internal/stack: add the leak test

This adds a package for dealing with stacks in tests.
The only function at this time is NoLeak which verifies that a test
does not leak any goroutines, and prints a stack summary if it does.

Change-Id: I284b846f422334b745bef0e96d84138295120a62
Reviewed-on: https://go-review.googlesource.com/c/tools/+/232681
Run-TryBot: Ian Cottrell <iancottrell@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

internal/jsonrpc2/jsonrpc2_test.go

@@ -18,6 +18,7 @@ import (
 
 	"golang.org/x/tools/internal/event/export/eventtest"
 	"golang.org/x/tools/internal/jsonrpc2"
+	"golang.org/x/tools/internal/stack/stacktest"
 )
 
 var logRPC = flag.Bool("logrpc", false, "Enable jsonrpc2 communication logging")
@@ -62,6 +63,7 @@ func (test *callTest) verifyResults(t *testing.T, results interface{}) {
 }
 
 func TestCall(t *testing.T) {
+	stacktest.NoLeak(t)
 	ctx := eventtest.NewContext(context.Background(), t)
 	for _, headers := range []bool{false, true} {
 		name := "Plain"

internal/jsonrpc2/serve_test.go

@@ -10,9 +10,12 @@ import (
 	"sync"
 	"testing"
 	"time"
+
+	"golang.org/x/tools/internal/stack/stacktest"
 )
 
 func TestIdleTimeout(t *testing.T) {
+	stacktest.NoLeak(t)
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 	defer cancel()
 

internal/stack/stacktest/stacktest.go

@@ -0,0 +1,50 @@
+// 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 stacktest
+
+import (
+	"testing"
+	"time"
+
+	"golang.org/x/tools/internal/stack"
+)
+
+//this is only needed to support pre 1.14 when testing.TB did not have Cleanup
+type withCleanup interface {
+	Cleanup(func())
+}
+
+// the maximum amount of time to wait for goroutines to clean themselves up.
+const maxWait = time.Second
+
+// NoLeak checks that a test (or benchmark) does not leak any goroutines.
+func NoLeak(t testing.TB) {
+	c, ok := t.(withCleanup)
+	if !ok {
+		return
+	}
+	before := stack.Capture()
+	c.Cleanup(func() {
+		var delta stack.Delta
+		start := time.Now()
+		delay := time.Millisecond
+		for {
+			after := stack.Capture()
+			delta = stack.Diff(before, after)
+			if len(delta.After) == 0 {
+				// no leaks
+				return
+			}
+			if time.Since(start) > maxWait {
+				break
+			}
+			time.Sleep(delay)
+			delay *= 2
+		}
+		// it's been long enough, and leaks are still present
+		summary := stack.Summarize(delta.After)
+		t.Errorf("goroutine leak detected:\n%+v", summary)
+	})
+}