gh-155717: use spawn as the default start method for read-only filesystems - #155827

…ly filesystems (GH-155827) (GH-155887)

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>

…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>

…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>

…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>

…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>