https://github.com/python/cpython/commit/44fb7c361cb24dcf9989a7a1cfee4f6aad5c81aa
commit: 44fb7c361cb24dcf9989a7a1cfee4f6aad5c81aa
branch: main
author: Duane Griffin <[email protected]>
committer: colesbury <[email protected]>
date: 2025-06-02T17:22:41Z
summary:
gh-134908: Protect `textiowrapper_iternext` with critical section (gh-134910)
The `textiowrapper_iternext` function called `_textiowrapper_writeflush`, but
did not
use a critical section, making it racy in free-threaded builds.
files:
A
Misc/NEWS.d/next/Core_and_Builtins/2025-05-30-15-56-19.gh-issue-134908.3a7PxM.rst
M Lib/test/test_io.py
M Modules/_io/textio.c
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 168e66c5a3f0e0..0c921ffbc2576a 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -1062,6 +1062,37 @@ def flush(self):
# Silence destructor error
R.flush = lambda self: None
+ @threading_helper.requires_working_threading()
+ def test_write_readline_races(self):
+ # gh-134908: Concurrent iteration over a file caused races
+ thread_count = 2
+ write_count = 100
+ read_count = 100
+
+ def writer(file, barrier):
+ barrier.wait()
+ for _ in range(write_count):
+ file.write("x")
+
+ def reader(file, barrier):
+ barrier.wait()
+ for _ in range(read_count):
+ for line in file:
+ self.assertEqual(line, "")
+
+ with self.open(os_helper.TESTFN, "w+") as f:
+ barrier = threading.Barrier(thread_count + 1)
+ reader = threading.Thread(target=reader, args=(f, barrier))
+ writers = [threading.Thread(target=writer, args=(f, barrier))
+ for _ in range(thread_count)]
+ with threading_helper.catch_threading_exception() as cm:
+ with threading_helper.start_threads(writers + [reader]):
+ pass
+ self.assertIsNone(cm.exc_type)
+
+ self.assertEqual(os.stat(os_helper.TESTFN).st_size,
+ write_count * thread_count)
+
class CIOTest(IOTest):
diff --git
a/Misc/NEWS.d/next/Core_and_Builtins/2025-05-30-15-56-19.gh-issue-134908.3a7PxM.rst
b/Misc/NEWS.d/next/Core_and_Builtins/2025-05-30-15-56-19.gh-issue-134908.3a7PxM.rst
new file mode 100644
index 00000000000000..3178f0aaf885f8
--- /dev/null
+++
b/Misc/NEWS.d/next/Core_and_Builtins/2025-05-30-15-56-19.gh-issue-134908.3a7PxM.rst
@@ -0,0 +1 @@
+Fix crash when iterating over lines in a text file on the :term:`free threaded
<free threading>` build.
diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
index 86328e46a7b131..3808ecdceb9b70 100644
--- a/Modules/_io/textio.c
+++ b/Modules/_io/textio.c
@@ -1578,6 +1578,8 @@ _io_TextIOWrapper_detach_impl(textio *self)
static int
_textiowrapper_writeflush(textio *self)
{
+ _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
+
if (self->pending_bytes == NULL)
return 0;
@@ -3173,8 +3175,9 @@ _io_TextIOWrapper_close_impl(textio *self)
}
static PyObject *
-textiowrapper_iternext(PyObject *op)
+textiowrapper_iternext_lock_held(PyObject *op)
{
+ _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(op);
PyObject *line;
textio *self = textio_CAST(op);
@@ -3210,6 +3213,16 @@ textiowrapper_iternext(PyObject *op)
return line;
}
+static PyObject *
+textiowrapper_iternext(PyObject *op)
+{
+ PyObject *result;
+ Py_BEGIN_CRITICAL_SECTION(op);
+ result = textiowrapper_iternext_lock_held(op);
+ Py_END_CRITICAL_SECTION();
+ return result;
+}
+
/*[clinic input]
@critical_section
@getter
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