gh-157048: check for buffer errors before mutating in BytesIO.__init__ by ngoldbaum · Pull Request #157049 · python/cpython

ngoldbaum · 2026-09-06T21:33:42Z

Issue: Failed BytesIO reinitialization mutates the stream's position and contents #157048

…_init__

kumaraditya303 · 2026-09-07T05:25:53Z

Thanks, I'll let you merge this :)

ngoldbaum · 2026-09-07T09:18:58Z

@kumaraditya303 thanks for the review! Can you help with my intuition: do you think this should be backported?

bedevere-bot · 2026-09-07T09:59:27Z

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Debian root 3.x (tier-1) has failed when building commit d125f00.

What do you need to do:

Don't panic.
Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/345/builds/15281) and take a look at the build logs.
Check if the failure is related to this commit (d125f00) or if it is a false positive.
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/15281

Failed tests:

test.test_multiprocessing_spawn.test_misc
test.test_multiprocessing_fork.test_misc

Failed subtests:

test_large_pool - test.test_multiprocessing_fork.test_misc.MiscTestCase.test_large_pool
test_large_pool - test.test_multiprocessing_spawn.test_misc.MiscTestCase.test_large_pool

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

==

Click to see traceback logs

Traceback (most recent call last):
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/_test_multiprocessing.py", line 7633, in test_large_pool
    rc, out, err = script_helper.assert_python_ok(testfn)
                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/support/script_helper.py", line 182, in assert_python_ok
    return _assert_python(True, *args, **env_vars)
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/support/script_helper.py", line 167, in _assert_python
    res.fail(cmd_line)
    ~~~~~~~~^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/support/script_helper.py", line 80, in fail
    raise AssertionError(f"Process return code is {exitcode}\n"
    ...<10 lines>...
                         f"---")
AssertionError: Process return code is 1
command line: ['/root/buildarea/3.x.angelico-debian-amd64/build/python', '-X', 'faulthandler', '-I', '@test_2432591_tmpæ']


Traceback (most recent call last):
  File "<string>", line 1, in <module>
    import sys; from multiprocessing.forkserver import main; main(10, 11, ['__main__'], sys_argv=sys.argv[1:], **{'sys_path': ['/root/buildarea/3.x.angelico-debian-amd64/build/target/lib/python316.zip', '/root/buildarea/3.x.angelico-debian-amd64/build/Lib', '/root/buildarea/3.x.angelico-debian-amd64/build/build/lib.linux-x86_64-3.16'], 'main_path': '/root/buildarea/3.x.angelico-debian-amd64/build/build/test_python_2432591æ/@test_2432591_tmpæ', 'authkey_r': 13})
                                                             ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/forkserver.py", line 406, in main
    pid = os.fork()
OSError: [Errno 12] Cannot allocate memory
Traceback (most recent call last):
  File "/root/buildarea/3.x.angelico-debian-amd64/build/build/test_python_2432591æ/@test_2432591_tmpæ", line 4, in <module>
    with multiprocessing.Pool(200) as p:
         ~~~~~~~~~~~~~~~~~~~~^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/context.py", line 120, in Pool
    return Pool(processes, initializer, initargs, maxtasksperchild,
                context=self.get_context())
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/pool.py", line 220, in __init__
    self._repopulate_pool()
    ~~~~~~~~~~~~~~~~~~~~~^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/pool.py", line 312, in _repopulate_pool
    return self._repopulate_pool_static(self._ctx, self.Process,
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
                                        self._processes,
                                        ^^^^^^^^^^^^^^^^
    ...<3 lines>...
                                        self._maxtasksperchild,
                                        ^^^^^^^^^^^^^^^^^^^^^^^
                                        self._wrap_exception)
                                        ^^^^^^^^^^^^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/pool.py", line 335, in _repopulate_pool_static
    w.start()
    ~~~~~~~^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
                  ~~~~~~~~~~~^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/context.py", line 310, in _Popen
    return Popen(process_obj)
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/popen_forkserver.py", line 35, in __init__
    super().__init__(process_obj)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/popen_fork.py", line 20, in __init__
    self._launch(process_obj)
    ~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/popen_forkserver.py", line 59, in _launch
    self.pid = forkserver.read_signed(self.sentinel)
               ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/forkserver.py", line 472, in read_signed
    raise EOFError('unexpected EOF')
EOFError: unexpected EOF
---


