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>

src/runtime/crash_test.go

@@ -10,6 +10,7 @@ import (
 	"errors"
 	"flag"
 	"fmt"
+	"internal/profile"
 	"internal/testenv"
 	traceparse "internal/trace"
 	"io"
@@ -1100,3 +1101,77 @@ func TestNetpollWaiters(t *testing.T) {
 		t.Fatalf("output is not %q\n%s", want, output)
 	}
 }
+
+// The runtime.runfinq frame should appear in panics, even if runtime frames
+// are normally hidden (GOTRACEBACK=all).
+func TestFinalizerDeadlockPanic(t *testing.T) {
+	t.Parallel()
+	output := runTestProg(t, "testprog", "FinalizerDeadlock", "GOTRACEBACK=all", "GO_TEST_FINALIZER_DEADLOCK=panic")
+
+	want := "runtime.runfinq()"
+	if !strings.Contains(output, want) {
+		t.Errorf("output does not contain %q:\n%s", want, output)
+	}
+}
+
+// The runtime.runfinq frame should appear in runtime.Stack, even though
+// runtime frames are normally hidden.
+func TestFinalizerDeadlockStack(t *testing.T) {
+	t.Parallel()
+	output := runTestProg(t, "testprog", "FinalizerDeadlock", "GO_TEST_FINALIZER_DEADLOCK=stack")
+
+	want := "runtime.runfinq()"
+	if !strings.Contains(output, want) {
+		t.Errorf("output does not contain %q:\n%s", want, output)
+	}
+}
+
+// The runtime.runfinq frame should appear in goroutine profiles.
+func TestFinalizerDeadlockPprofProto(t *testing.T) {
+	t.Parallel()
+	output := runTestProg(t, "testprog", "FinalizerDeadlock", "GO_TEST_FINALIZER_DEADLOCK=pprof_proto")
+
+	p, err := profile.Parse(strings.NewReader(output))
+	if err != nil {
+		// Logging the binary proto data is not very nice, but it might
+		// be a text error message instead.
+		t.Logf("Output: %s", output)
+		t.Fatalf("Error parsing proto output: %v", err)
+	}
+
+	want := "runtime.runfinq"
+	for _, s := range p.Sample {
+		for _, loc := range s.Location {
+			for _, line := range loc.Line {
+				if line.Function.Name == want {
+					// Done!
+					return
+				}
+			}
+		}
+	}
+
+	t.Errorf("Profile does not contain %q:\n%s", want, p)
+}
+
+// The runtime.runfinq frame should appear in goroutine profiles (debug=1).
+func TestFinalizerDeadlockPprofDebug1(t *testing.T) {
+	t.Parallel()
+	output := runTestProg(t, "testprog", "FinalizerDeadlock", "GO_TEST_FINALIZER_DEADLOCK=pprof_debug1")
+
+	want := "runtime.runfinq+"
+	if !strings.Contains(output, want) {
+		t.Errorf("output does not contain %q:\n%s", want, output)
+	}
+}
+
+// The runtime.runfinq frame should appear in goroutine profiles (debug=2).
+func TestFinalizerDeadlockPprofDebug2(t *testing.T) {
+	t.Parallel()
+	output := runTestProg(t, "testprog", "FinalizerDeadlock", "GO_TEST_FINALIZER_DEADLOCK=pprof_debug2")
+
+	want := "runtime.runfinq()"
+	if !strings.Contains(output, want) {
+		t.Errorf("output does not contain %q:\n%s", want, output)
+	}
+}

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)
+	}
+}

src/runtime/traceback.go

@@ -1131,6 +1131,21 @@ func showfuncinfo(sf srcFunc, firstFrame bool, calleeID abi.FuncID) bool {
 		return false
 	}
 
+	// Always show runtime.runfinq as context that this goroutine is
+	// running finalizers, otherwise there is no obvious indicator.
+	//
+	// TODO(prattmic): A more general approach would be to always show the
+	// outermost frame (besides runtime.goexit), even if it is a runtime.
+	// Hiding the outermost frame allows the apparent outermost frame to
+	// change across different traces, which seems impossible.
+	//
+	// Unfortunately, implementing this requires looking ahead at the next
+	// frame, which goes against traceback's incremental approach (see big
+	// coment in traceback1).
+	if sf.funcID == abi.FuncID_runfinq {
+		return true
+	}
+
 	name := sf.name()
 
 	// Special case: always show runtime.gopanic frame