fix(epoll): publish a driver conn before arming its descriptor, so its first edge is never dropped (celeris#770)

[FumingPower3925]

Fixes #770

The defect

Loop.RegisterConn armed the descriptor (EPOLL_CTL_ADD, edge triggered) and only then put the conn in driverConns and set hasDriverConns. The worker looks a driver conn up only while hasDriverConns is set. On a loop with no other driver conn, a worker that took the new conn's first event in between skipped the lookup and handed the event to the HTTP path, which found no connection and read nothing. Edge triggered, the event does not come again: bytes the peer wrote before (or right at) the register never reached onRecv until more arrived. With another driver conn on the loop, hasDriverConns is already set and lookupDriver waits on driverMu (held by RegisterConn) until the conn is in the map, so the window is the first driver conn on a loop, or the first after the others were unregistered.

Failing-first

engine/epoll/driver_register_first_edge_linux_test.go, TestRegisterConnDeliversBytesQueuedBeforeIt: 5000 registrations on worker loop 0, which has no other driver conn (asserted before each), of a conn whose peer has already written one byte; onRecv must run within 200 ms. Another goroutine signals the loop's wakeup eventfd in a loop, so the worker keeps returning from epoll_wait. The window is a few instructions wide, so the test counts rather than forces.

Test-only commit aa2331d (main 698bed6 + the test), 5 runs, each its own go test -race process, Docker linux/arm64, --cpus 4, memlock 8 MiB, kernel 7.0.12:

	registrations whose byte never reached onRecv, per run (of 5000)	runs FAIL
main + test (aa2331d)	10, 13, 17, 19, 26	5/5
head	0, 0, 0, 0, 0	0/5

(The head's runs signal fewer wakeups, 0.29-0.32 M against 2.6-7.7 M, because they finish in about a tenth of a second instead of waiting 200 ms for every lost byte.)

A first version of the test kept the worker awake with HTTP connections opening and closing on the engine instead. It reproduced the loss too (a probe: 3 of 2000), but the race detector then reported closeConn writing l.conns[fd] without driverMu while RegisterConn reads it under driverMu, on every run: a separate defect, filed as #775.

The fix

RegisterConn puts the conn in driverConns and sets hasDriverConns before the EPOLL_CTL_ADD, all under the driverMu it already holds, and takes both back if the EPOLL_CTL_ADD fails. A racing worker then finds hasDriverConns set and waits in lookupDriver for driverMu until the conn is in the map. No lock is added and none is held longer: the same critical section, in a different order. The refusal paths (invalid fd, HTTP collision, already registered) return before anything is published, as before; the epoll_ctl refusal after shutdown (#655) now also removes the entry it published. No test checks that rollback (see Suites and #787).

Controls

Mutants, applied with go test -overlay (the tree is never edited), 3 runs each on the head:

mutant	result
the fix reverted (main's driver.go)	killed 3/3
the conn put in the map before the arm, but hasDriverConns set only after it	killed 3/3

The second is the half-fix: the worker checks hasDriverConns before it looks at the map, so publishing the entry early is not enough.

Suites

./engine/epoll, go test -race -count=1 -v, arm64, at the head: memlock 8 MiB 153 PASS, 0 FAIL, 3 SKIP; unlimited memlock 153 PASS, 0 FAIL, 3 SKIP. The skips are gated by the environment and predate this branch (GOTEST_BACKPRESSURE, and two tests that need net.ipv4.tcp_synack_retries=0). No race report. TestRegisterConnAfterShutdownDoesNotTouchTheClosedEpollFD (#655), which reaches the refused epoll_ctl path this change touches, passes in both. It asserts only the interest set and the returned error, not the rollback: the review found that a mutant without the rollback passes the whole suite. A test for it is #787, item 1.

linux/amd64 (qemu emulation, a compile and a quick run without -race): the new test, TestDriverRegisterUnregister and the #655 test PASS.

Host: GOOS=linux build of the module, go test -c ./engine/epoll/, go vet and golangci-lint (the repo's config), for amd64 and arm64: clean.

CI

CI run 36353281352 on fc40b42: success. The Coverage run 36353281374 failed on its first attempt in ./engine/iouring, which this branch does not touch: TestDriverRetireClosesOutsideItsLock (#696) stopped on its own apparatus check, apparatus: the close did not linger (its lingering-TCP rig did not linger on that runner). The re-run of that job (attempt 2) succeeded. Log: 770/logs/ci-36353281374-coverage.log.

Cost

None measurable by construction: the same two stores and one map insert as before, moved ahead of the epoll_ctl inside the same critical section, once per RegisterConn (a driver conn's setup, not a request).

Follow-ups

The review's minor findings and nits are in #787: a test for the rollback, a failing-first run of the test on amd64 (every one so far was on arm64), a 1 s bound instead of 200 ms, and the Coverage flake above (recorded on #744).

Evidence

Every number above comes from a script under the maintainer's evidence tree, evidence/lanes-20260927/EPOLL-HIJACK/770/ and the lane's queue scripts (queue10.sh), with logs in 770/logs/.

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📝 Walkthrough

Walkthrough

RegisterConn now publishes driver connection state before adding the FD to epoll. A Linux test checks delivery of bytes queued before registration while another goroutine signals the loop’s wakeup eventfd.

Changes

Epoll driver connection registration

Layer / File(s)	Summary
Publish connections before epoll registration engine/epoll/driver.go, engine/epoll/driver_register_first_edge_linux_test.go	RegisterConn publishes the connection before EPOLL_CTL_ADD and removes it if the add fails. The Linux test performs 5,000 registrations with prequeued bytes and checks callback delivery. It also signals the wakeup eventfd during registration.

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title uses the required Conventional Commit form, identifies the epoll fix, states the registration-order change, and ends with the issue reference (celeris#770).
Description check	✅ Passed	The description directly explains the RegisterConn race, the fix, the regression test, rollback behavior, and validation results. It is fully related to the changeset.
Linked Issues check	✅ Passed	Issue #770 requires publication before EPOLL_CTL_ADD, with driverMu protection and rollback on add failure. engine/epoll/driver.go now inserts driverConns[fd] and sets hasDriverConns before …
Out of Scope Changes check	✅ Passed	The PR changes only RegisterConn and adds a Linux regression test for the first edge described by issue #770. The rollback code supports the required registration behavior. No unrelated change is sh…

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Inline comments:
Review comments at @engine/epoll/driver_register_first_edge_linux_test.go:
- Around line 69-73: Replace the 200 ms per-registration timeout in the test’s
`got` select with a generous deadline in seconds, and fail immediately on the
first missed delivery instead of counting losses; cleanly stop and join the wake
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