Handle upstream multi-line SASL

References: https://todo.sr.ht/~emersion/soju/173

downstream.go

@@ -983,7 +983,7 @@ func (dc *downstreamConn) handleAuthenticateCommand(msg *irc.Message) (result *d
 
 		dc.sasl.pendingResp.WriteString(chunk)
 
-		if len(chunk) == 400 {
+		if len(chunk) == maxSASLLength {
 			return nil, nil // Multi-line response, wait for the next command
 		}
 

irc.go

@@ -25,6 +25,7 @@ const (
 const (
 	maxMessageLength = 512
 	maxMessageParams = 15
+	maxSASLLength    = 400
 )
 
 // The server-time layout, as defined in the IRCv3 spec.

upstream.go

@@ -619,16 +619,26 @@ func (uc *upstreamConn) handleMessage(ctx context.Context, msg *irc.Message) err
 			return err
 		}
 
-		// TODO: send response in multiple chunks if >= 400 bytes
+		// <= instead of < because we need to send a final empty response if
+		// the last chunk is exactly 400 bytes long
+		for i := 0; i <= len(resp); i += maxSASLLength {
+			j := i + maxSASLLength
+			if j > len(resp) {
+				j = len(resp)
+			}
+
+			chunk := resp[i:j]
+
 			var respStr = "+"
-		if len(resp) != 0 {
-			respStr = base64.StdEncoding.EncodeToString(resp)
+			if len(chunk) != 0 {
+				respStr = base64.StdEncoding.EncodeToString(chunk)
 			}
 
 			uc.SendMessage(ctx, &irc.Message{
 				Command: "AUTHENTICATE",
 				Params:  []string{respStr},
 			})
+		}
 	case irc.RPL_LOGGEDIN:
 		if err := parseMessageParams(msg, nil, nil, &uc.account); err != nil {
 			return err