Traceback (most recent call last):
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/_test_multiprocessing.py", line 7633, in test_large_pool
    rc, out, err = script_helper.assert_python_ok(testfn)
                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/support/script_helper.py", line 182, in assert_python_ok
    return _assert_python(True, *args, **env_vars)
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/support/script_helper.py", line 167, in _assert_python
    res.fail(cmd_line)
    ~~~~~~~~^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/support/script_helper.py", line 80, in fail
    raise AssertionError(f"Process return code is {exitcode}\n"
    ...<10 lines>...
                         f"---")
AssertionError: Process return code is 1
command line: ['/root/buildarea/3.x.angelico-debian-amd64/build/python', '-X', 'faulthandler', '-I', '@test_2400747_tmpæ']


Traceback (most recent call last):
  File "<string>", line 1, in <module>
    import sys; from multiprocessing.forkserver import main; main(10, 11, ['__main__'], sys_argv=sys.argv[1:], **{'sys_path': ['/root/buildarea/3.x.angelico-debian-amd64/build/target/lib/python316.zip', '/root/buildarea/3.x.angelico-debian-amd64/build/Lib', '/root/buildarea/3.x.angelico-debian-amd64/build/build/lib.linux-x86_64-3.16'], 'main_path': '/root/buildarea/3.x.angelico-debian-amd64/build/build/test_python_2410098æ/@test_2410098_tmpæ', 'authkey_r': 13})
                                                             ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/forkserver.py", line 406, in main
    pid = os.fork()
OSError: [Errno 12] Cannot allocate memory
Traceback (most recent call last):
  File "/root/buildarea/3.x.angelico-debian-amd64/build/build/test_python_2410098æ/@test_2410098_tmpæ", line 4, in <module>
    with multiprocessing.Pool(200) as p:
         ~~~~~~~~~~~~~~~~~~~~^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/context.py", line 120, in Pool
    return Pool(processes, initializer, initargs, maxtasksperchild,
                context=self.get_context())
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/pool.py", line 220, in __init__
    self._repopulate_pool()
    ~~~~~~~~~~~~~~~~~~~~~^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/pool.py", line 312, in _repopulate_pool
    return self._repopulate_pool_static(self._ctx, self.Process,
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
                                        self._processes,
                                        ^^^^^^^^^^^^^^^^
    ...<3 lines>...
                                        self._maxtasksperchild,
                                        ^^^^^^^^^^^^^^^^^^^^^^^
                                        self._wrap_exception)
                                        ^^^^^^^^^^^^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/pool.py", line 335, in _repopulate_pool_static
    w.start()
    ~~~~~~~^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
                  ~~~~~~~~~~~^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/context.py", line 310, in _Popen
    return Popen(process_obj)
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/popen_forkserver.py", line 35, in __init__
    super().__init__(process_obj)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/popen_fork.py", line 20, in __init__
    self._launch(process_obj)
    ~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/popen_forkserver.py", line 59, in _launch
    self.pid = forkserver.read_signed(self.sentinel)
               ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/forkserver.py", line 472, in read_signed
    raise EOFError('unexpected EOF')
EOFError: unexpected EOF
---


Traceback (most recent call last):
  File "<string>", line 1, in <module>
    import sys; from multiprocessing.forkserver import main; main(10, 11, ['__main__'], sys_argv=sys.argv[1:], **{'sys_path': ['/root/buildarea/3.x.angelico-debian-amd64/build/target/lib/python316.zip', '/root/buildarea/3.x.angelico-debian-amd64/build/Lib', '/root/buildarea/3.x.angelico-debian-amd64/build/build/lib.linux-x86_64-3.16'], 'main_path': '/root/buildarea/3.x.angelico-debian-amd64/build/build/test_python_2400747æ/@test_2400747_tmpæ', 'authkey_r': 13})
                                                             ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/forkserver.py", line 406, in main
    pid = os.fork()
OSError: [Errno 12] Cannot allocate memory
Traceback (most recent call last):
  File "/root/buildarea/3.x.angelico-debian-amd64/build/build/test_python_2400747æ/@test_2400747_tmpæ", line 4, in <module>
    with multiprocessing.Pool(200) as p:
         ~~~~~~~~~~~~~~~~~~~~^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/context.py", line 120, in Pool
    return Pool(processes, initializer, initargs, maxtasksperchild,
                context=self.get_context())
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/pool.py", line 220, in __init__
    self._repopulate_pool()
    ~~~~~~~~~~~~~~~~~~~~~^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/pool.py", line 312, in _repopulate_pool
    return self._repopulate_pool_static(self._ctx, self.Process,
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
                                        self._processes,
                                        ^^^^^^^^^^^^^^^^
    ...<3 lines>...
                                        self._maxtasksperchild,
                                        ^^^^^^^^^^^^^^^^^^^^^^^
                                        self._wrap_exception)
                                        ^^^^^^^^^^^^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/pool.py", line 335, in _repopulate_pool_static
    w.start()
    ~~~~~~~^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
                  ~~~~~~~~~~~^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/context.py", line 310, in _Popen
    return Popen(process_obj)
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/popen_forkserver.py", line 35, in __init__
    super().__init__(process_obj)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/popen_fork.py", line 20, in __init__
    self._launch(process_obj)
    ~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/popen_forkserver.py", line 59, in _launch
    self.pid = forkserver.read_signed(self.sentinel)
               ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/multiprocessing/forkserver.py", line 472, in read_signed
    raise EOFError('unexpected EOF')
EOFError: unexpected EOF
---


