runtime/traceback: support tracking goroutine ancestor tracebacks with GODEBUG="tracebackancestors=N"

Currently, collecting a stack trace via runtime.Stack captures the stack for the
immediately running goroutines. This change extends those tracebacks to include
the tracebacks of their ancestors. This is done with a low memory cost and only
utilized when debug option tracebackancestors is set to a value greater than 0.

Resolves #22289

Change-Id: I7edacc62b2ee3bd278600c4a21052c351f313f3a
Reviewed-on: https://go-review.googlesource.com/70993
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 53 insertions, 0 deletions

diff --git a/src/runtime/testdata/testprog/traceback_ancestors.go b/src/runtime/testdata/testprog/traceback_ancestors.go
new file mode 100644
index 0000000000..fe57c1c157
--- /dev/null
+++ b/src/runtime/testdata/testprog/traceback_ancestors.go
@@ -0,0 +1,53 @@
+// Copyright 2018 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 (
+	"fmt"
+	"runtime"
+)
+
+func init() {
+	register("TracebackAncestors", TracebackAncestors)
+}
+
+const numGoroutines = 3
+const numFrames = 2
+
+func TracebackAncestors() {
+	w := make(chan struct{})
+	recurseThenCallGo(w, numGoroutines, numFrames)
+	<-w
+	printStack()
+	close(w)
+}
+
+func printStack() {
+	buf := make([]byte, 1024)
+	for {
+		n := runtime.Stack(buf, true)
+		if n < len(buf) {
+			fmt.Print(string(buf[:n]))
+			return
+		}
+		buf = make([]byte, 2*len(buf))
+	}
+}
+
+func recurseThenCallGo(w chan struct{}, frames int, goroutines int) {
+	if frames == 0 {
+		// Signal to TracebackAncestors that we are done recursing and starting goroutines.
+		w <- struct{}{}
+		<-w
+		return
+	}
+	if goroutines == 0 {
+		// Start the next goroutine now that there are no more recursions left
+		// for this current goroutine.
+		go recurseThenCallGo(w, frames-1, numFrames)
+		return
+	}
+	recurseThenCallGo(w, frames, goroutines-1)
+}