[issue31935] subprocess.run() timeout not working with grandchildren and stdout=PIPE

[Martin Panter]

Martin Panter  added the comment:

Closing in faviour of Issue 30154, which suggests documentation or adjusting 
the timeout implementation. There is also Issue 26534 proposing a new 
“kill_group” option when using the timeout feature.

--
resolution:  -> duplicate
stage:  -> resolved
status: open -> closed
superseder:  -> subprocess.run with stderr connected to a pipe won't timeout 
when killing a never-ending shell commanad

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[issue31935] subprocess.run() timeout not working with grandchildren and stdout=PIPE

[STINNER Victor]

Change by STINNER Victor :


--
nosy: +haypo

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[issue31935] subprocess.run() timeout not working with grandchildren and stdout=PIPE

[Martin Panter]

Martin Panter  added the comment:

This proposal sounds like a race condition. Closing the output pipe as a child 
exits means you risk missing recent output. On the other hand, if you don’t 
care about the output any more, close the pipe first and then wait for the 
child.

Related discussions:

Issue 30154: Similar problem, but the grandchild does not write any output
Issue 26534: kill_group=True option for timeout
Issue 31447: Windows-specific (?)

--
nosy: +martin.panter

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[issue31935] subprocess.run() timeout not working with grandchildren and stdout=PIPE

[Martin Ritter]

New submission from Martin Ritter :

Hi,

I tried to use 

subprocess.run(..., stdout=subprocess.PIPE, timeout=N)

to run some test scripts with a given timeout. 

This works as expected with simple scripts. However if the script itself 
creates other children which write to stdout then `subprocess.run()` seems to 
wait for all of the children to finish. I've attached a minimal example.

I looked into subprocess.py and `subprocess.run()` calls 
`process.communicate()` again without timeout when handling the TimeoutExpired 
exception which then in turn waits for the pipes to be closed by all children.

If communicate() would check if the process is still alive while waiting for 
output and close the pipes once the process has finished the timeout feature 
should work as expected and descendants would get a SIGPIPE when writing to 
stdout/stderr.

--
components: Library (Lib)
files: test_killsub.py
messages: 305487
nosy: Martin Ritter
priority: normal
severity: normal
status: open
title: subprocess.run() timeout not working with grandchildren and stdout=PIPE
type: behavior
versions: Python 3.6
Added file: https://bugs.python.org/file47251/test_killsub.py

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