Traceback (most recent call last):
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/_test_multiprocessing.py", line 7633, in test_large_pool
    rc, out, err = script_helper.assert_python_ok(testfn)
                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/support/script_helper.py", line 182, in assert_python_ok
    return _assert_python(True, *args, **env_vars)
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/support/script_helper.py", line 167, in _assert_python
    res.fail(cmd_line)
    ~~~~~~~~^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/support/script_helper.py", line 80, in fail
    raise AssertionError(f"Process return code is {exitcode}\n"
    ...<10 lines>...
                         f"---")
AssertionError: Process return code is 1
command line: ['/root/buildarea/3.x.angelico-debian-amd64/build/python', '-X', 'faulthandler', '-I', '@test_2410098_tmpæ']

kumaraditya303 · 2026-09-07T10:45:40Z

do you think this should be backported?

Yes, seems fine to backport

miss-islington-app · 2026-09-07T15:29:11Z

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

miss-islington-app · 2026-09-07T15:29:11Z

Thanks @ngoldbaum for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

miss-islington-app · 2026-09-07T15:29:11Z

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

miss-islington-app · 2026-09-07T15:29:31Z

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

cherry_picker d125f009dc2a698de62deb11a0c50d3ef8e33d62 3.13

bedevere-app · 2026-09-07T15:29:35Z

GH-157068 is a backport of this pull request to the 3.14 branch.

bedevere-app · 2026-09-07T15:29:49Z

GH-157069 is a backport of this pull request to the 3.15 branch.

bedevere-app · 2026-09-07T15:35:18Z

GH-157070 is a backport of this pull request to the 3.13 branch.

…__init__ (GH-157049) (#157068)

…__init__ (GH-157049) (#157070)

…_init__ (python#157049)

…__init__ (GH-157049) (GH-157069) (cherry picked from commit d125f00) Co-authored-by: Nathan Goldbaum <nathan.goldbaum@gmail.com>

pythongh-157048: check for buffer errors before mutating in BytesIO._…

3038d44

…_init__

bedevere-app Bot mentioned this pull request Sep 6, 2026

Failed BytesIO reinitialization mutates the stream's position and contents #157048

Closed

bedevere-app Bot added the awaiting core review label Sep 6, 2026

kumaraditya303 approved these changes Sep 7, 2026

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bedevere-app Bot added awaiting merge and removed awaiting core review labels Sep 7, 2026

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ngoldbaum merged commit d125f00 into python:main Sep 7, 2026
60 checks passed

bedevere-app Bot removed the awaiting merge label Sep 7, 2026

ngoldbaum added needs backport to 3.13 only security fixes needs backport to 3.14 bugs and security fixes needs backport to 3.15 bugs and security fixes labels Sep 7, 2026

miss-islington-app Bot assigned ngoldbaum Sep 7, 2026

bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label Sep 7, 2026

bedevere-app Bot removed the needs backport to 3.15 bugs and security fixes label Sep 7, 2026

bedevere-app Bot removed the needs backport to 3.13 only security fixes label Sep 7, 2026

ngoldbaum pushed a commit that referenced this pull request Sep 7, 2026

[3.14] gh-157048: check for buffer errors before mutating in BytesIO.…

c6af0c4

…__init__ (GH-157049) (#157068)

ngoldbaum added a commit that referenced this pull request Sep 7, 2026

[3.13] gh-157048: check for buffer errors before mutating in BytesIO.…

88f6017

…__init__ (GH-157049) (#157070)

clin1234 pushed a commit to clin1234/cpython that referenced this pull request Sep 12, 2026

pythongh-157048: check for buffer errors before mutating in BytesIO._…

5fec438

…_init__ (python#157049)

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ngoldbaum commented Sep 6, 2026 •

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