daemon: fix hang on SSH disconnect during remote builds#14667
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When an SSH connection dies during a remote build, MonitorFdHup correctly detects the disconnect and calls triggerInterrupt(). However, without ReceiveInterrupts instantiated, no SIGUSR1 is sent to interrupt the blocking read() syscall. This causes the daemon to hang indefinitely while holding file locks, blocking subsequent builds. The fix instantiates ReceiveInterrupts in processConnection(), which registers a callback to send SIGUSR1 to the current thread when triggerInterrupt() is called. This allows the blocking read() to return with EINTR, causing checkInterrupt() to throw and the daemon to exit cleanly. This pattern is already used in ThreadPool::doWork() and SubstitutionGoal for the same purpose.
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I wasn't able to reproduce the bug after this fix being deployed in two moderate busy CIs. I created little c programs with the old vs the new approach to verify the behavior the signal blocking + read(). |
Ericson2314
approved these changes
Nov 27, 2025
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When an SSH connection dies during a remote build, MonitorFdHup correctly detects the disconnect and calls triggerInterrupt(). However, without ReceiveInterrupts instantiated, no SIGUSR1 is sent to interrupt the blocking read() syscall. This causes the daemon to hang indefinitely while holding file locks, blocking subsequent builds.
The fix instantiates ReceiveInterrupts in processConnection(), which registers a callback to send SIGUSR1 to the current thread when triggerInterrupt() is called. This allows the blocking read() to return with EINTR, causing checkInterrupt() to throw and the daemon to exit cleanly.
This pattern is already used in ThreadPool::doWork() and SubstitutionGoal for the same purpose.
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