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Fix CliRunner error with logging message #3140

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Fix CliRunner error with logging message #3140

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6d8dac7
fix CliRunner error with logging message
getzze Nov 12, 2025
d71fa41
deal with copy_to stream being closed
getzze Nov 13, 2025
59ed117
add description in CHANGES.rst
getzze Nov 13, 2025
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2 changes: 2 additions & 0 deletions CHANGES.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -15,6 +15,8 @@ Unreleased
empty. :issue:`3019` :pr:`3021`
- When ``Sentinel.UNSET`` is found during parsing, it will skip calls to
``type_cast_value``. :issue:`3069` :pr:`3090`
- Fix regression related to ``click.testing.StreamMixer`` finalization in ``CliRunner``
that introduced a race condition. :issue:`3110` :pr:`3140`

Version 8.3.0
--------------
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18 changes: 11 additions & 7 deletions src/click/testing.py
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Expand Up @@ -72,16 +72,20 @@ class BytesIOCopy(io.BytesIO):
.. versionadded:: 8.2
"""

copy_to: io.BytesIO

def __init__(self, copy_to: io.BytesIO) -> None:
super().__init__()
self.copy_to = copy_to

def flush(self) -> None:
if not self.copy_to.closed:
self.copy_to.flush()
Comment on lines +82 to +83
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@neutrinoceros neutrinoceros Nov 13, 2025

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unfortunately this LBYL (Look Before You Leap) approach is infamously inappropriate to guard against races; in a concurrent execution model, self.copy_to.closed could change between the moment the guard clause (if ...) is evaluated, and the rest of the block is, and then you're back to square one, but the code is more complex and the race is potentially harder to reproduce (which isn't necessarily a good thing when you're try to debug it).

super().flush()
self.copy_to.flush()

def write(self, b: ReadableBuffer) -> int:
self.copy_to.write(b)
if not self.copy_to.closed:
self.copy_to.write(b)
return super().write(b)


Expand All @@ -100,15 +104,15 @@ def __init__(self) -> None:

def __del__(self) -> None:
"""
Guarantee that embedded file-like objects are closed in a
Guarantee that embedded file-like objects are deleted in a
predictable order, protecting against races between
self.output being closed and other streams being flushed on close
self.output being deleted and other streams being flushed on deletion.

.. versionadded:: 8.2.2
"""
self.stderr.close()
self.stdout.close()
self.output.close()
del self.stdout
del self.stderr
del self.output


class _NamedTextIOWrapper(io.TextIOWrapper):
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