misc/android: serialize adb commands on android emulators

Android emulator builders are soon to join the trybot set. To avoid
flaky runs, work around a longstanding adb bug where concurrent adb
commands sometimes fail.

I haven't seen the problem on actual devices until recently. It seems
that the recently added "adb wait-for-device" can introduce flakyness
with errors such as:

adb: error: failed to get feature set: protocol fault (couldn't read status): Connection reset by peer

Instead of working around that, give up and serialize use of adb
everywhere.

Fixes #23795
Updates #23824

Change-Id: If347c9981fa32ff8a1e14b7454f122ef682450a6
Reviewed-on: https://go-review.googlesource.com/c/163625Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/android/go_android_exec.go

@@ -60,6 +60,19 @@ func main() {
 	log.SetFlags(0)
 	log.SetPrefix("go_android_exec: ")
 
+	// Concurrent use of adb is flaky, so serialize adb commands.
+	// See https://github.com/golang/go/issues/23795 or
+	// https://issuetracker.google.com/issues/73230216.
+	lockPath := filepath.Join(os.TempDir(), "go_android_exec-adb-lock")
+	lock, err := os.OpenFile(lockPath, os.O_CREATE|os.O_RDWR, 0666)
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer lock.Close()
+	if err := syscall.Flock(int(lock.Fd()), syscall.LOCK_EX); err != nil {
+		log.Fatal(err)
+	}
+
 	// In case we're booting a device or emulator alongside androidtest.bash
 	// wait for it to be ready. adb wait-for-device is not enough, we have to
 	// wait for sys.boot_completed.
@@ -87,15 +100,7 @@ func main() {
 	// E.g. template.test from the {html,text}/template packages.
 	binName := fmt.Sprintf("%s-%d", filepath.Base(os.Args[1]), os.Getpid())
 	deviceBin := fmt.Sprintf("%s/%s", deviceGotmp, binName)
-
-	// The push of the binary happens in parallel with other tests.
-	// Unfortunately, a simultaneous call to adb shell hold open
-	// file descriptors, so it is necessary to push then move to
-	// avoid a "text file busy" error on execution.
-	// https://code.google.com/p/android/issues/detail?id=65857
-	run("push", os.Args[1], deviceBin+"-tmp")
-	run("shell", "cp '"+deviceBin+"-tmp' '"+deviceBin+"'")
-	run("shell", "rm '"+deviceBin+"-tmp'")
+	run("push", os.Args[1], deviceBin)
 
 	// Forward SIGQUIT from the go command to show backtraces from
 	// the binary instead of from this wrapper.