Repository navigation
Conversation
…rectory _has_writeable_tempdir() called tempfile.gettempdir() outside its try/except OSError, so when no temporary directory was usable, import multiprocessing raised FileNotFoundError instead of making spawn the default start method.
gpshead
added a commit
that referenced
this pull request
Oct 12, 2026
…H-159254) gh-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 gh-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 #155717 (comment) Co-authored-by: Vyron Vasileiadis <hi@fedonman.com>
Member
|
Thanks! I incorporated this into #159254 and gave you credit there. |
gpshead
added a commit
to miss-islington/cpython
that referenced
this pull request
Oct 12, 2026
…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>
gpshead
added a commit
that referenced
this pull request
Oct 12, 2026
…zily (GH-159254) (GH-159271) gh-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 gh-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 #155717 (comment) (cherry picked from commit 48ddda1) Co-authored-by: Vyron Vasileiadis <hi@fedonman.com> * gh-155717: Reword the NEWS entry for the default start method change Say that the change applies when there is no writable temporary directory and that starting a process previously failed there. --------- Co-authored-by: Vyron Vasileiadis <hi@fedonman.com>
gpshead
added a commit
that referenced
this pull request
Oct 13, 2026
…ly filesystems (GH-155827) (#155886) * gh-155717: use `spawn` as the default start method for read-only filesystems (GH-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> * Update Lib/test/_test_multiprocessing.py * [3.14] gh-155717: Determine multiprocessing's default start method lazily (GH-159254) gh-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 gh-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 #155717 (comment) (cherry picked from commit 48ddda1) Co-authored-by: Vyron Vasileiadis <hi@fedonman.com> * gh-155717: Reword the NEWS entry for the default start method change Say that the change applies when there is no writable temporary directory and that starting a process previously failed there. --------- Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com> Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com> Co-authored-by: Vyron Vasileiadis <hi@fedonman.com> Co-authored-by: Gregory P. Smith <greg@krypto.org>
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
When
tempfilefinds no usable temporary directory, which is the setup this issue is about,import multiprocessingon main raisesFileNotFoundError((ulimit -f 0; ./python -c 'import multiprocessing')shows it), because_has_writeable_tempdir()callstempfile.gettempdir()outside itstry. This moves the call inside it, so the import succeeds and the default start method isspawn.No NEWS entry, since GH-155827 has not been released yet.