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gh-160181: Don't modify a hashed bytes buffer in place in io.BytesIO - #160182
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needs backport to 3.14
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October 22, 2026 04:26
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The behavior here feels a little sus. This seems better... but bytes. immutable. overwriting. scary. |
…tesIO Treat the internal buffer as shared once its hash is cached, so the next modification copies it, as pythongh-158219 did for bytearray.
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It would be good to add a _PyBytes_IsMutable assertion in write_bytes_lock_held to validate we've got all the cases here. For the previous fixes to bytearray allowing mutating immutable things found an underlying _PyBytes_Resize issue by adding that shape check.
(resize_unshared_buffer_lock_held doesn't need one because it uses _PyBytes_ResizeKeepOnError which already does)
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October 22, 2026 22:25
vstinner
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Thanks @gpshead for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14, 3.15. |
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GH-160408 is a backport of this pull request to the 3.15 branch. |
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GH-160409 is a backport of this pull request to the 3.14 branch. |
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Treat the internal buffer as shared once its hash is cached, so the next write,
getbuffer()export or resize copies it instead of modifying abytesobject that may already be in a set or dict.SHARED_BUF()is the copy-on-write check every modifying path uses, so this covers all of them on both the default and free-threaded builds. The fix for gh-158219 did the same forbytearray.Fixes #160181
ANT-2026-IZN6JY41