fix(server): shut down on every context cancel, and return only after it (celeris#673)

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Summary

StartWithContext / StartWithListenerAndContext could skip Server.Shutdown on a context cancel. The #673 "flake" was this product bug, not a timing problem in the test.

Fixes #673

Mechanism

The watcher goroutine chose between ctx.Done() and listenDone in one select. When it got listenDone, it returned without shutting down.

• The context handed to Listen is derived from ctx, so a cancel also makes Listen return and close listenDone.
• If the watcher had not reached its select by then, both cases were ready and select picked one at random. Half the time Shutdown never ran:
  • no OnShutdown hooks
  • the CPU monitor's descriptor stayed open
  • the settle re-opener (adaptive dispatch never re-times a settled route: a store-backed handler that turns slow runs inline on the engine worker forever (#493 item 4, measured) #592) kept running for the life of the process

That leaked re-opener is exactly what the CI failure reported (run 35066319491 attempt 1: 1 settle-re-opener goroutine(s) alive after shutdown, want 0).

When the watcher did shut down, it did so on its own goroutine after Start had returned. A main that exits as soon as Start returns could lose its hooks.

Changes

• Both entry points now share listenUntilCancelled.
  • Its watcher decides from state (ctx.Err()), not from which case select took.
  • Start*Context waits for the watcher before it returns. This is documented on both methods.
• A Shutdown call counter stops the watcher from repeating a Shutdown that the caller already made during the run. Without it, the usual srv.Shutdown(x); cancel() would run every hook twice.
  • Before this PR, that double run already happened whenever the parked watcher woke on ctx.Done() while Listen was still tearing down, which is the native engines' case.
• OnShutdown's doc now says that the hooks run before a cancelled Start*Context returns, so a hook must not wait for that call to return. StartWithContext and StartWithListenerAndContext point to it. See "Behaviour change" below.
• No hot-path change: this is the start/stop path only.

Why the new join cannot deadlock on its own

Start*Context now blocks on <-watcherDone after Listen has returned. A deadlock needs the watcher's Server.Shutdown to wait on something that happens only after Start*Context returns.

At that point the Start*Context goroutine holds nothing:

• doPrepare's startOnce finished before Listen began.
• listenUntilCancelled holds no mutex.
• Its only deferred call is the cancelListen of listenCtx. The watcher does not need it: the watcher shuts down only when ctx is cancelled, and ctx is listenCtx's parent, so listenCtx is already cancelled.

Each step of the watcher's Server.Shutdown(shutCtx), with shutCtx bounded by Config.ShutdownTimeout:

• router.stopSettleReopener: closes a channel under reopenMu, and waits for nothing.
• Engine.Shutdown:
  • std: once.Do(http.Server.Shutdown(shutCtx)). If Listen's own ctx.Done branch ran that once.Do, it finished before Listen returned. Otherwise the watcher runs it, and it is bounded by shutCtx.
  • epoll: returns nil.
  • io_uring: takes e.mu. Listen holds e.mu only around publishing its workers (engine.go), never across its run, so e.mu is free.
  • adaptive: waits for its own Listen's deferred close(done) (already closed when Listen returned) or shutCtx, then shuts down its sub-engines as above.
• cancelListen and closeCPUMonitor: short critical sections on lifecycleMu and cpuMonMu, and they wait for nothing.
• The OnShutdown hooks: user code. This is the one step that can wait on Start*Context's return (next section).

Behaviour change

Before this PR, Start*Context returned without waiting for the cancel-triggered Shutdown. So an OnShutdown hook that waited for Start*Context to return worked: Start returned first.

Now that hook and Start*Context wait on each other:

• A hook that returns when its ctx is done ends the wait after Config.ShutdownTimeout.
• A hook that ignores ctx never ends it.

Measured with hook-waits-on-start.sh (std, darwin; ShutdownTimeout 300 ms; the ctx-ignoring hook is released by the test after 3 s so the run can finish):

Tree	Hook honours ctx	Hook ignores ctx
9f4d89b	Start returned at once, and the hook saw it return	Start returned at once, and the hook saw it return
this PR (5a05c2e)	Start returned after 300 ms, when the hook gave up on its ctx	Start returned after 3 s, when the test released the hook

No OnShutdown hook in this repository waits for Start. Besides the definition and two test-function names, git grep -n 'OnShutdown(' finds 8 call sites, all in tests: server_test.go:1413, :1435-1437, :1454, :1463, and the two new tests, start_context_shutdown_test.go:94 and :161. That the grep finds those two new call sites is the positive control. The hooks set a flag, append to a slice, close a channel, count, or do nothing. middleware/session's Close doc suggests calling it from a hook: it waits for the store's write-behind worker, not for Start. Evidence: onshutdown-hooks.txt.

