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gh-79805: Clarify thread-safety limitations in shelve module #137997

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gh-79805: Clarify thread-safety limitations in shelve module #137997

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12 changes: 9 additions & 3 deletions Doc/library/shelve.rst
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Expand Up @@ -133,11 +133,17 @@
database should be fairly small, and in rare cases key collisions may cause
the database to refuse updates.

* The :mod:`shelve` module does not support *concurrent* read/write access to

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py:attr reference target not found: writeback [ref.attr]
shelved objects. (Multiple simultaneous read accesses are safe.) When a
shelved objects, whether from different programs or different threads within
the same program. (Multiple simultaneous read accesses are safe.) When a
program has a shelf open for writing, no other program should have it open for
reading or writing. Unix file locking can be used to solve this, but this
differs across Unix versions and requires knowledge about the database
reading or writing. Additionally, within a single program, concurrent access
from multiple threads is not supported and may lead to data corruption,
unexpected behavior, or crashes. This affects all writing operations including
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Out of the loop here, but at a glance, is there any way to prevent the crashes (e.g. with some simple locking)? We typically don't want pure-Python code to be able to crash, unless it's in ctypes.

assignment, deletion, :meth:`~Shelf.sync`, and :meth:`~Shelf.reorganize`.
Even with :attr:`writeback` set to :const:`True`, thread-safety is not
guaranteed. Unix file locking can be used to solve cross-program concurrency,
but this differs across Unix versions and requires knowledge about the database
implementation used.

* On macOS :mod:`dbm.ndbm` can silently corrupt the database file on updates,
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