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gh-129005: Fix buffer expansion in _pyio.FileIO.readall#129541

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cmaloney wants to merge 1 commit into from
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gh-129005: Fix buffer expansion in _pyio.FileIO.readall #129541

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4 changes: 3 additions & 1 deletion Lib/_pyio.py
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Original file line numberDiff line numberDiff line change
Expand Up@@ -10,6 +10,7 @@
import sys
# Import _thread instead of threading to reduce startup cost
from _thread import allocate_lock as Lock
from itertools import repeat
if sys.platform in{'win32', 'cygwin'}:
from msvcrt import setmode as _setmode
else:
Expand DownExpand Up@@ -1686,7 +1687,8 @@ def readall(self):
if addend < DEFAULT_BUFFER_SIZE:
addend = DEFAULT_BUFFER_SIZE
bufsize += addend
result[bytes_read:bufsize] = b'\0'
result.extend(repeat(0, addend))
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@vstinnervstinnerFeb 1, 2025

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Why not just b'\0' * addend?

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@cmaloneycmaloneyFeb 1, 2025•
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That uses a lot more memory (And overall goal is to get _io and _pyio to use the same). result is expanded in length by addend bytes, a new bytes is constructed which is of length addend containing just null, then there's a memcpy from the new temporary bytes object to result... That means get 2x addend memory usage, rather than just expanding the buffer by addend bytes.

In this case, bytearray.extend also doesn't prefer working in place / creates a PyByteArray_FromStringAndSize that it copies the data out of a iterator on the object passed to it as an argument...

My preference is definitely bytearray.resize which does what I'd need here, but didn't want that to be a blocker for getting bots back to green (#129560). Reading through the slice, extend, append, etc. code in bytearray haven't found any other efficient ways to just "expand capacity, in place if possible without multiplying existing space" and (ideally) without requiring a by-byte write to every byte (the read loop is about to do that anyways).

assert len(result) == bufsize, "Should have expanded in size"
assert bufsize - bytes_read > 0, "Should always try and read at least one byte"
try:
n = os.readinto(self._fd, memoryview(result)[bytes_read:])
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2025-01-31-19-07-31.gh-issue-129005.II6go0.rst
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@@ -0,0 +1 @@
:mod:`!pyio`: Fix expansion of buffer in _pyio.FileIO.readall
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