Test Plan

All tests are in-process on the std engine, with no Docker. Evidence and scripts: evidence/celeris-673-679-653-424/lane-20260926/673/ (per-finding index for round 2: ROUND2.md).

• TestStartWithContextShutsDownWhenListenReturnsFirst forces the order. The context is cancelled before Start and GOMAXPROCS is 1, so Listen returns and listenDone closes before the watcher looks at either channel.

  • On 9f4d89b: FAIL, with the test text as pushed (base-9f4d89b-pushedtext.log, from base-pushedtext.sh). The run uses the same command line as the first base run: run-forced.sh, go test -v -race -count=1 -run TestStartWithContextShutsDownWhenListenReturnsFirst ..
    • The pushed file (sha256 f7ccbd7b…) is copied onto a detached 9f4d89b worktree with one test left out, TestStartContextWatcherDoesNotRepeatADirectShutdown, and its teardownEngine type. That test calls listenUntilCancelled, which only this PR adds, so the file does not compile on 9f4d89b with it in. Its two now-unused import lines are blanked, so every line of the forced-order test keeps its pushed line number. The log's messages are at start_context_shutdown_test.go:124/:127/:131, as in the pushed file.
    • Tally: 0 PASS, 3 FAIL (the test and both subtests), 0 SKIP.
    • StartWithContext: Shutdown never ran in 9/16 iterations, ran only after Start returned in 6, and ran before Start returned in 1.
    • StartWithListenerAndContext: never ran in 4/16, ran late in 12, and ran before Start returned in none.
    • The first base run (base-9f4d89b.log: 8/16 and 10/16 never) did not save the text it ran, and its messages sit 3 lines higher (:121/:124/:128). Deleting the two unused import lines, instead of blanking them, gives exactly those line numbers (base-9f4d89b-pushedtext.imports-deleted.log). So that run may have used this same text. The run above removes the doubt either way.
  • Fixed: -race -count=20 PASS, 16/16 shut down before Start returned in every run.

• TestStartContextWatcherDoesNotRepeatADirectShutdown is the control for the new guard. It uses an engine whose Listen outlives its cancel, as the native engines' teardown does.

• Mutants, each run -race -count=3 and killed 3/3:

  Mutant	Test that kills it	Failure
  M1: Start does not wait for the watcher	forced-order test	16/16 late
  M2: the watcher returns on listenDone (the old decision)	forced-order test	3-10/16 never shut down (per subtest and run: 7, 6 / 3, 7 / 10, 9; recounted by recount-M2.sh → recount-M2.txt)
  M3: the repeat guard is removed	guard test	the hook runs 2 times

• Root package on darwin at 5a05c2e, go test -v -race -count=1 .: ok. 432 PASS, 0 FAIL, 1 SKIP (TestRouteAdaptive_SettleReopenCost, an opt-in cost measurement). go vet passes natively and for linux/amd64 and linux/arm64 (round2-root-pkg-darwin-race-v.log).

• CI run 36254685292 on 5a05c2e: 9/9 jobs green. The Linux root -race step in its Unit job reports ok github.com/goceleris/celeris. That covers both new tests and TestRouteAdaptive_NoReopenerWhenEngineCreationFails (ci-36254685292-unit.log). On 556087f, CI run 36241169403 was also 9/9 green.

Tested on: [x] std [ ] epoll [ ] io_uring — [ ] amd64 [x] arm64 (darwin, native)

Release notes

• Breaking change? Behaviour, not API. Start*Context now returns only after the cancel-triggered Shutdown has finished, OnShutdown hooks included. An OnShutdown hook that waits for Start*Context to return used to work and now hangs until it gives up on its ctx (Config.ShutdownTimeout), or forever if it ignores ctx. See "Behaviour change".
• Labeled for release notes (bug)

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📝 Summary

Summary by CodeRabbit

• Bug Fixes

  • Starting the server with a context now reliably completes shutdown hooks before returning when the context is canceled. Shutdown uses the configured timeout, defaulting to 30 seconds.
  • Prevented an extra shutdown when the server has already been shut down directly or listening ends while the context remains active.
  • Adaptive-route re-openers now stop during context-triggered shutdown.

