python · yoney · Dec 18, 2025 · Dec 19, 2025 · Dec 22, 2025 · Dec 22, 2025
diff --git a/Lib/test/test_free_threading/test_lzma.py b/Lib/test/test_free_threading/test_lzma.py
@@ -45,11 +45,24 @@ def worker():
             data = lzd.decompress(compressed, chunk_size)
             self.assertEqual(len(data), chunk_size)
             output.append(data)
+            # Read attributes concurrently with other threads decompressing
+            self.assertEqual(lzd.check, lzma.CHECK_CRC64)
+            self.assertIsInstance(lzd.eof, bool)
+            self.assertIsInstance(lzd.needs_input, bool)
+            self.assertIsInstance(lzd.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.assertSetEqual(set(output), set(chunks))
+        self.assertEqual(lzd.check, lzma.CHECK_CRC64)
+        self.assertTrue(lzd.eof)
+        self.assertFalse(lzd.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(lzd.unused_data), len(compressed) * (NTHREADS - 1)
+        )
 
 
 if __name__ == "__main__":

diff --git a/Misc/NEWS.d/next/Library/2025-12-19-12-38-01.gh-issue-116738.iMt3Ol.rst b/Misc/NEWS.d/next/Library/2025-12-19-12-38-01.gh-issue-116738.iMt3Ol.rst
@@ -0,0 +1,2 @@
+Make the attributes in :mod:`lzma` thread-safe on the :term:`free threaded
+<free threading>` build.
diff --git a/Modules/_lzmamodule.c b/Modules/_lzmamodule.c
@@ -20,6 +20,7 @@
 #include "pycore_long.h"          // _PyLong_UInt32_Converter()
 // Blocks output buffer wrappers
 #include "pycore_blocks_output_buffer.h"
+#include "pycore_pyatomic_ft_wrappers.h" // FT_ATOMIC_STORE_*_RELAXED
 
 #if OUTPUT_BUFFER_MAX_BLOCK_SIZE > SIZE_MAX
     #error "The maximum block size accepted by liblzma is SIZE_MAX."
@@ -948,10 +949,10 @@ decompress_buf(Decompressor *d, Py_ssize_t max_length)
             goto error;
         }
         if (lzret == LZMA_GET_CHECK || lzret == LZMA_NO_CHECK) {
-            d->check = lzma_get_check(&d->lzs);
+            FT_ATOMIC_STORE_INT_RELAXED(d->check, lzma_get_check(&d->lzs));
         }
         if (lzret == LZMA_STREAM_END) {
-            d->eof = 1;
+            FT_ATOMIC_STORE_CHAR_RELAXED(d->eof, 1);
             break;
         } else if (lzs->avail_out == 0) {
             /* Need to check lzs->avail_out before lzs->avail_in.
@@ -1038,13 +1039,14 @@ decompress(Decompressor *d, uint8_t *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 (lzs->avail_in > 0) {
-            Py_XSETREF(d->unused_data,
-                      PyBytes_FromStringAndSize((char *)lzs->next_in, lzs->avail_in));
-            if (d->unused_data == NULL) {
+            PyObject *unused_data = PyBytes_FromStringAndSize(
+                (char *)lzs->next_in, lzs->avail_in);
+            if (unused_data == NULL) {
                 goto error;
             }
+            Py_XSETREF(d->unused_data, unused_data);
         }
     }
     else if (lzs->avail_in == 0) {
@@ -1054,17 +1056,17 @@ decompress(Decompressor *d, uint8_t *data, size_t len, Py_ssize_t max_length)
             /* (avail_in==0 && avail_out==0)
                Maybe lzs's internal state still have a few bytes can
                be output, try to output them next time. */
-            d->needs_input = 0;
+            FT_ATOMIC_STORE_CHAR_RELAXED(d->needs_input, 0);
 
             /* If max_length < 0, lzs->avail_out always > 0 */
             assert(max_length >= 0);
         } else {
             /* Input buffer exhausted, output buffer has space. */
-            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
@@ -1314,15 +1316,33 @@ PyDoc_STRVAR(Decompressor_needs_input_doc,
 PyDoc_STRVAR(Decompressor_unused_data_doc,
 "Data found after the end of the compressed stream.");
 
+static PyObject *
+Decompressor_unused_data_get(PyObject *op, void *Py_UNUSED(ignored))
+{
+    Decompressor *self = Decompressor_CAST(op);
+    if (!FT_ATOMIC_LOAD_CHAR_RELAXED(self->eof)) {
+        return Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);
+    }
+    PyMutex_Lock(&self->mutex);
+    assert(self->unused_data != NULL);
+    PyObject *result = Py_NewRef(self->unused_data);
+    PyMutex_Unlock(&self->mutex);
+    return result;
+}
+
+static PyGetSetDef Decompressor_getset[] = {
+    {"unused_data", Decompressor_unused_data_get, NULL,
+     Decompressor_unused_data_doc},
+    {NULL},
+};
+
 static PyMemberDef Decompressor_members[] = {
     {"check", Py_T_INT, offsetof(Decompressor, check), Py_READONLY,
      Decompressor_check_doc},
     {"eof", Py_T_BOOL, offsetof(Decompressor, eof), Py_READONLY,
      Decompressor_eof_doc},
     {"needs_input", Py_T_BOOL, offsetof(Decompressor, needs_input), Py_READONLY,
      Decompressor_needs_input_doc},
-    {"unused_data", Py_T_OBJECT_EX, offsetof(Decompressor, unused_data), Py_READONLY,
-     Decompressor_unused_data_doc},
     {NULL}
 };
 
@@ -1332,6 +1352,7 @@ static PyType_Slot lzma_decompressor_type_slots[] = {
     {Py_tp_new, _lzma_LZMADecompressor},
     {Py_tp_doc, (char *)_lzma_LZMADecompressor__doc__},
     {Py_tp_members, Decompressor_members},
+    {Py_tp_getset, Decompressor_getset},
     {0, 0}
 };
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Make the attributes in :mod:`lzma` thread-safe on the :term:`free threaded
		<free threading>` build.