fix: resynchronize UART stream on frame magic

cmd/groundstation/main.go

@@ -57,7 +57,7 @@ func main() {
 func run(ctx context.Context, input io.Reader, output io.Writer) error {
 	buf := make([]byte, protocol.RCSize)
 	for {
-		if err := readFull(ctx, input, buf); err != nil {
+		if err := readRCFrame(ctx, input, buf); err != nil {
 			return err
 		}
 		frame, err := protocol.DecodeRC(buf)
@@ -72,6 +72,25 @@ func run(ctx context.Context, input io.Reader, output io.Writer) error {
 	}
 }
 
+func readRCFrame(ctx context.Context, input io.Reader, dst []byte) error {
+	if len(dst) != protocol.RCSize {
+		return fmt.Errorf("invalid RC frame buffer size %d", len(dst))
+	}
+
+	var previous byte
+	for {
+		var current [1]byte
+		if err := readFull(ctx, input, current[:]); err != nil {
+			return err
+		}
+		if previous == protocol.Magic0 && current[0] == protocol.Magic1 {
+			dst[0], dst[1] = protocol.Magic0, protocol.Magic1
+			return readFull(ctx, input, dst[2:])
+		}
+		previous = current[0]
+	}
+}
+
 func readFull(ctx context.Context, input io.Reader, dst []byte) error {
 	read := 0
 	for read < len(dst) {

cmd/groundstation/main_test.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"bytes"
+	"context"
+	"testing"
+
+	"git.piksar.eu/piotr/groundstation/internal/protocol"
+)
+
+func TestReadRCFrameResynchronizesAfterPartialFrame(t *testing.T) {
+	packet := protocol.EncodeRC(protocol.RCFrame{Sequence: 7})
+	input := bytes.NewReader(append([]byte{0xaa, 0xbb, 'G'}, packet...))
+	got := make([]byte, protocol.RCSize)
+	if err := readRCFrame(context.Background(), input, got); err != nil {
+		t.Fatal(err)
+	}
+	frame, err := protocol.DecodeRC(got)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if frame.Sequence != 7 {
+		t.Fatalf("sequence=%d want 7", frame.Sequence)
+	}
+}