gdbserial,debugger: better error message when debugserver not installed

When gdbserial can not find debugserver or lldb-server the error
message is always the same and it complains about lldb-server not being
found.

This is fine on linux (where the backend is unnecessary) but incomplete
on macOS (where the backend is actually used).

Make the error message clearer so that users who do not bother reading
install instructions are not confused.

pkg/proc/gdbserial/gdbserver.go

@@ -135,6 +135,14 @@ type Thread struct {
 	setbp             bool // thread was stopped because of a breakpoint
 }
 
+// ErrBackendUnavailable is returned when the stub program can not be found.
+type ErrBackendUnavailable struct {
+}
+
+func (err *ErrBackendUnavailable) Error() string {
+	return "backend unavailable"
+}
+
 // gdbRegisters represents the current value of the registers of a thread.
 // The storage space for all the registers is allocated as a single memory
 // block in buf, the value field inside an individual gdbRegister will be a
@@ -392,6 +400,9 @@ func LLDBLaunch(cmd []string, wd string) (*Process, error) {
 
 		proc = exec.Command(debugserverExecutable, args...)
 	} else {
+		if _, err := exec.LookPath("lldb-server"); err != nil {
+			return nil, &ErrBackendUnavailable{}
+		}
 		port = unusedPort()
 		args := make([]string, 0, len(cmd)+3)
 		args = append(args, "gdbserver")
@@ -452,6 +463,9 @@ func LLDBAttach(pid int, path string) (*Process, error) {
 		}
 		proc = exec.Command(debugserverExecutable, "-R", fmt.Sprintf("127.0.0.1:%d", listener.Addr().(*net.TCPAddr).Port), "--attach="+strconv.Itoa(pid))
 	} else {
+		if _, err := exec.LookPath("lldb-server"); err != nil {
+			return nil, &ErrBackendUnavailable{}
+		}
 		port = unusedPort()
 		proc = exec.Command("lldb-server", "gdbserver", "--attach", strconv.Itoa(pid), port)
 	}

pkg/proc/gdbserial/rr.go

@@ -15,6 +15,10 @@ import (
 // Record uses rr to record the execution of the specified program and
 // returns the trace directory's path.
 func Record(cmd []string, wd string, quiet bool) (tracedir string, err error) {
+	if _, err := exec.LookPath("rr"); err != nil {
+		return "", &ErrBackendUnavailable{}
+	}
+
 	rfd, wfd, err := os.Pipe()
 	if err != nil {
 		return "", err
@@ -49,6 +53,10 @@ func Record(cmd []string, wd string, quiet bool) (tracedir string, err error) {
 // Replay starts an instance of rr in replay mode, with the specified trace
 // directory, and connects to it.
 func Replay(tracedir string, quiet bool) (*Process, error) {
+	if _, err := exec.LookPath("rr"); err != nil {
+		return nil, &ErrBackendUnavailable{}
+	}
+
 	rrcmd := exec.Command("rr", "replay", "--dbgport=0", tracedir)
 	rrcmd.Stdout = os.Stdout
 	stderr, err := rrcmd.StderrPipe()

service/debugger/debugger.go

@@ -113,13 +113,13 @@ func (d *Debugger) Launch(processArgs []string, wd string) (proc.Process, error)
 	case "native":
 		return native.Launch(processArgs, wd)
 	case "lldb":
-		return gdbserial.LLDBLaunch(processArgs, wd)
+		return betterGdbserialLaunchError(gdbserial.LLDBLaunch(processArgs, wd))
 	case "rr":
 		p, _, err := gdbserial.RecordAndReplay(processArgs, wd, false)
 		return p, err
 	case "default":
 		if runtime.GOOS == "darwin" {
-			return gdbserial.LLDBLaunch(processArgs, wd)
+			return betterGdbserialLaunchError(gdbserial.LLDBLaunch(processArgs, wd))
 		}
 		return native.Launch(processArgs, wd)
 	default:
@@ -137,10 +137,10 @@ func (d *Debugger) Attach(pid int, path string) (proc.Process, error) {
 	case "native":
 		return native.Attach(pid)
 	case "lldb":
-		return gdbserial.LLDBAttach(pid, path)
+		return betterGdbserialLaunchError(gdbserial.LLDBAttach(pid, path))
 	case "default":
 		if runtime.GOOS == "darwin" {
-			return gdbserial.LLDBAttach(pid, path)
+			return betterGdbserialLaunchError(gdbserial.LLDBAttach(pid, path))
 		}
 		return native.Attach(pid)
 	default:
@@ -148,6 +148,19 @@ func (d *Debugger) Attach(pid int, path string) (proc.Process, error) {
 	}
 }
 
+var macOSBackendUnavailableErr = errors.New("debugserver or lldb-server not found: install XCode's command line tools or lldb-server")
+
+func betterGdbserialLaunchError(p proc.Process, err error) (proc.Process, error) {
+	if runtime.GOOS != "darwin" {
+		return p, err
+	}
+	if _, isUnavailable := err.(*gdbserial.ErrBackendUnavailable); !isUnavailable {
+		return p, err
+	}
+
+	return p, macOSBackendUnavailableErr
+}
+
 // ProcessPid returns the PID of the process
 // the debugger is debugging.
 func (d *Debugger) ProcessPid() int {