cmd/tailscale: make debug profiling output - mean stdout

Because the macOS CLI runs in the sandbox, including the filesystem,
so users would be confused that -cpu-profile=prof.cpu succeeds but doesn't
write to their current directory, but rather in some random Library/Containers
directory somewhere on the machine (which varies depending on the Mac build
type: App Store vs System Extension)

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/debug.go

@@ -36,8 +36,8 @@ var debugCmd = &ffcli.Command{
 		fs.BoolVar(&debugArgs.netMap, "netmap", true, "whether to include netmap in --ipn mode")
 		fs.BoolVar(&debugArgs.localCreds, "local-creds", false, "print how to connect to local tailscaled")
 		fs.StringVar(&debugArgs.file, "file", "", "get, delete:NAME, or NAME")
-		fs.StringVar(&debugArgs.cpuFile, "cpu-profile", "", "if non-empty, grab a CPU profile for --profile-sec seconds and write it to this file")
-		fs.StringVar(&debugArgs.memFile, "mem-profile", "", "if non-empty, grab a memory profile and write it to this file")
+		fs.StringVar(&debugArgs.cpuFile, "cpu-profile", "", "if non-empty, grab a CPU profile for --profile-sec seconds and write it to this file; - for stdout")
+		fs.StringVar(&debugArgs.memFile, "mem-profile", "", "if non-empty, grab a memory profile and write it to this file; - for stdout")
 		fs.IntVar(&debugArgs.cpuSec, "profile-seconds", 15, "number of seconds to run a CPU profile for, when --cpu-profile is non-empty")
 		return fs
 	})(),
@@ -57,6 +57,24 @@ var debugArgs struct {
 	memFile    string
 }
 
+func writeProfile(dst string, v []byte) error {
+	if dst == "-" {
+		_, err := os.Stdout.Write(v)
+		return err
+	}
+	return os.WriteFile(dst, v, 0600)
+}
+
+func outName(dst string) string {
+	if dst == "-" {
+		return "stdout"
+	}
+	if runtime.GOOS == "darwin" {
+		return fmt.Sprintf("%s (warning: sandboxed macOS binaries write to Library/Containers; use - to write to stdout and redirect to file instead)", dst)
+	}
+	return dst
+}
+
 func runDebug(ctx context.Context, args []string) error {
 	if len(args) > 0 {
 		return errors.New("unknown arguments")
@@ -79,10 +97,10 @@ func runDebug(ctx context.Context, args []string) error {
 		if v, err := tailscale.Profile(ctx, "profile", debugArgs.cpuSec); err != nil {
 			return err
 		} else {
-			if err := os.WriteFile(out, v, 0600); err != nil {
+			if err := writeProfile(out, v); err != nil {
 				return err
 			}
-			log.Printf("CPU profile written to %s", out)
+			log.Printf("CPU profile written to %s", outName(out))
 		}
 	}
 	if out := debugArgs.memFile; out != "" {
@@ -90,10 +108,10 @@ func runDebug(ctx context.Context, args []string) error {
 		if v, err := tailscale.Profile(ctx, "heap", 0); err != nil {
 			return err
 		} else {
-			if err := os.WriteFile(out, v, 0600); err != nil {
+			if err := writeProfile(out, v); err != nil {
 				return err
 			}
-			log.Printf("Memory profile written to %s", out)
+			log.Printf("Memory profile written to %s", outName(out))
 		}
 	}
 	if debugArgs.prefs {