Request invite-notify to upstreams

... and do not forward INVITEs to downstreams that do not support the
capability.

The downstream capability can be permanent because there is no way for a
client to get the list of people invited to a channel, thus no state can
be corrupted.

downstream.go

@@ -64,12 +64,13 @@ const illegalNickChars = " :@!*?"
 // permanentDownstreamCaps is the list of always-supported downstream
 // capabilities.
 var permanentDownstreamCaps = map[string]string{
-	"batch":        "",
-	"cap-notify":   "",
-	"echo-message": "",
-	"message-tags": "",
-	"sasl":         "PLAIN",
-	"server-time":  "",
+	"batch":         "",
+	"cap-notify":    "",
+	"echo-message":  "",
+	"invite-notify": "",
+	"message-tags":  "",
+	"sasl":          "PLAIN",
+	"server-time":   "",
 }
 
 // needAllDownstreamCaps is the list of downstream capabilities that

upstream.go

@@ -27,6 +27,7 @@ var permanentUpstreamCaps = map[string]bool{
 	"away-notify":      true,
 	"batch":            true,
 	"extended-join":    true,
+	"invite-notify":    true,
 	"labeled-response": true,
 	"message-tags":     true,
 	"multi-prefix":     true,
@@ -1279,7 +1280,12 @@ func (uc *upstreamConn) handleMessage(msg *irc.Message) error {
 			return err
 		}
 
+		weAreInvited := nick == uc.nick
+
 		uc.forEachDownstream(func(dc *downstreamConn) {
+			if !weAreInvited && !dc.caps["invite-notify"] {
+				return
+			}
 			dc.SendMessage(&irc.Message{
 				Prefix:  dc.marshalUserPrefix(uc.network, msg.Prefix),
 				Command: "INVITE",