On 2026-03-03 09:24, Fabian Grünbichler wrote: > On March 3, 2026 8:15 am, Hannes Laimer wrote: >> If the worker finishes right after we `waitpid` but before we add it to >> `WORKER_PIDS` the `worker_reaper` won't `waitpid` it cause it iterates >> over `WORKER_PIDS`. So > > it would be interesting to get more details how this happens in practice > (with your reproducer)? >
I do have a reproducer for task processes sticking around as zombies when they are done, but this change unfortunately did not fix that. I just noticed this in the process of finding the cause for the "original" problem, so I guess this is not a problem in practice, cause of the tight timings? But technically it would be possible (I think) > the sequence when forking a worker is: > - fork > - child executes some setup code > - child tells parent it is ready > - child waits for parent to tell it it can continue > > register_worker is called by the parent in between the last two steps > (after receiving the notification form the child, but before sending the > notification to the child), so why does the child disappear inbetween? > > I think this might actually (also?) be missing error handling in > fork_worker? all the POSIX::close/read/write calls there don't check for > failure, which means we attempt to register a worker that has already > failed at that point? > could be, but I don't think that should influence if a `SIGCHLD` is sent when the child is done? Cause the handler for `SIGCHLD` in the parent is never called... I'll take a look at that, thanks for the pointer! > and, somewhat tangentially related - should we switch this code over to > use pidfds and waitid to close PID reuse races? > @Wolfgang also mentioned that, would probably make sense >> - the clean-up triggered by the SIGCHLD won't catch it cause it needs it to >> be in `WORKER_PIDS` >> - and, `register_worker` won't because it was still running when it >> `waitpid`'ed it >> >> Moving the insertion into `WORKER_PIDS` before the `waitpid` solves >> this by making sure it is >> - always in the var for `worker_reaper` >> - and, if SIGCHILD should trigger `worker_reaper` before we add it to >> `WORKER_PIDS`, the `waitpid` in `register_worker` itself will catch >> it >> >> Signed-off-by: Hannes Laimer <[email protected]> >> --- >> src/PVE/RESTEnvironment.pm | 11 ++++++----- >> 1 file changed, 6 insertions(+), 5 deletions(-) >> >> diff --git a/src/PVE/RESTEnvironment.pm b/src/PVE/RESTEnvironment.pm >> index 4ed5c05..4677687 100644 >> --- a/src/PVE/RESTEnvironment.pm >> +++ b/src/PVE/RESTEnvironment.pm >> @@ -99,17 +99,18 @@ my $register_worker = sub { >> >> return if !$pid; >> >> - # do not register if already finished >> + $WORKER_PIDS->{$pid} = { >> + user => $user, >> + upid => $upid, >> + }; >> + >> + # remove immediately if already finished >> my $waitpid = waitpid($pid, WNOHANG); >> if (defined($waitpid) && ($waitpid == $pid)) { >> delete($WORKER_PIDS->{$pid}); >> return; >> } >> >> - $WORKER_PIDS->{$pid} = { >> - user => $user, >> - upid => $upid, >> - }; >> }; >> >> # initialize environment - must be called once at program startup >> -- >> 2.47.3 >> >> >> >> >> >>
