misc/ios: ensure deferred cleanup functions are run even on error

log.Fatal exits the process and doesn't allow deferred functions
to run. Extract log.Fatal calls to main where all deferred functions
have completed.

For the iOS builder.

Change-Id: Id1ef9955bed19944a819d6137a611d6ecbe624a6
Reviewed-on: https://go-review.googlesource.com/110955
Run-TryBot: Elias Naur <elias.naur@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/ios/go_darwin_arm_exec.go

@@ -86,14 +86,18 @@ func main() {
 		bundleID = parts[1]
 	}
 
-	os.Exit(runMain())
+	exitCode, err := runMain()
+	if err != nil {
+		log.Fatalf("%v\n", err)
+	}
+	os.Exit(exitCode)
 }
 
-func runMain() int {
+func runMain() (int, error) {
 	var err error
 	tmpdir, err = ioutil.TempDir("", "go_darwin_arm_exec_")
 	if err != nil {
-		log.Fatal(err)
+		return 1, err
 	}
 	if !debug {
 		defer os.RemoveAll(tmpdir)
@@ -103,7 +107,7 @@ func runMain() int {
 	os.RemoveAll(appdir)
 
 	if err := assembleApp(appdir, os.Args[1]); err != nil {
-		log.Fatal(err)
+		return 1, err
 	}
 
 	// This wrapper uses complicated machinery to run iOS binaries. It
@@ -115,28 +119,28 @@ func runMain() int {
 	lockName := filepath.Join(os.TempDir(), "go_darwin_arm_exec-"+deviceID+".lock")
 	lock, err = os.OpenFile(lockName, os.O_CREATE|os.O_RDONLY, 0666)
 	if err != nil {
-		log.Fatal(err)
+		return 1, err
 	}
 	if err := syscall.Flock(int(lock.Fd()), syscall.LOCK_EX); err != nil {
-		log.Fatal(err)
+		return 1, err
 	}
 
 	if err := install(appdir); err != nil {
-		log.Fatal(err)
+		return 1, err
 	}
 
 	deviceApp, err := findDeviceAppPath(bundleID)
 	if err != nil {
-		log.Fatal(err)
+		return 1, err
 	}
 
 	if err := mountDevImage(); err != nil {
-		log.Fatal(err)
+		return 1, err
 	}
 
 	closer, err := startDebugBridge()
 	if err != nil {
-		log.Fatal(err)
+		return 1, err
 	}
 	defer closer()
 
@@ -144,13 +148,12 @@ func runMain() int {
 		// If the lldb driver completed with an exit code, use that.
 		if err, ok := err.(*exec.ExitError); ok {
 			if ws, ok := err.Sys().(interface{ ExitStatus() int }); ok {
-				return ws.ExitStatus()
+				return ws.ExitStatus(), nil
 			}
 		}
-		fmt.Fprintf(os.Stderr, "go_darwin_arm_exec: %v\n", err)
-		return 1
+		return 1, err
 	}
-	return 0
+	return 0, nil
 }
 
 func getenv(envvar string) string {