From 89d4bcabab5ad51455ea43780ed04d106dbf5b0f Mon Sep 17 00:00:00 2001
From: Stefan Wang <1fannnw@gmail.com>
Date: Tue, 8 Sep 2026 12:36:10 -0700
Subject: [PATCH 1/2] GH-51229: [Python] Check MemoryMappedFile before resize
Reject resize calls on directly constructed MemoryMappedFile objects before dereferencing the native handle.
Generated-by: GitHub Copilot CLI (Claude Opus 5)
Signed-off-by: Stefan Wang <1fannnw@gmail.com>
---
 python/pyarrow/io.pxi &#124; 1 +
 python/pyarrow/tests/test_io.py &#124; 19 +++++++++++++++++++
 2 files changed, 20 insertions(+)
diff --git a/python/pyarrow/io.pxi b/python/pyarrow/io.pxi
index b648fbf66980..bc3ee1841990 100644
--- a/python/pyarrow/io.pxi
+++ b/python/pyarrow/io.pxi
@@ -1101,6 +1101,7 @@ cdef class MemoryMappedFile(NativeFile):
 ----------
 new_size : new size in bytes
 """
+ self._assert_open()
 check_status(self.handle.get().Resize(new_size))
 
 def fileno(self):
diff --git a/python/pyarrow/tests/test_io.py b/python/pyarrow/tests/test_io.py
index 8494a0ee66b4..5e96a3a36772 100644
--- a/python/pyarrow/tests/test_io.py
+++ b/python/pyarrow/tests/test_io.py
@@ -27,6 +27,7 @@
 import pathlib
 import pytest
 import random
+import subprocess
 import sys
 import tempfile
 import weakref
@@ -1216,6 +1217,24 @@ def test_memory_map_resize(tmpdir):
 assert f.read() == bytes(arr[:SIZE])
 
 
+def test_memory_map_resize_uninitialized():
+ code = """if 1:
+ import pyarrow as pa
+
+ try:
+ pa.MemoryMappedFile().resize(0)
+ except ValueError as exc:
+ assert str(exc) == "I/O operation on closed file"
+ else:
+ raise AssertionError("expected ValueError")
+ """
+ res = subprocess.run([sys.executable, "-c", code],
+ universal_newlines=True, stderr=subprocess.PIPE)
+ if res.returncode != 0:
+ print(res.stderr, file=sys.stderr)
+ res.check_returncode()
+
+
 def test_memory_zero_length(tmpdir):
 path = os.path.join(str(tmpdir), guid())
 f = open(path, 'wb')
From 1a2052d26950354bccaab926fcef05184e9f2ddc Mon Sep 17 00:00:00 2001
From: 1fanwang <1fannnw@gmail.com>
Date: 2026年9月10日 04:54:52 -0700
Subject: [PATCH 2/2] GH-51229: Assert the value error with pytest.raises
The subprocess wrapper guarded against the crash this change removes, so
the in-process form reads better now.
Signed-off-by: 1fanwang <1fannnw@gmail.com>
---
 python/pyarrow/tests/test_io.py &#124; 16 +---------------
 1 file changed, 1 insertion(+), 15 deletions(-)
diff --git a/python/pyarrow/tests/test_io.py b/python/pyarrow/tests/test_io.py
index 5e96a3a36772..79f14eef7dff 100644
--- a/python/pyarrow/tests/test_io.py
+++ b/python/pyarrow/tests/test_io.py
@@ -27,7 +27,6 @@
 import pathlib
 import pytest
 import random
-import subprocess
 import sys
 import tempfile
 import weakref
@@ -1218,21 +1217,8 @@ def test_memory_map_resize(tmpdir):
 
 
 def test_memory_map_resize_uninitialized():
- code = """if 1:
- import pyarrow as pa
-
- try:
+ with pytest.raises(ValueError, match="I/O operation on closed file"):
 pa.MemoryMappedFile().resize(0)
- except ValueError as exc:
- assert str(exc) == "I/O operation on closed file"
- else:
- raise AssertionError("expected ValueError")
- """
- res = subprocess.run([sys.executable, "-c", code],
- universal_newlines=True, stderr=subprocess.PIPE)
- if res.returncode != 0:
- print(res.stderr, file=sys.stderr)
- res.check_returncode()
 
 
 def test_memory_zero_length(tmpdir):
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