https://github.com/python/cpython/commit/fb26d9c2ef739cbfdc134da5ab89470511f1f5fd
commit: fb26d9c2ef739cbfdc134da5ab89470511f1f5fd
branch: main
author: Alper <[email protected]>
committer: colesbury <[email protected]>
date: 2025-11-21T11:22:31-05:00
summary:
gh-116738: Make csv module thread-safe (gh-141365)
Added a critical section to protect the states of `ReaderObj` and `WriterObj`
in the free-threading build. Without the critical sections, both new
free-threading tests were crashing.
files:
A Lib/test/test_free_threading/test_csv.py
A
Misc/NEWS.d/next/Core_and_Builtins/2025-11-10-00-14-20.gh-issue-116738.IxliC_.rst
M Modules/_csv.c
diff --git a/Lib/test/test_free_threading/test_csv.py
b/Lib/test/test_free_threading/test_csv.py
new file mode 100644
index 00000000000000..beb4510a1281b8
--- /dev/null
+++ b/Lib/test/test_free_threading/test_csv.py
@@ -0,0 +1,50 @@
+import csv
+import io
+import unittest
+
+from test.support import threading_helper
+from test.support.threading_helper import run_concurrently
+
+
+NTHREADS = 10
+
+
+@threading_helper.requires_working_threading()
+class TestCSV(unittest.TestCase):
+ def test_concurrent_reader_next(self):
+ input_rows = [f"{i},{i},{i}" for i in range(50)]
+ input_stream = io.StringIO("\n".join(input_rows))
+ reader = csv.reader(input_stream)
+ output_rows = []
+
+ def read_row():
+ for row in reader:
+ self.assertEqual(len(row), 3)
+ output_rows.append(",".join(row))
+
+ run_concurrently(worker_func=read_row, nthreads=NTHREADS)
+ self.assertSetEqual(set(input_rows), set(output_rows))
+
+ def test_concurrent_writer_writerow(self):
+ output_stream = io.StringIO()
+ writer = csv.writer(output_stream)
+ row_per_thread = 10
+ expected_rows = []
+
+ def write_row():
+ for i in range(row_per_thread):
+ writer.writerow([i, i, i])
+ expected_rows.append(f"{i},{i},{i}")
+
+ run_concurrently(worker_func=write_row, nthreads=NTHREADS)
+
+ # Rewind to the start of the stream and parse the rows
+ output_stream.seek(0)
+ output_rows = [line.strip() for line in output_stream.readlines()]
+
+ self.assertEqual(len(output_rows), NTHREADS * row_per_thread)
+ self.assertListEqual(sorted(output_rows), sorted(expected_rows))
+
+
+if __name__ == "__main__":
+ unittest.main()
diff --git
a/Misc/NEWS.d/next/Core_and_Builtins/2025-11-10-00-14-20.gh-issue-116738.IxliC_.rst
b/Misc/NEWS.d/next/Core_and_Builtins/2025-11-10-00-14-20.gh-issue-116738.IxliC_.rst
new file mode 100644
index 00000000000000..8b08bccafd73eb
--- /dev/null
+++
b/Misc/NEWS.d/next/Core_and_Builtins/2025-11-10-00-14-20.gh-issue-116738.IxliC_.rst
@@ -0,0 +1,2 @@
+Make csv module thread-safe on the :term:`free threaded <free threading>`
+build.
diff --git a/Modules/_csv.c b/Modules/_csv.c
index 87be7a8f1fb136..1f41976e95fdb1 100644
--- a/Modules/_csv.c
+++ b/Modules/_csv.c
@@ -918,7 +918,7 @@ parse_reset(ReaderObj *self)
}
static PyObject *
-Reader_iternext(PyObject *op)
+Reader_iternext_lock_held(PyObject *op)
{
ReaderObj *self = _ReaderObj_CAST(op);
@@ -985,6 +985,16 @@ Reader_iternext(PyObject *op)
return fields;
}
+static PyObject *
+Reader_iternext(PyObject *op)
+{
+ PyObject *result;
+ Py_BEGIN_CRITICAL_SECTION(op);
+ result = Reader_iternext_lock_held(op);
+ Py_END_CRITICAL_SECTION();
+ return result;
+}
+
static void
Reader_dealloc(PyObject *op)
{
@@ -1303,15 +1313,8 @@ join_append_lineterminator(WriterObj *self)
return 1;
}
-PyDoc_STRVAR(csv_writerow_doc,
-"writerow($self, row, /)\n"
-"--\n\n"
-"Construct and write a CSV record from an iterable of fields.\n"
-"\n"
-"Non-string elements will be converted to string.");
-
static PyObject *
-csv_writerow(PyObject *op, PyObject *seq)
+csv_writerow_lock_held(PyObject *op, PyObject *seq)
{
WriterObj *self = _WriterObj_CAST(op);
DialectObj *dialect = self->dialect;
@@ -1414,6 +1417,23 @@ csv_writerow(PyObject *op, PyObject *seq)
return result;
}
+PyDoc_STRVAR(csv_writerow_doc,
+"writerow($self, row, /)\n"
+"--\n\n"
+"Construct and write a CSV record from an iterable of fields.\n"
+"\n"
+"Non-string elements will be converted to string.");
+
+static PyObject *
+csv_writerow(PyObject *op, PyObject *seq)
+{
+ PyObject *result;
+ Py_BEGIN_CRITICAL_SECTION(op);
+ result = csv_writerow_lock_held(op, seq);
+ Py_END_CRITICAL_SECTION();
+ return result;
+}
+
PyDoc_STRVAR(csv_writerows_doc,
"writerows($self, rows, /)\n"
"--\n\n"
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