fix(GODT-2021): Indefinite parser stall after new line

Can't consume new line or we will be stuck waiting on more data which
does not arrive until the client has received the response.

imap/command/parser.go

@@ -115,8 +115,14 @@ func (p *Parser) Parse() (Command, error) {
 
 	result.Payload = payload
 
-	if err := p.parser.ConsumeNewLine(); err != nil {
-		return result, err
+	if err := p.parser.Consume(rfcparser.TokenTypeCR, "expected CR"); err != nil {
+		return Command{}, err
+	}
+
+	// Can't fully consume the last new line here or we will hang forever as the clients don't send the next token.
+	// In the next loop, the call to advance will ensure the next token in the stream gets loaded properly.
+	if !p.parser.Check(rfcparser.TokenTypeLF) {
+		return Command{}, p.parser.MakeError("expected LF after CR")
 	}
 
 	return result, nil

imap/command/parser_test.go

@@ -120,3 +120,54 @@ func TestParser_LiteralWithContinuationSubmission(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, expected, cmd)
 }
+
+func TestParser_TwoCommandsInSuccession(t *testing.T) {
+	// Run one go routine that submits bytes until the continuation call has been received
+	continueCh := make(chan struct{})
+	reader, writer := io.Pipe()
+
+	go func() {
+		defer writer.Close()
+
+		firstLine := toIMAPLine(`A003 CAPABILITY`)
+
+		secondLine := toIMAPLine(`B002 LIST "" %`)
+
+		if l, err := writer.Write(firstLine); err != nil || l != len(firstLine) {
+			writer.CloseWithError(fmt.Errorf("failed to write first line: %w", err))
+			return
+		}
+
+		<-continueCh
+
+		if l, err := writer.Write(secondLine); err != nil || l != len(secondLine) {
+			writer.CloseWithError(fmt.Errorf("failed to write second line: %w", err))
+			return
+		}
+	}()
+
+	s := rfcparser.NewScanner(reader)
+	p := NewParser(s)
+
+	// First command
+	{
+		expected := Command{Tag: "A003", Payload: &Capability{}}
+
+		cmd, err := p.Parse()
+		require.NoError(t, err)
+		require.Equal(t, expected, cmd)
+	}
+
+	// Submit next command.
+	close(continueCh)
+	{
+		expected := Command{Tag: "B002", Payload: &List{
+			Mailbox:     "",
+			ListMailbox: "%",
+		}}
+
+		cmd, err := p.Parse()
+		require.NoError(t, err)
+		require.Equal(t, expected, cmd)
+	}
+}