add regression tests for GIL ordering bug (issue #370)

Adds two reproducers that exercise the go2py/C.CString-without-GIL crash:
1. A 5000-iteration stress loop in the cgo example (Hi/Hello string returns).
2. A new gilstring example covering struct string fields, slice elements,
   and map values under repeated calls.

Author: b-long

SUPPORT_MATRIX.md

@@ -11,6 +11,7 @@ _examples/consts | yes
 _examples/cstrings | yes
 _examples/empty | yes
 _examples/funcs | yes
+_examples/gilstring | yes
 _examples/gobytes | yes
 _examples/gopygc | yes
 _examples/gostrings | yes

_examples/cgo/test.py

@@ -11,4 +11,12 @@
 print("cgo.Hi()= %s" % repr(cgo.Hi()).lstrip('u'))
 print("cgo.Hello(you)= %s" % repr(cgo.Hello("you")).lstrip('u'))
 
+# Regression test for GIL ordering bug (issue #370 / PR #386):
+# go functions returning string (go2py=C.CString) previously called C.CString
+# without holding the GIL, causing crashes under repeated calls.
+for _ in range(5000):
+    assert cgo.Hi() == 'hi from go\n'
+    assert cgo.Hello("world") == 'hello world from go\n'
+print("stress OK")
+
 print("OK")

_examples/gilstring/gilstring.go

@@ -0,0 +1,27 @@
+// Copyright 2026 The go-python Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package gilstring is a regression test for the GIL ordering bug (issue #370).
+// It exercises go2py string conversion through functions, struct fields,
+// slice elements, and map values — all of which previously ran C.CString
+// without holding the GIL, causing crashes under repeated calls.
+package gilstring
+
+// Item is a struct with a string field to exercise struct member string getters.
+type Item struct {
+	Label string
+	Count int
+}
+
+// MakeItem returns an Item with the given label.
+func MakeItem(s string) Item { return Item{Label: s, Count: len(s)} }
+
+// GetLabel returns the Label field of an Item.
+func GetLabel(i Item) string { return i.Label }
+
+// StringSlice returns a slice of strings.
+func StringSlice() []string { return []string{"alpha", "beta", "gamma"} }
+
+// StringMap returns a map with string values.
+func StringMap() map[string]string { return map[string]string{"key": "value"} }

_examples/gilstring/test.py

@@ -0,0 +1,27 @@
+# Copyright 2026 The go-python Authors.  All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file.
+
+## py2/py3 compat
+from __future__ import print_function
+
+import gilstring
+
+# Regression test for GIL ordering bug (issue #370 / PR #386).
+# Struct string fields, slice elements, and map values all previously used
+# C.CString (go2py) without the GIL held, causing crashes under repeated calls.
+N = 5000
+
+for _ in range(N):
+    item = gilstring.MakeItem("hello")
+    assert item.Label == "hello", item.Label
+
+for _ in range(N):
+    s = gilstring.StringSlice()
+    assert s[0] == "alpha", s[0]
+
+for _ in range(N):
+    m = gilstring.StringMap()
+    assert m["key"] == "value", m["key"]
+
+print("OK")

main_test.go

@@ -49,6 +49,7 @@ var (
 		"_examples/cstrings":    []string{"py3"},
 		"_examples/pkgconflict": []string{"py3"},
 		"_examples/variadic":    []string{"py3"},
+		"_examples/gilstring":   []string{"py3"},
 	}
 
 	testEnvironment = os.Environ()
@@ -555,6 +556,7 @@ func TestBindCgoPackage(t *testing.T) {
 		want: []byte(`cgo.doc: '\nPackage cgo tests bindings of CGo-based packages.\n\n'
 cgo.Hi()= 'hi from go\n'
 cgo.Hello(you)= 'hello you from go\n'
+stress OK
 OK
 `),
 	})
@@ -783,6 +785,18 @@ OK
 	})
 }
 
+func TestGilString(t *testing.T) {
+	// t.Parallel()
+	path := "_examples/gilstring"
+	testPkg(t, pkg{
+		path:   path,
+		lang:   features[path],
+		cmd:    "build",
+		extras: nil,
+		want:   []byte("OK\n"),
+	})
+}
+
 func TestPackagePrefix(t *testing.T) {
 	// t.Parallel()
 	path := "_examples/package/mypkg"