runtime: always show runfinq in traceback

Today, runtime.runfinq is hidden whenever runtime frames are hidden.

However this frame serves as a hint that this goroutine is running
finalizers, which is otherwise unclear, but can be useful when debugging
issues with finalizers.

Fixes #73011.

Change-Id: I6a6a636cb63951fbe1fefc3554fe9cea5d0a0fb6
Reviewed-on: https://go-review.googlesource.com/c/go/+/660295
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

1 files changed, 74 insertions, 0 deletions

diff --git a/src/runtime/testdata/testprog/finalizer_deadlock.go b/src/runtime/testdata/testprog/finalizer_deadlock.go
new file mode 100644
index 0000000000..a55145fa15
--- /dev/null
+++ b/src/runtime/testdata/testprog/finalizer_deadlock.go
@@ -0,0 +1,74 @@
+// Copyright 2025 The Go 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 main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"runtime"
+	"runtime/pprof"
+)
+
+var finalizerDeadlockMode = flag.String("finalizer-deadlock-mode", "panic", "Trigger mode of FinalizerDeadlock")
+
+func init() {
+	register("FinalizerDeadlock", FinalizerDeadlock)
+}
+
+func FinalizerDeadlock() {
+	flag.Parse()
+
+	started := make(chan struct{})
+	b := new([16]byte)
+	runtime.SetFinalizer(b, func(*[16]byte) {
+		started <- struct{}{}
+		select {}
+	})
+	b = nil
+
+	runtime.GC()
+
+	<-started
+	// We know the finalizer has started running. The goroutine might still
+	// be running or it may now be blocked. Either is fine, the goroutine
+	// should appear in stacks either way.
+
+	mode := os.Getenv("GO_TEST_FINALIZER_DEADLOCK")
+	switch mode {
+	case "panic":
+		panic("panic")
+	case "stack":
+		buf := make([]byte, 4096)
+		for {
+			n := runtime.Stack(buf, true)
+			if n >= len(buf) {
+				buf = make([]byte, 2*len(buf))
+				continue
+			}
+			buf = buf[:n]
+			break
+		}
+		fmt.Printf("%s\n", string(buf))
+	case "pprof_proto":
+		if err := pprof.Lookup("goroutine").WriteTo(os.Stdout, 0); err != nil {
+			fmt.Fprintf(os.Stderr, "Error writing profile: %v\n", err)
+			os.Exit(1)
+		}
+	case "pprof_debug1":
+		if err := pprof.Lookup("goroutine").WriteTo(os.Stdout, 1); err != nil {
+			fmt.Fprintf(os.Stderr, "Error writing profile: %v\n", err)
+			os.Exit(1)
+		}
+	case "pprof_debug2":
+		if err := pprof.Lookup("goroutine").WriteTo(os.Stdout, 2); err != nil {
+			fmt.Fprintf(os.Stderr, "Error writing profile: %v\n", err)
+			os.Exit(1)
+		}
+	default:
+		fmt.Fprintf(os.Stderr, "Unknown mode %q. GO_TEST_FINALIZER_DEADLOCK must be one of panic, stack, pprof_proto, pprof_debug1, pprof_debug2\n", mode)
+		os.Exit(1)
+	}
+}