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[3.14] gh-155717: use spawn as the default start method for read-only filesystems (GH-155827) - #155886
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Waiting until #155891 is merged. |
picnixz
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August 16, 2026 09:45
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🤖 New build scheduled with the buildbot fleet by @picnixz for commit 0e1100c 🤖 Results will be shown at: https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F155886%2Fmerge The command will test the builders whose names match following regular expression: The builders matched are:
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picnixz
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I have other matters to attend so this will need to wait. |
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see the issue, this causes a startup time performance regression. holding off is good for now. |
…y filesystems (pythonGH-155827) The "forkserver" start method (the default start method on non-Windows systems) requires the ability to write temporary files, which is not possible if TMPDIR is read-only (e.g., k8s containers mounted with `readOnlyRootFilesystem=True`). On such filesystems, the default start method changes from "forkserver" to "spawn". (cherry picked from commit c144799) Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
…hod lazily (pythonGH-159254) pythongh-155827 made `import multiprocessing` check whether the temporary directory is writeable in order to choose between "forkserver" and "spawn" as the default start method. That check ran at import time and pulled in `multiprocessing.util`, `subprocess`, `tempfile`, `shutil` and `random`, noticeably slowing down the import for every program, including ones that never start a process and every spawned child, the fork server and the resource tracker. The default start method is now determined the first time it is needed instead: on the first `get_context()`, `get_start_method()`, `get_all_start_methods()` or default `Process` start. `multiprocessing.util` goes back to importing `tempfile` only where it is used. Also include the fix from pythongh-158410 by Vyron Vasileiadis, along with a test adapted from that PR: when `tempfile` finds no usable temporary directory at all, the check now reports that instead of raising `FileNotFoundError`, so that "spawn" is chosen. The regression was reported, and the lazy approach suggested, by Leandro Damascena in python#155717 (comment) (cherry picked from commit 48ddda1) Co-authored-by: Vyron Vasileiadis <hi@fedonman.com>
…hange Say that the change applies when there is no writable temporary directory and that starting a process previously failed there.
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The "forkserver" start method (the default start method on non-Windows systems)
requires the ability to write temporary files, which is not possible if TMPDIR
is read-only (e.g., k8s containers mounted with
readOnlyRootFilesystem=True).On such filesystems, the default start method changes from "forkserver" to "spawn".
(cherry picked from commit c144799)
Co-authored-by: Bénédikt Tran 10796600+picnixz@users.noreply.github.com
I rebased this backport on current 3.14 and updated it to also include the changes to avoid the performance regression from #159271 via #159254. --@gpshead