gh-116738: Make _bz2 module thread-safe (gh-142756)

Make the attributes in _bz2 module thread-safe on the free-threading build.
Attributes (eof, needs_input, unused_data) are now stored atomically or
accessed via mutex-protected getters.

Lib/test/test_free_threading/test_bz2.py

@@ -42,11 +42,22 @@ class TestBZ2(unittest.TestCase):
             data = bz2d.decompress(compressed, chunk_size)
             self.assertEqual(len(data), chunk_size)
             output.append(data)
+            # Read attributes concurrently with other threads decompressing
+            self.assertIsInstance(bz2d.eof, bool)
+            self.assertIsInstance(bz2d.needs_input, bool)
+            self.assertIsInstance(bz2d.unused_data, bytes)
 
         run_concurrently(worker_func=worker, nthreads=NTHREADS)
         self.assertEqual(len(output), NTHREADS)
         # Verify the expected chunks (order doesn't matter due to append race)
         self.assertEqual(set(output), set(chunks))
+        self.assertTrue(bz2d.eof)
+        self.assertFalse(bz2d.needs_input)
+        # Each thread added full compressed data to the buffer, but only 1 copy
+        # is consumed to produce the output. The rest remains as unused_data.
+        self.assertEqual(
+            len(bz2d.unused_data), len(compressed) * (NTHREADS - 1)
+        )
 
 
 if __name__ == "__main__":

Misc/NEWS.d/next/Core_and_Builtins/2025-12-15-03-20-24.gh-issue-116738.NNHiTK.rst

@@ -0,0 +1,2 @@
+Make the attributes in :mod:`bz2` thread-safe on the :term:`free threaded
+<free threading>` build.

Modules/_bz2module.c

@@ -12,6 +12,7 @@
 
 // Blocks output buffer wrappers
 #include "pycore_blocks_output_buffer.h"
+#include "pycore_pyatomic_ft_wrappers.h" // FT_ATOMIC_STORE_CHAR_RELAXED
 
 #if OUTPUT_BUFFER_MAX_BLOCK_SIZE > UINT32_MAX
     #error "The maximum block size accepted by libbzip2 is UINT32_MAX."
@@ -437,7 +438,7 @@ decompress_buf(BZ2Decompressor *d, Py_ssize_t max_length)
         if (catch_bz2_error(bzret))
             goto error;
         if (bzret == BZ_STREAM_END) {
-            d->eof = 1;
+            FT_ATOMIC_STORE_CHAR_RELAXED(d->eof, 1);
             break;
         } else if (d->bzs_avail_in_real == 0) {
             break;
@@ -521,7 +522,7 @@ decompress(BZ2Decompressor *d, char *data, size_t len, Py_ssize_t max_length)
     }
 
     if (d->eof) {
-        d->needs_input = 0;
+        FT_ATOMIC_STORE_CHAR_RELAXED(d->needs_input, 0);
         if (d->bzs_avail_in_real > 0) {
             Py_XSETREF(d->unused_data,
                       PyBytes_FromStringAndSize(bzs->next_in, d->bzs_avail_in_real));
@@ -531,10 +532,10 @@ decompress(BZ2Decompressor *d, char *data, size_t len, Py_ssize_t max_length)
     }
     else if (d->bzs_avail_in_real == 0) {
         bzs->next_in = NULL;
-        d->needs_input = 1;
+        FT_ATOMIC_STORE_CHAR_RELAXED(d->needs_input, 1);
     }
     else {
-        d->needs_input = 0;
+        FT_ATOMIC_STORE_CHAR_RELAXED(d->needs_input, 0);
 
         /* If we did not use the input buffer, we now have
            to copy the tail from the caller's buffer into the
@@ -682,11 +683,28 @@ PyDoc_STRVAR(BZ2Decompressor_unused_data__doc__,
 PyDoc_STRVAR(BZ2Decompressor_needs_input_doc,
 "True if more input is needed before more decompressed data can be produced.");
 
+static PyObject *
+BZ2Decompressor_unused_data_get(PyObject *op, void *Py_UNUSED(ignored))
+{
+    BZ2Decompressor *self = _BZ2Decompressor_CAST(op);
+    PyMutex_Lock(&self->mutex);
+    PyObject *result = Py_XNewRef(self->unused_data);
+    PyMutex_Unlock(&self->mutex);
+    if (result == NULL) {
+        PyErr_SetString(PyExc_AttributeError, "unused_data");
+    }
+    return result;
+}
+
+static PyGetSetDef BZ2Decompressor_getset[] = {
+    {"unused_data", BZ2Decompressor_unused_data_get, NULL,
+     BZ2Decompressor_unused_data__doc__},
+    {NULL},
+};
+
 static PyMemberDef BZ2Decompressor_members[] = {
     {"eof", Py_T_BOOL, offsetof(BZ2Decompressor, eof),
      Py_READONLY, BZ2Decompressor_eof__doc__},
-    {"unused_data", Py_T_OBJECT_EX, offsetof(BZ2Decompressor, unused_data),
-     Py_READONLY, BZ2Decompressor_unused_data__doc__},
     {"needs_input", Py_T_BOOL, offsetof(BZ2Decompressor, needs_input), Py_READONLY,
      BZ2Decompressor_needs_input_doc},
     {NULL}
@@ -697,6 +715,7 @@ static PyType_Slot bz2_decompressor_type_slots[] = {
     {Py_tp_methods, BZ2Decompressor_methods},
     {Py_tp_doc, (char *)_bz2_BZ2Decompressor__doc__},
     {Py_tp_members, BZ2Decompressor_members},
+    {Py_tp_getset, BZ2Decompressor_getset},
     {Py_tp_new, _bz2_BZ2Decompressor},
     {0, 0}
 };