Add a sasl set-plain command

This allows to manually set the SASL credentials for a network.

doc/soju.1.scd

@@ -173,6 +173,9 @@ abbreviated form, for instance *network* can be abbreviated as *net* or just
 *certfp reset* <network name>
 	Disable SASL EXTERNAL authentication and remove stored certificate.
 
+*sasl set-plain* <network name> <username> <password>
+	Set SASL PLAIN credentials.
+
 *user create* -username <username> -password <password> [-admin]
 	Create a new soju user. Only admin users can create new accounts.
 

service.go

@@ -187,6 +187,15 @@ func init() {
 				},
 			},
 		},
+		"sasl": {
+			children: serviceCommandSet{
+				"set-plain": {
+					usage:  "<network name> <username> <password>",
+					desc:   "set SASL PLAIN credentials",
+					handle: handleServiceSASLSetPlain,
+				},
+			},
+		},
 		"user": {
 			children: serviceCommandSet{
 				"create": {
@@ -588,6 +597,28 @@ func handleServiceCertfpReset(dc *downstreamConn, params []string) error {
 	return nil
 }
 
+func handleServiceSASLSetPlain(dc *downstreamConn, params []string) error {
+	if len(params) != 3 {
+		return fmt.Errorf("expected exactly 3 arguments")
+	}
+
+	net := dc.user.getNetwork(params[0])
+	if net == nil {
+		return fmt.Errorf("unknown network %q", params[0])
+	}
+
+	net.SASL.Plain.Username = params[1]
+	net.SASL.Plain.Password = params[2]
+	net.SASL.Mechanism = "PLAIN"
+
+	if err := dc.srv.db.StoreNetwork(net.Username, &net.Network); err != nil {
+		return err
+	}
+
+	sendServicePRIVMSG(dc, "credentials saved")
+	return nil
+}
+
 func handlePasswordChange(dc *downstreamConn, params []string) error {
 	if len(params) != 1 {
 		return fmt.Errorf("expected exactly one argument")