[Docker] Fix ssh zombie processes issue #3295
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SSHTunnel.open() starts the ssh process with -f option, which causes it to become a daemon (it calls daemon(3) after authentication). Then, SSHTunnel.close() uses `ssh -O exit` to ask that daemon process to exit (the processes communicate via the control socket, -S option). The daemon exits, and then PID 1 should reap it. Before #3165, PID 1 was bash, which handles SIGCHLD, properly wait(2)ing children, but in that pull request exec was added, making `dstack server` PID 1. dstack does nothing with SIGCHLD (neither handles nor explicitly ignores it), thus the default disposition (ignore the signal but do not discard children) leads to an evergrowing number of unreaped zombies. Fixes: #3291
r4victor
approved these changes
Nov 17, 2025
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SSHTunnel.open() starts the ssh process with -f option, which causes it to become a daemon (it calls daemon(3) after authentication).
Then, SSHTunnel.close() uses
ssh -O exitto ask that daemon process to exit (the processes communicate via the control socket, -S option).The daemon exits, and then PID 1 should reap it.
Before #3165, PID 1 was bash, which handles SIGCHLD, properly wait(2)ing children, but in that pull request exec was added, making
dstack serverPID 1. dstack does nothing with SIGCHLD (neither handles nor explicitly ignores it), thus the default disposition (ignore the signal but do not discard children) leads to an evergrowing number of unreaped zombies.Fixes: #3291