'s empty-set retraction never comes and TransplantResidual* keeps the residue it published last, with nothing behind it, until the next ResumeAccept. On epoll that is any tick-gate checkTimeouts close (or a detach-queue, dirty-flush or H2-queue close); on io_uring, whose only checkTimeouts runs after sweep(), every Read, Idle or Write timeout close of the worker's last connection. The park now retracts itself when the live set is empty, as shutdown already does. Both sweep.go comments that called the retraction always reached are corrected, and the four new tests join the CI interlocks.

coderabbitai · 2026-09-27T20:50:10Z

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📝 Walkthrough

Walkthrough

When an epoll loop or io_uring worker parks with no live connections, it retracts its sweep contribution. Linux regression tests cover post-sweep and pre-sweep closes in both engines. CI test inventories include the new tests.

Changes

Park-time residual gauge retraction

Layer / File(s)	Summary
Retract gauges before parking `engine/epoll/loop.go`, `engine/epoll/sweep.go`, `engine/iouring/worker.go`, `engine/iouring/sweep.go`	Both engines call `sweepRetract` before parking when no live connections remain. Comments document retraction during parking and shutdown.
Regression tests and CI registration `engine/epoll/park_retracts_residue_test.go`, `engine/iouring/park_retracts_residue_test.go`, `.github/workflows/ci.yml`	Tests cover post-sweep closes and pre-sweep controls. CI inventories and named test selections include the four tests.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Low

Merge Risk: ⚪ Minimal · up to 38848

The change removes stale residual-gauge values when empty loops or workers park, without changing connection handling. Regression tests cover both close timings in both engines, and no actionable merge risk remains.

Architecture Summary

Architecture risk: 🔵 Low · up to 38848

The change affects 1 system.

Changed systems: engine

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

observed — engine (service) was modified; 6 changed files map to changed impact.

Before / after behavior

observed — Modified behavior in engine/epoll/loop.go: Before suspending, the loop now retracts its sweep contribution when liveConns is empty. Previously, this suspension path had no retraction, so residual gauges could retain the loop’s last published values until it ran again.
observed — Modified behavior in engine/epoll/park_retracts_residue_test.go: Adds a Linux-only test fixture with an async route handler that writes a simple response and satisfies stream.AsyncRouteResolver.
observed — Modified behavior in engine/epoll/park_retracts_residue_test.go: Adds a transplant target that counts and closes adopted file descriptors, plus a polling helper that waits for a condition until a deadline.
observed — Modified behavior in engine/epoll/park_retracts_residue_test.go: Adds the shared scenario for pre-sweep and post-sweep connection closure: it starts a two-worker HTTP/1 engine, disarms loop timerfds, sends an incomplete request, waits for Busy residue, and pauses accepts. It then closes the client early for the pre-sweep case or waits for the header deadline for the post-sweep case. After the loops park, it checks closure and disconnect counts and reports an error if Busy residue is nonzero with no active connections.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title uses the required conventional-commit format, accurately describes the residual-gauge retraction change, and ends with the issue reference `(celeris#711)`.
Description check	✅ Passed	The description directly explains the stale-gauge defect, the park-time fix, regression tests, controls, and validation results.
Linked Issues check	✅ Passed	`#711` requires park-time retraction in epoll loops and io_uring workers, corrected sweep comments, and post-sweep plus pre-sweep regression tests. `engine/epoll/loop.go` and `engine/iouring/worker.go` …
Out of Scope Changes check	✅ Passed	The changed code and tests address `#711`. The CI inventory updates register the four regression tests. No unrelated functional change is established by the supplied whole-PR summary.

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codecov · 2026-09-27T20:55:01Z

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…ried At CI's 8 MiB memlock the kernel gives a closed ring's pages back 12-23 ms after the close, and startFDLEngine does not wait that out: an engine started right after another ring closed failed with ENOMEM (measured on the celeris#713 branch, 3 of 10 runs per shape right after a fixture ring), and its cleanup then waited 5 s on a Listen result it had already consumed. The tests now start through startRingRetried662, as the other back-to-back engine tests do.

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Inline comments:
Review comments at @engine/epoll/park_retracts_residue_test.go:
- Around line 174-179: Give the pre-sweep test arm a separate, generous
ReadHeaderTimeout so the partial-request and Busy-publication waits cannot
consume its deadline; keep the short timeout for the post-sweep arm. In the
preSweep path, close the client immediately after PauseAccept returns instead of
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.github/workflows/ci.yml
engine/epoll/loop.go
engine/epoll/park_retracts_residue_test.go
engine/epoll/sweep.go
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engine/iouring/sweep.go
engine/iouring/worker.go

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FumingPower3925 added 2 commits September 27, 2026 22:12

FumingPower3925 added this to the v1.6.0 milestone Sep 27, 2026

FumingPower3925 added bug Something isn't working area/engine Engine interface or implementation engine/epoll Epoll engine specifics engine/iouring io_uring engine specifics labels Sep 27, 2026

FumingPower3925 marked this pull request as ready for review September 28, 2026 09:37

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Comment thread engine/epoll/park_retracts_residue_test.go

Merge branch 'main' into fix/celeris-711-park-retract-residual

38848d8

FumingPower3925 merged commit dbbaaee into main Sep 28, 2026
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FumingPower3925 deleted the fix/celeris-711-park-retract-residual branch September 28, 2026 09:56

FumingPower3925 mentioned this pull request Sep 29, 2026

fix(epoll, iouring, std): graceful shutdown waits for the HTTP/2 streams on the shared worker pool, and std for its h2c streams (celeris#759) #808

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fix(engine): retract a loop's or worker's residual gauges where it parks (celeris#711) - #766

fix(engine): retract a loop's or worker's residual gauges where it parks (celeris#711)#766
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Summary

Failing-first on main

The fix

Controls

Suites

Cost

Relation to #712 and #713

Evidence

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