From 012b2b45cf97bb73350622c29a932f63d17fbb6f Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Thu, 23 Apr 2026 18:12:34 +0300
Subject: [PATCH] cgen: fix sporadic runtime failure of assertion with `-gc
 boehm_full_opt` on windows (the default gc mode) (fixes #17916)

---
 vlib/v/gen/c/dumpexpr.v                            | 14 ++++++++++++++
 .../gen/c/testdata/dump_gc_keep_alive.c.must_have  |  2 ++
 vlib/v/gen/c/testdata/dump_gc_keep_alive.vv        |  6 ++++++
 3 files changed, 22 insertions(+)
 create mode 100644 vlib/v/gen/c/testdata/dump_gc_keep_alive.c.must_have
 create mode 100644 vlib/v/gen/c/testdata/dump_gc_keep_alive.vv

diff --git a/vlib/v/gen/c/dumpexpr.v b/vlib/v/gen/c/dumpexpr.v
index 779fa970a..1a058a245 100644
--- a/vlib/v/gen/c/dumpexpr.v
+++ b/vlib/v/gen/c/dumpexpr.v
@@ -4,6 +4,13 @@ import v.ast
 import v.util
 import strings
 
+fn (mut g Gen) dump_arg_needs_gc_pin(typ ast.Type) bool {
+	if g.pref.gc_mode !in [.boehm_full, .boehm_incr, .boehm_full_opt, .boehm_incr_opt] {
+		return false
+	}
+	return typ.is_any_kind_of_pointer() || g.contains_ptr(typ)
+}
+
 fn (mut g Gen) dump_expr(node ast.DumpExpr) {
 	sexpr := ctoslit(node.expr.str())
 	fpath := cestring(g.file.path)
@@ -418,6 +425,13 @@ fn (mut g Gen) dump_expr_definitions() {
 		}
 		dump_fns.writeln('\tbuiltin___writeln_to_fd(2, value);')
 		dump_fns.writeln('\tbuiltin__flush_stderr();')
+		if g.dump_arg_needs_gc_pin(typ) {
+			if is_ptr {
+				dump_fns.writeln('\tGC_reachable_here(dump_arg);')
+			} else {
+				dump_fns.writeln('\tGC_reachable_here(&dump_arg);')
+			}
+		}
 		surrounder.builder_write_afters(mut dump_fns)
 		if is_fixed_arr_ret && !is_ptr {
 			tmp_var := g.new_tmp_var()
diff --git a/vlib/v/gen/c/testdata/dump_gc_keep_alive.c.must_have b/vlib/v/gen/c/testdata/dump_gc_keep_alive.c.must_have
new file mode 100644
index 000000000..9ad2acd98
--- /dev/null
+++ b/vlib/v/gen/c/testdata/dump_gc_keep_alive.c.must_have
@@ -0,0 +1,2 @@
+Array_int _v_dump_expr_Array_int(string fpath, int line, string sexpr, Array_int dump_arg) {
+GC_reachable_here(&dump_arg);
diff --git a/vlib/v/gen/c/testdata/dump_gc_keep_alive.vv b/vlib/v/gen/c/testdata/dump_gc_keep_alive.vv
new file mode 100644
index 000000000..79d795d04
--- /dev/null
+++ b/vlib/v/gen/c/testdata/dump_gc_keep_alive.vv
@@ -0,0 +1,6 @@
+// vtest vflags: -gc boehm_full_opt
+
+fn main() {
+	arr := [1, 2, 3]
+	dump(arr)
+}
-- 
2.39.5