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bpo-30643: Fix race condition in signal wakeup in forkserver (followup to PR #1989) #2139

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Merged
merged 2 commits into from
Jun 13, 2017
Merged

bpo-30643: Fix race condition in signal wakeup in forkserver (followup to PR #1989) #2139

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0513fe5
Fix race condition in signal wakeup in forkserver (followup to PR #1989)
pitrou Jun 12, 2017
aedadb3
Reset wakeup fd in child
pitrou Jun 12, 2017
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12 changes: 8 additions & 4 deletions Lib/multiprocessing/forkserver.py
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Original file line number Diff line number Diff line change
Expand Up @@ -150,22 +150,25 @@ def main(listener_fd, alive_r, preload, main_path=None, sys_path=None):
util._close_stdin()

sig_r, sig_w = os.pipe()
os.set_blocking(sig_r, False)
os.set_blocking(sig_w, False)

def sigchld_handler(*_unused):
try:
os.write(sig_w, b'.')
except BlockingIOError:
pass
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I like this change :-) Using the existing C signal handler for the write is probably more reliable ;-)

# Dummy signal handler, doesn't do anything
pass

# letting SIGINT through avoids KeyboardInterrupt tracebacks
# unblocking SIGCHLD allows the wakeup fd to notify our event loop
handlers = {
signal.SIGCHLD: sigchld_handler,
signal.SIGINT: signal.SIG_DFL,
}
old_handlers = {sig: signal.signal(sig, val)
for (sig, val) in handlers.items()}

# calling os.write() in the Python signal handler is racy
signal.set_wakeup_fd(sig_w)

# map child pids to client fds
pid_to_fd = {}

Expand Down Expand Up @@ -252,6 +255,7 @@ def sigchld_handler(*_unused):

def _serve_one(child_r, fds, unused_fds, handlers):
# close unnecessary stuff and reset signal handlers
signal.set_wakeup_fd(-1)
for sig, val in handlers.items():
signal.signal(sig, val)
for fd in unused_fds:
Expand Down