From 21e8cf56c8fdbc71d752b75857964f26570547a0 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 25 Mar 2026 16:42:18 +0300
Subject: [PATCH] json: fix decode() segfault on recursive struct with ?&t
 (fixes #26385)

---
 vlib/json/tests/json_decode_struct_ptr_test.v | 20 +++++++++++++++++++
 vlib/v/gen/c/json.v                           |  9 ++++++++-
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/vlib/json/tests/json_decode_struct_ptr_test.v b/vlib/json/tests/json_decode_struct_ptr_test.v
index 954d025c2..509f06031 100644
--- a/vlib/json/tests/json_decode_struct_ptr_test.v
+++ b/vlib/json/tests/json_decode_struct_ptr_test.v
@@ -7,6 +7,13 @@ mut:
 	reply_to &Message
 }
 
+struct OptionalMessage {
+mut:
+	id       int
+	text     string
+	reply_to ?&OptionalMessage
+}
+
 fn test_main() {
 	mut json_data := '{"id": 1, "text": "Hello", "reply_to": {"id": 2, "text": "Hi"}}'
 	mut message := json.decode(Message, json_data)!
@@ -20,3 +27,16 @@ fn test_main() {
 	message = json.decode(Message, json_data)!
 	assert message.reply_to.reply_to.id == 5
 }
+
+fn test_recursive_optional_struct_ptr() {
+	json_data := '{"id": 1, "text": "Hello", "reply_to": {"id": 2, "text": "Hi", "reply_to": null}}'
+	message := json.decode(OptionalMessage, json_data)!
+	reply := message.reply_to or {
+		assert false
+		return
+	}
+
+	assert reply.id == 2
+	assert reply.text == 'Hi'
+	assert reply.reply_to == none
+}
diff --git a/vlib/v/gen/c/json.v b/vlib/v/gen/c/json.v
index 8a277fcdc..acbcb0759 100644
--- a/vlib/v/gen/c/json.v
+++ b/vlib/v/gen/c/json.v
@@ -129,7 +129,14 @@ ${dec_fn_dec} {
 
 			base_type := utyp.clear_flag(.option)
 			base_type_str := g.styp(base_type)
-			dec.writeln('\tbuiltin___option_ok(&(${base_type_str}[]){ ${g.type_default(base_type)} }, (${styp}*)&res, sizeof(${base_type_str}));\n')
+			// Optional struct pointers need storage before field decoding writes through them.
+			base_value := if base_type.is_ptr() && sym.info is ast.Struct {
+				ptr_styp := g.styp(base_type.set_nr_muls(base_type.nr_muls() - 1))
+				'HEAP(${ptr_styp}, {0})'
+			} else {
+				g.type_default(base_type)
+			}
+			dec.writeln('\tbuiltin___option_ok(&(${base_type_str}[]){ ${base_value} }, (${styp}*)&res, sizeof(${base_type_str}));\n')
 		}
 
 		extern_str := if g.pref.parallel_cc { 'extern ' } else { '' }
-- 
2.39.5