• Documentation

  • Clarified that start methods wait for shutdown hooks and that shutdown hooks should not wait for the start call to return.

Walkthrough

Both context-based start methods use shared shutdown handling. Cancellation-triggered shutdown uses the configured timeout or a 30-second default. Each start call waits for shutdown handling to finish.

Changes

Context-driven shutdown

Layer / File(s)	Summary
Shared context shutdown lifecycle server.go	Server tracks direct and watcher-triggered shutdown. Both context-based start methods use shared lifecycle handling and wait for the watcher. API comments describe hook completion and warn hooks not to wait for the start call to return.
Shutdown ordering and duplicate-call tests start_context_shutdown_test.go	Tests cover both context-based start methods, shutdown-hook completion, adaptive-route reopener state, and avoiding repeated shutdown after direct or watcher-triggered calls.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 3c6c8

The duplicate-shutdown race is addressed, and no remaining issue identified here prevents merging after normal checks.

Security Architecture Review

Security architecture risk: 🟡 Moderate · up to 3c6c8

A direct shutdown can still let a context-based start return before its cleanup hooks finish. A shutdown racing startup can also leave server resources running. No new remotely callable shutdown path is established, but these gaps matter to applications relying on clean termination.

Retained concerns

• Medium · reliability · inferred: When direct Shutdown wins ownership, the context-based start call can return before that shutdown finishes its hooks, contrary to the new completion contract.
• Medium · reliability · inferred: A direct shutdown racing preparation can finish before resources are published; its new ownership flag then prevents cancellation from cleaning up resources created afterward.

Security review details

Security Blast Radius

• inferred — The demonstrated failure scope is a server instance's shutdown hooks and owned resources. The reviewed change does not establish a new network, tenant, credential, or identity transition.

Trust Boundaries and Controls

• observed — The repository production caller derives its lifecycle context from process signals. Attacker control of contexts or hooks supplied by external applications is not established by the available source.

Resilience and Maintainability Implications

• inferred — The new join makes Start wait for watcher-owned hooks. The shutdown timeout supplies a context to hooks but cannot itself make a hook that ignores that context return; the API documentation warns about this dependency.

Hardening Proposals

• proposed — Represent direct and watcher shutdown with one completion state, and coordinate that state with resource publication, so every cancellation path can join the actual cleanup owner.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title uses the required Conventional Commit format, describes the server shutdown fix, and ends with the issue reference (celeris#673).
Description check	✅ Passed	The description directly explains the context-cancellation bug, lifecycle changes, behavior change, and test results.
Linked Issues check	✅ Passed	The changes address [#673]. listenUntilCancelled checks ctx.Err() after either wake-up, claims watcher shutdown under lifecycleMu, runs s.shutdown, and waits for watcherDone before returning…
Out of Scope Changes check	✅ Passed	The changes stay within [#673]. Shutdown arbitration, start-method documentation, and tests prevent duplicate hooks and verify cleanup caused by the reported lifecycle race. No unrelated change is est…

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Inline comments:
In @server.go:
- Line 889: Replace the shutdownCalls counter check with a shared atomic
shutdown claim used by both the direct caller and the watcher before either
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Codecov Report

❌ Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
server.go	96.42%	1 Missing ⚠️

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[codspeed]

Merging this PR will improve performance by 17.57%

⚡ 6 improved benchmarks
✅ 59 untouched benchmarks
⏩ 5 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	4producers	11 ns	8 ns	+37.5%
⚡	BenchmarkContextQuery	71 ns	61 ns	+16.39%
⚡	BenchmarkContextBlob	489 ns	423 ns	+15.6%
⚡	BenchmarkChainBaseline	2.1 µs	1.8 µs	+14%
⚡	BenchmarkChainPreRoutingOnly	3.9 µs	3.5 µs	+12.76%
⚡	BenchmarkChainPreRouting	4 µs	3.6 µs	+11.07%

Tip

Curious why performance improved? Comment @codspeedbot explain why performance improved on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing fix/673-start-watcher-shutdown (023930d) with main (a842109)²}

Footnotes

1. 5 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (7c123da) during the generation of this report, so a842109 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