gdbserial: fixes for rr 5.7.0

The following fixes have been applied to make delve work with rr 5.7.0

- added a new launch prefix to exclude from stderr output
- allow the thread selection packet to be sent for 'c' commands even
  when the stub supports thread suffixes, because the specification is
  unclear over what should be done with bc and bs packets with thread
  suffixes.
- changed the way qRRCmd are escaped and added a thread selector to
  them to match changes to rr codebase

pkg/proc/gdbserial/gdbserver_conn.go

@@ -940,7 +940,10 @@ func (conn *gdbConn) queryThreads(first bool) (threads []string, err error) {
 }
 
 func (conn *gdbConn) selectThread(kind byte, threadID string, context string) error {
-	if conn.threadSuffixSupported {
+	if conn.threadSuffixSupported && kind != 'c' {
+		// kind == 'c' is still allowed because 'rr' is weird about it's support
+		// for thread suffixes and the specification for it doesn't really say how
+		// it should be used with packets 'bc' and 'bs'.
 		panic("selectThread when thread suffix is supported")
 	}
 	conn.outbuf.Reset()
@@ -1144,9 +1147,15 @@ func (conn *gdbConn) qRRCmd(args ...string) (string, error) {
 	}
 	conn.outbuf.Reset()
 	fmt.Fprint(&conn.outbuf, "$qRRCmd")
-	for _, arg := range args {
+	for i, arg := range args {
 		fmt.Fprint(&conn.outbuf, ":")
-		writeAsciiBytes(&conn.outbuf, []byte(arg))
+		if i == 0 && conn.threadSuffixSupported {
+			// newer versions of RR require the command to be followed by a thread id
+			// and the command name to be unescaped.
+			fmt.Fprintf(&conn.outbuf, "%s:-1", arg)
+		} else {
+			writeAsciiBytes(&conn.outbuf, []byte(arg))
+		}
 	}
 	resp, err := conn.exec(conn.outbuf.Bytes(), "qRRCmd")
 	if err != nil {

pkg/proc/gdbserial/rr.go

@@ -217,9 +217,10 @@ type rrInit struct {
 
 const (
 	rrGdbCommandLegacyPrefix = "  gdb "
-	rrGdbCommandPrefix = "  'gdb' "
-	rrGdbLaunchPrefix  = "Launch gdb with"
-	targetCmd          = "target extended-remote "
+	rrGdbCommandPrefix       = "  'gdb' "
+	rrGdbLaunchLegacyPrefix  = "Launch gdb with"
+	rrGdbLaunchPrefix        = "Launch debugger with"
+	targetCmd                = "target extended-remote "
 )
 
 func rrStderrParser(stderr io.ReadCloser, initch chan<- rrInit, quiet bool) {
@@ -245,7 +246,7 @@ func rrStderrParser(stderr io.ReadCloser, initch chan<- rrInit, quiet bool) {
 			break
 		}
 
-		if strings.HasPrefix(line, rrGdbLaunchPrefix) {
+		if strings.HasPrefix(line, rrGdbLaunchPrefix) || strings.HasPrefix(line, rrGdbLaunchLegacyPrefix) {
 			continue
 		}