From 9be36ca8bb06c684052cb2021e58df316fe335d7 Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Fri, 9 May 2025 20:17:25 +0100
Subject: [PATCH 1/2] Make zstd types immutable

---
 Modules/_zstd/_zstdmodule.c  | 3 +++
 Modules/_zstd/decompressor.c | 3 ++-
 Modules/_zstd/zstddict.c     | 3 ++-
 3 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/Modules/_zstd/_zstdmodule.c b/Modules/_zstd/_zstdmodule.c
index 4004bbb3461393..0af2045fd01434 100644
--- a/Modules/_zstd/_zstdmodule.c
+++ b/Modules/_zstd/_zstdmodule.c
@@ -682,6 +682,9 @@ do {                                                                         \
     ADD_INT_CONST_TO_TYPE(mod_state->ZstdCompressor_type,
                           "FLUSH_FRAME", ZSTD_e_end);
 
+    /* Make ZstdCompressor immutable */
+    PyType_Freeze(mod_state->ZstdCompressor_type);
+
 #undef ADD_TYPE
 #undef ADD_INT_MACRO
 #undef ADD_ZSTD_COMPRESSOR_INT_CONST
diff --git a/Modules/_zstd/decompressor.c b/Modules/_zstd/decompressor.c
index d141e68efded26..d172d54e3e86dd 100644
--- a/Modules/_zstd/decompressor.c
+++ b/Modules/_zstd/decompressor.c
@@ -886,6 +886,7 @@ static PyType_Slot ZstdDecompressor_slots[] = {
 PyType_Spec zstd_decompressor_type_spec = {
     .name = "_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,
 };
diff --git a/Modules/_zstd/zstddict.c b/Modules/_zstd/zstddict.c
index b411eadfc95625..61d198d5a37090 100644
--- a/Modules/_zstd/zstddict.c
+++ b/Modules/_zstd/zstddict.c
@@ -281,6 +281,7 @@ static PyType_Slot zstddict_slots[] = {
 PyType_Spec zstd_dict_type_spec = {
     .name = "_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,
 };

From a02c4274b70733eb3e43b607123d3bd336156c3c Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Sat, 10 May 2025 23:11:55 +0100
Subject: [PATCH 2/2] Add comment

---
 Modules/_zstd/_zstdmodule.c | 2 +-
 Modules/_zstd/compressor.c  | 3 +++
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/Modules/_zstd/_zstdmodule.c b/Modules/_zstd/_zstdmodule.c
index 36a891a04bd336..c3852fe89732bc 100644
--- a/Modules/_zstd/_zstdmodule.c
+++ b/Modules/_zstd/_zstdmodule.c
@@ -679,7 +679,7 @@ do {                                                                         \
     ADD_INT_CONST_TO_TYPE(mod_state->ZstdCompressor_type,
                           "FLUSH_FRAME", ZSTD_e_end);
 
-    /* Make ZstdCompressor immutable */
+    /* Make ZstdCompressor immutable (set Py_TPFLAGS_IMMUTABLETYPE) */
     PyType_Freeze(mod_state->ZstdCompressor_type);
 
 #undef ADD_TYPE
diff --git a/Modules/_zstd/compressor.c b/Modules/_zstd/compressor.c
index e70eb637b29f3e..355a27d2734a1b 100644
--- a/Modules/_zstd/compressor.c
+++ b/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,
 };