Only check for ninja stuckness after it's been running for a bit

Bug: 62580037
Test: rm -rf out && m -j showcommands | grep "ninja may be stuck" || echo ok

Change-Id: I8ff1bd216b5f8349ce9e06e5465a9f8d0663f8c0

ui/build/exec.go

@@ -30,9 +30,6 @@ type Cmd struct {
 	ctx    Context
 	config Config
 	name   string
-
-	// doneChannel closes to signal the command's termination
-	doneChannel chan bool
 }
 
 func Command(ctx Context, config Config, name string, executable string, args ...string) *Cmd {
@@ -44,7 +41,6 @@ func Command(ctx Context, config Config, name string, executable string, args ..
 		ctx:    ctx,
 		config: config,
 		name:   name,
-		doneChannel: make(chan bool),
 	}
 
 	return ret
@@ -61,10 +57,6 @@ func (c *Cmd) prepare() {
 	c.ctx.Verboseln(c.Path, c.Args)
 }
 
-func (c *Cmd) teardown() {
-	close(c.doneChannel)
-}
-
 func (c *Cmd) Start() error {
 	c.prepare()
 	return c.Cmd.Start()
@@ -72,21 +64,18 @@ func (c *Cmd) Start() error {
 
 func (c *Cmd) Run() error {
 	c.prepare()
-	defer c.teardown()
 	err := c.Cmd.Run()
 	return err
 }
 
 func (c *Cmd) Output() ([]byte, error) {
 	c.prepare()
-	defer c.teardown()
 	bytes, err := c.Cmd.Output()
 	return bytes, err
 }
 
 func (c *Cmd) CombinedOutput() ([]byte, error) {
 	c.prepare()
-	defer c.teardown()
 	bytes, err := c.Cmd.CombinedOutput()
 	return bytes, err
 }
@@ -133,13 +122,3 @@ func (c *Cmd) CombinedOutputOrFatal() []byte {
 	c.reportError(err)
 	return ret
 }
-
-// Done() tells whether this command has finished executing
-func (c *Cmd) Done() bool {
-	select {
-	case <-c.doneChannel:
-		return true
-	default:
-		return false
-	}
-}

ui/build/ninja.go

@@ -81,11 +81,19 @@ func runNinja(ctx Context, config Config) {
 		}
 	}
 	// Poll the ninja log for updates; if it isn't updated enough, then we want to show some diagnostics
+	done := make(chan struct{})
+	defer close(done)
+	ticker := time.NewTicker(ninjaHeartbeatDuration)
+	defer ticker.Stop()
 	checker := &statusChecker{}
 	go func() {
-		for !cmd.Done() {
+		for {
+			select {
+			case <-ticker.C:
 				checker.check(ctx, config, logPath)
-			time.Sleep(ninjaHeartbeatDuration)
+			case <-done:
+				return
+			}
 		}
 	}()
 
@@ -127,5 +135,5 @@ func dumpStucknessDiagnostics(ctx Context, config Config, statusPath string, las
 	output := cmd.CombinedOutputOrFatal()
 	ctx.Verbose(string(output))
 
-	ctx.Printf("done\n")
+	ctx.Verbosef("done\n")
 }