runtime: limit total frames traversed during stack walking

src/runtime/testdata/testprog/deepcontext.go

@@ -0,0 +1,58 @@
+// 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 (
+	"context"
+	"fmt"
+	"runtime"
+	"time"
+)
+
+func init() {
+	register("DeepContextChain", DeepContextChain)
+}
+
+// DeepContextChain tests that traceback completes in reasonable time
+// even with very deep context chains (issue #75583).
+func DeepContextChain() {
+	// Create a context chain deep enough to trigger the frame limit.
+	// We use 2000 layers to ensure we exceed the limit (1024) while
+	// keeping the test fast.
+	const depth = 2000
+	ctx := context.Background()
+	for i := 0; i < depth; i++ {
+		ctx = context.WithValue(ctx, i, i)
+	}
+
+	// Start profiling to trigger stack walking with deep context
+	start := time.Now()
+
+	// Get a stack trace multiple times
+	// This simulates what happens during CPU profiling
+	for i := 0; i < 10; i++ {
+		var pcs [64]uintptr
+		n := runtime.Callers(0, pcs[:])
+		if n == 0 {
+			fmt.Println("FAIL: got 0 callers")
+			return
+		}
+
+		// Call Deadline to ensure the deep context chain is traversed
+		// during any potential stack walking
+		_, _ = ctx.Deadline()
+	}
+
+	elapsed := time.Since(start)
+
+	// The test should complete quickly. If it takes more than 1 second,
+	// something is wrong (likely walking too many frames).
+	if elapsed > time.Second {
+		fmt.Printf("FAIL: test took %v, expected < 1s\n", elapsed)
+		return
+	}
+
+	fmt.Println("OK")
+}

src/runtime/traceback.go

@@ -620,7 +620,18 @@ func (u *unwinder) cgoCallers(pcBuf []uintptr) int {
 func tracebackPCs(u *unwinder, skip int, pcBuf []uintptr) int {
 	var cgoBuf [32]uintptr
 	n := 0
+	// maxTotalFrames limits the total number of frames we'll walk through,
+	// including wrapper frames. This prevents excessive CPU time when
+	// unwinding stacks with very deep context chains (issue #75583).
+	// The limit of 1024 is high enough for normal stacks while preventing
+	// multi-second delays from walking millions of wrapper frames.
+	const maxTotalFrames = 1024
+	totalFrames := 0
 	for ; n < len(pcBuf) && u.valid(); u.next() {
+		totalFrames++
+		if totalFrames >= maxTotalFrames {
+			break
+		}
 		f := u.frame.fn
 		cgoN := u.cgoCallers(cgoBuf[:])
 

src/runtime/traceback_test.go

@@ -882,3 +882,16 @@ func TestSetCgoTracebackNoCgo(t *testing.T) {
 		t.Fatalf("want %s, got %s\n", want, output)
 	}
 }
+
+// TestDeepContextChainTraceback tests that tracebackPCs completes in reasonable
+// time even with very deep context chains (issue #75583).
+// This test creates a context chain deep enough to trigger the frame limit
+// and verifies that tracing completes quickly.
+func TestDeepContextChainTraceback(t *testing.T) {
+	output := runTestProg(t, "testprog", "DeepContextChain")
+	if !strings.Contains(output, "OK") {
+		t.Fatalf("expected OK, got:\n%s", output)
+	}
+	// The test should complete in reasonable time. If it hangs or takes
+	// multiple seconds, the frame limit is not working.
+}