gh-145301: Fix double-free in hmac module initialization

Author: krylosov-aa

Misc/NEWS.d/next/Library/2026-02-27-19-00-26.gh-issue-145301.2Wih4b.rst

@@ -1,3 +1 @@
-Fix a double-free bug in :mod:`_hashlib` module initialization when
-``_Py_hashtable_set()`` fails while adding an algorithm alias to the hash
-table after the primary name was already added
+Fix a crash when :mod:`hashlib` or :mod:`hmac` C extension module initialization fails.

Modules/hmacmodule.c

@@ -1457,7 +1457,10 @@ py_hmac_hinfo_ht_new(void)
         do {                                                    \
             int rc = py_hmac_hinfo_ht_add(table, KEY, value);   \
             if (rc < 0) {                                       \
-                PyMem_Free(value);                              \
+                /* entry may already be in ht, will be freed by _Py_hashtable_destroy() */ \
+                if (value->refcnt == 0) {                       \
+                    PyMem_Free(value);                          \
+                }                                               \
                 goto error;                                     \
             }                                                   \
             else if (rc == 1) {                                 \