gh-132983: Make zstd types immutable (#133784)

Modules/_zstd/_zstdmodule.c

@@ -679,6 +679,9 @@ do {                                                                         \
     ADD_INT_CONST_TO_TYPE(mod_state->ZstdCompressor_type,
                           "FLUSH_FRAME", ZSTD_e_end);
 
+    /* Make ZstdCompressor immutable (set Py_TPFLAGS_IMMUTABLETYPE) */
+    PyType_Freeze(mod_state->ZstdCompressor_type);
+
 #undef ADD_TYPE
 #undef ADD_INT_MACRO
 #undef ADD_ZSTD_COMPRESSOR_INT_CONST

Modules/_zstd/compressor.c

@@ -729,6 +729,9 @@ static PyType_Slot zstdcompressor_slots[] = {
 PyType_Spec zstd_compressor_type_spec = {
     .name = "compression.zstd.ZstdCompressor",
     .basicsize = sizeof(ZstdCompressor),
+    // Py_TPFLAGS_IMMUTABLETYPE is not used here as several
+    // associated constants need to be added to the type.
+    // PyType_Freeze is called later to set the flag.
     .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
     .slots = zstdcompressor_slots,
 };

Modules/_zstd/decompressor.c

@@ -902,6 +902,7 @@ static PyType_Slot ZstdDecompressor_slots[] = {
 PyType_Spec zstd_decompressor_type_spec = {
     .name = "compression.zstd.ZstdDecompressor",
     .basicsize = sizeof(ZstdDecompressor),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE
+             | Py_TPFLAGS_HAVE_GC,
     .slots = ZstdDecompressor_slots,
 };

Modules/_zstd/zstddict.c

@@ -278,6 +278,7 @@ static PyType_Slot zstddict_slots[] = {
 PyType_Spec zstd_dict_type_spec = {
     .name = "compression.zstd.ZstdDict",
     .basicsize = sizeof(ZstdDict),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE
+             | Py_TPFLAGS_HAVE_GC,
     .slots = zstddict_slots,
 };