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

I'll merge this after #149597 which I forgot to add tests for. I'm doing it now.

tempfile was a deferred import presumably for startup time reasons but it's going to be used at import time now so moving it back to be up top makes sense.

Yes. That's was only for those reasons but I think we can just make it a regular one. It doesn't hurt.

Thanks @picnixz for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

Thanks @picnixz for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

Thanks @picnixz for the PR 🌮🎉.. I'm working now to backport this PR to: 3.15.
🐍🍒⛏🤖

Sorry, @picnixz, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker c1447994a42004a81a8caca026218daf69d451c8 3.13

GH-155886 is a backport of this pull request to the 3.14 branch.

Well I messed up my merge order and merged this one first... well, it's not a problem, it's not really a dependency of the other PR.

GH-155887 is a backport of this pull request to the 3.15 branch.

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot s390x Fedora Stable 3.x (tier-3) has failed when building commit c144799.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1636/builds/3269) and take a look at the build logs.
4. Check if the failure is related to this commit (c144799) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1636/builds/3269

Summary of the results of the build (if available):

Note: switching to 'c1447994a42004a81a8caca026218daf69d451c8'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
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do so (now or later) by using -c with the switch command. Example:

  git switch -c <new-branch-name>

Or undo this operation with:

  git switch -

Turn off this advice by setting config variable advice.detachedHead to false

HEAD is now at c1447994a42004 gh-155717: use `spawn` as the default start method for read-only filesystems (#155827)
Switched to and reset branch 'main'

/tmp/ccwUkvsI.s: Assembler messages:
/tmp/ccwUkvsI.s: Fatal error: Objects/dictobject.o: No space left on device
make: *** [Makefile:3382: Objects/dictobject.o] Error 1
make: *** Waiting for unfinished jobs....
Assembler messages:
Fatal error: can't create Objects/unionobject.o: No space left on device
make: *** [Makefile:3382: Objects/unionobject.o] Error 1
/tmp/ccFkb9sx.s: Assembler messages:
/tmp/ccFkb9sx.s: Fatal error: Objects/typevarobject.o: No space left on device
make: *** [Makefile:3382: Objects/typevarobject.o] Error 1
Assembler messages:
Fatal error: can't create Objects/weakrefobject.o: No space left on device
make: *** [Makefile:3382: Objects/weakrefobject.o] Error 1
Assembler messages:
Fatal error: can't create Objects/obmalloc.o: No space left on device
make: *** [Makefile:3382: Objects/obmalloc.o] Error 1
Assembler messages:
Fatal error: can't create Objects/typeobject.o: No space left on device
make: *** [Makefile:3382: Objects/typeobject.o] Error 1
Assembler messages:
Fatal error: can't create Parser/parser.o: No space left on device
make: *** [Makefile:3382: Parser/parser.o] Error 1
Assembler messages:
Fatal error: can't create Objects/unicodeobject.o: No space left on device
make: *** [Makefile:3382: Objects/unicodeobject.o] Error 1

find: ‘build’: No such file or directory
find: ‘build’: No such file or directory
find: ‘build’: No such file or directory
find: ‘build’: No such file or directory
make: [Makefile:3476: clean-retain-profile] Error 1 (ignored)

Default method was changed from fork to forkserver in Python 3.14 so I don't think I need to do the change in Python 3.13.

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Debian root 3.x (tier-1) has failed when building commit c144799.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/345/builds/15094) and take a look at the build logs.
4. Check if the failure is related to this commit (c144799) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/345/builds/15094

Failed tests:

• test.test_multiprocessing_forkserver.test_misc
• test.test_multiprocessing_fork.test_misc
• test.test_multiprocessing_spawn.test_misc

Failed subtests:

• test_forkserver_requires_writeable_tempdir - test.test_multiprocessing_spawn.test_misc.TestStartMethod.test_forkserver_requires_writeable_tempdir
• test_forkserver_requires_writeable_tempdir - test.test_multiprocessing_forkserver.test_misc.TestStartMethod.test_forkserver_requires_writeable_tempdir
• test_forkserver_requires_writeable_tempdir - test.test_multiprocessing_fork.test_misc.TestStartMethod.test_forkserver_requires_writeable_tempdir

Summary of the results of the build (if available):

==

Traceback (most recent call last):
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/support/__init__.py", line 1121, in wrapper
    func(self, *args, **kwargs, **subtest_kwargs)
    ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/_test_multiprocessing.py", line 6389, in test_forkserver_requires_writeable_tempdir
    self.assertRaises(OSError, file.touch)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
AssertionError: OSError not raised by touch