bpo-31530: fix crash when multiple threads iterate over a file, round 2 #5060
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… file on multiple threads. (#3672)"
benjaminp
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Lib/test/test_file2k.py
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| def test_iteration_seek(self): |
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Why this test is removed?
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Because this test no longer raises any exceptions.
| } | ||
| if ((f->f_buf = (char *)PyMem_Malloc(bufsize)) == NULL) { | ||
| if ((rab->buf = PyMem_MALLOC(bufsize)) == NULL) { |
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Is changing PyMem_Malloc to PyMem_MALLOC required?
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No, but I prefer them over the functions.
| cache the file buffer state locally and only set it back on the file | ||
| object when we're done. | ||
| */ | ||
| readaheadbuffer rab = {f->f_buf, f->f_bufptr, f->f_bufend}; |
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drop_readaheadbuffer() frees rab->buf which is f->f_buf. If two threads call drop_readaheadbuffer() simultaneously f->f_buf will be freed twice.
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Other threads can't access f->f_buf because it's set to NULL for the duration of the method.
pitrou
reviewed
Dec 31, 2017
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| Prevent crashes when multiple threads iterate of a file at once. | |||
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No, actually this file is supposed to be gone.
…s, round 2 Multiple threads iterating over a file can corrupt the file's internal readahead buffer resulting in crashes. To fix this, cache buffer state thread-locally for the duration of a file_iternext call and only update the file's internal state after reading completes. No attempt is made to define or provide "reasonable" semantics for iterating over a file on multiple threads. (Non-crashing) races are still present. Duplicated, corrupt, and missing data will happen. This was originally fixed by 6401e56, which raised an exception from seek() and next() when concurrent operations were detected. Alas, this simpler solution breaks legitimate use cases such as capturing the standard streams when multiple threads are logging.
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This resurrects #3670 and mostly reverts 6401e56. See https://bugs.python.org/msg309265 for why.
https://bugs.python.org/issue31530