From 2f8c03024172d88570f842d7ec763fa2d7e8571e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Janek=20Nouvertn=C3=A9?= <provinzkraut@posteo.de>
Date: Sun, 26 Oct 2025 13:16:13 +0100
Subject: [PATCH 1/2] fix

---
 msgspec/_core.c           |  7 ++++++-
 tests/test_struct_meta.py | 23 +++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/msgspec/_core.c b/msgspec/_core.c
index 6b6ad1c4..72957290 100644
--- a/msgspec/_core.c
+++ b/msgspec/_core.c
@@ -1886,6 +1886,7 @@ Meta_new(PyTypeObject *type, PyObject *args, PyObject *kwargs) {
     Meta *out = (Meta *)Meta_Type.tp_alloc(&Meta_Type, 0);
     if (out == NULL) return NULL;
 
+// set fields on Meta and increase their refcount
 #define SET_FIELD(x) do { Py_XINCREF(x); out->x = x; } while(0)
     SET_FIELD(gt);
     SET_FIELD(ge);
@@ -1893,7 +1894,6 @@ Meta_new(PyTypeObject *type, PyObject *args, PyObject *kwargs) {
     SET_FIELD(le);
     SET_FIELD(multiple_of);
     SET_FIELD(pattern);
-    SET_FIELD(regex);
     SET_FIELD(min_length);
     SET_FIELD(max_length);
     SET_FIELD(tz);
@@ -1903,6 +1903,11 @@ Meta_new(PyTypeObject *type, PyObject *args, PyObject *kwargs) {
     SET_FIELD(extra_json_schema);
     SET_FIELD(extra);
 #undef SET_FIELD
+
+    // set fields on Meta without increasing their refcount
+    // regex was created by a PyObject_CallOneArg call, so refcount started out as 1; no need to increase
+    out->regex = regex;
+
     return (PyObject *)out;
 }
 
diff --git a/tests/test_struct_meta.py b/tests/test_struct_meta.py
index ebd9b0b1..6e32469e 100644
--- a/tests/test_struct_meta.py
+++ b/tests/test_struct_meta.py
@@ -1,4 +1,8 @@
 """Tests for the exposed StructMeta metaclass."""
+import gc
+import re
+import secrets
+import uuid
 
 import pytest
 import msgspec
@@ -362,3 +366,22 @@ class Container(metaclass=EncoderMeta):
     assert decoded.count == 1
     assert decoded.item.id == 123
     assert decoded.item.name == "test"
+
+
+def test_struct_meta_pattern_ref_leak():
+    # ensure that we're not keeping around references to re.Pattern longer than necessary
+    # see https://github.com/jcrist/msgspec/pull/899 for details
+
+    # clear cache to get a baseline
+    re.purge()
+
+    # use a random string to create a pattern, to ensure there can never be an overlap
+    # with any cached pattern
+    pattern_string = secrets.token_hex()
+    msgspec.Meta(pattern=pattern_string)
+    # purge cache and gc again
+    re.purge()
+    gc.collect()
+    # there should be no re.Pattern any more with our pattern anymore. if there is, it's
+    # being kept alive by some reference
+    assert not any(o for o in gc.get_objects() if isinstance(o, re.Pattern) and o.pattern == pattern_string)

From f3c3acc5d8a5df961dbd9c7691e33b2f4ef94921 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Janek=20Nouvertn=C3=A9?= <provinzkraut@posteo.de>
Date: Sun, 26 Oct 2025 19:24:50 +0100
Subject: [PATCH 2/2] formatting

---
 tests/test_struct_meta.py | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/tests/test_struct_meta.py b/tests/test_struct_meta.py
index 6e32469e..ed0ab740 100644
--- a/tests/test_struct_meta.py
+++ b/tests/test_struct_meta.py
@@ -1,8 +1,8 @@
 """Tests for the exposed StructMeta metaclass."""
+
 import gc
 import re
 import secrets
-import uuid
 
 import pytest
 import msgspec
@@ -384,4 +384,8 @@ def test_struct_meta_pattern_ref_leak():
     gc.collect()
     # there should be no re.Pattern any more with our pattern anymore. if there is, it's
     # being kept alive by some reference
-    assert not any(o for o in gc.get_objects() if isinstance(o, re.Pattern) and o.pattern == pattern_string)
+    assert not any(
+        o
+        for o in gc.get_objects()
+        if isinstance(o, re.Pattern) and o.pattern == pattern_string
+    )