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Fix Python controller hang at exit with stdout/stderr redirection on Windows - #7049
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…berbotics#7040) On Windows, Python controllers started with --stdout-redirect or --stderr-redirect hung indefinitely upon script completion. The helper thread in StdStreamRedirect was non-daemon and blocked in readline() on the pipe, while Python waits for non-daemon threads before running atexit handlers where the write end was supposed to close. Resolve the deadlock by making the helper thread a daemon thread, adding an explicit close() method registered with atexit, and restoring original stream handles upon cleanup.
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🟡 Changes recommended
Shutdown can still deadlock when pending output fills the native redirect pipe.
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What changed in this PR
Addresses #7040 by updating Windows Python controller output-redirection shutdown.
Changes:
- Makes redirect helpers daemon threads and registers explicit cleanup at exit.
- Restores original streams before closing and joining helpers.
- Documents the fix in the changelog.
| File | Description |
|---|---|
| lib/controller/python/controller/robot.py | Updates redirect-thread lifecycle and stream cleanup. |
| docs/reference/changelog-r2025.md | Records the Windows controller shutdown fix. |
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| elif self._stream == 2: | ||
| sys.stderr = self._orig | ||
| self._w.close() | ||
| self._thread.join() |
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Description
On Windows, Python controllers run with
--stdout-redirector--stderr-redirect(or withWEBOTS_STDOUT_REDIRECT/WEBOTS_STDERR_REDIRECTset) hung indefinitely upon script completion.This was caused by a circular deadlock in
StdStreamRedirect:daemon=False) and blocked onself._r.readline()on the anonymous pipe.atexitcallbacks.self._wremained open), Python hung permanently beforeatexitcould close it.Fix:
daemon=True) so Python proceeds into theatexitstage.close()method registered directly viaatexit.register(...). Closing the write end sends EOF down the pipe, allowingself._thread.join()to safely flush all remaining output toucrtbasebefore process exit.sys.stdout/sys.stderrto the original streams before closing the pipe to preventValueError: I/O operation on closed fileduring late interpreter shutdown.Related Issues
Fixes #7040
Tasks
Additional context
Verified on Windows 11 with Python 3.13 / 3.14: scripts now flush completely and terminate with exit code
0immediately. Conforms to PEP8 and repository source check standards.