fix(time): keep alarm(0) disarmed when no timer exists - #2308
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fslongjin merged 1 commit intoSep 21, 2026
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The empty alarm slot path created a timer before checking whether seconds was zero. A first alarm(0), or another cancellation after disarming, thus queued an immediately expiring timer and delivered an unexpected SIGALRM. Git upload-pack calls alarm(0) during local object transfer, so the sender could terminate and leave clone/fetch reporting early EOF or bad pack headers. Return zero from the existing locked empty-slot branch for a zero duration. Leave nonzero scheduling and the existing timer cancellation/replacement paths unchanged. The generic timer queue and Git behavior need no changes. Add a whitelisted dunitest suite for first cancellation, repeated cancellation, cancellation after disarming, and nonzero alarm delivery. Isolate signal state in child processes and use a monotonic parent-side watchdog independent of alarm(). Validation: - Linux: all four new cases pass. - Before the fix in DragonOS: all three zero-cancellation cases terminate with SIGALRM; the nonzero delivery case passes. - After the fix in DragonOS: all four pass, including three additional runs. - Adjacent poll timeout cases: 4/4; POSIX relative timer cases: 9/9. - Original alarm(0) reproducer exits normally after the fix. - Git 2.43.0 file:// clone and incremental pull pass with matching HEADs and successful fsck; the previously failing local-path pull also passes. - Git commit/merge/stash/push/gc/archive/bundle and HTTPS shallow clone pass. - make kernel, FMT_CHECK=1 make fmt, and git diff --check pass. This deliberately does not change existing alarm/setitimer interoperability, remaining-time accounting, or timer cancellation races. Signed-off-by: longjin <longjin@dragonos.org>
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This was referenced Sep 23, 2026
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Summary
Fix
alarm(0)incorrectly arming an immediately expiring timer when the current alarm slot is empty. This unexpected SIGALRM can terminategit-upload-pack, causing local clone/fetch/pull to report early EOF or bad pack headers.Validation
Validation used x86_64 DragonOS in QEMU/KVM with two vCPUs and an ext4 root filesystem. Independent plan, implementation and test reviews found no blocking issues.
Scope
This is a focused fix for an empty alarm slot and zero seconds. It does not claim complete alarm/setitimer interoperability, remaining-time accounting or cancellation-race coverage. The full Git and full kernel test suites were not run.
Issue linkage
Fixes #2330. Parent LMbench blocker tracker: #2286.