STINNER Victor <victor.stin...@gmail.com> added the comment:
> In a REPL on my Fedora 26, os.waitpid(0, 0) raises "ChildProcessError: [Errno
> 10] No child processes". I'm not sure that waitpid() is the cause of the
> hang, (...)
Oh wait, now I understood the full picture.
Summary:
* 2 new tests were added to test_subprocess and these tests call waitpid(0, 0)
by mistake
* In general, waitpid(0, 0) returns immediately and the code handles it properly
* Sometimes, a previous test leaks a child process and so waitpid(0, 0) takes a
few seconds or can even block
--
Running tests leak randomly child processes. See for example my recent
test_socketserver fix in Python 3.6: commit
fdcf3e9629201ef725af629d99e02215a2d657c8. This commit is *not* part of the
recent Python 3.6.3 release, tested by my colleague.
This fix is for the bug bpo-31593: test_socketserver leaked *randomly* child
processes. Depending on the system load, waitpid() was called or not called to
read the child process exit status.
If you run "./python -m test test_socketserver test_subprocess" and
test_socketserver() doesn't call waitpid() on a single process, it's possible
that test_subprocess hangs on waitpid(0, 0): waiting on the process spawned by
test_socketserver.
test_socketserver is just one example, I fixed many other bugs in the Python
test suite. Running Python tests in subprocesses using "./python -m test -jN
...", at least -j1, reduces the effect of the bug.
Short script to explain the bug:
---
import subprocess, sys, os, time
args = [sys.executable, '-c', 'import time; time.sleep(2)']
proc = subprocess.Popen(args)
t0 = time.monotonic()
print("waitpid(0, 0)...")
pid, status = os.waitpid(0, 0)
dt = time.monotonic() - t0
print("%.1f sec later: waitpid(0, 0) -> %s" % (dt, (pid, status)))
proc.wait()
---
This script takes 3 seconds, since a test leaked a child process which takes
time to complete.
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<https://bugs.python.org/issue31178>
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