Fix channel membership prefixes in cached WHO replies

Channel membership prefixes in WHO replies (RPL_WHOREPLY and
RPL_WHOSPCRPL) were cached in the user's flags, which meant those same
prefixes were returned on future cache hits, even though the flags are
channel specific.

Strip the channel membership prefixes from the user's flags before
adding a user to the cache and add the prefixes back when reading from
the cache (using the membership info from the NAMES reply).

downstream.go

@@ -2230,6 +2230,23 @@ func (dc *downstreamConn) handleMessageRegistered(ctx context.Context, msg *irc.
 				}
 				if uc.isChannel(mask) {
 					info.Channel = mask
+
+					// Set channel membership prefixes from cached NAMES reply
+					ch := uc.channels.Get(info.Channel)
+					memberships := ch.Members.Get(info.Nickname)
+					prefixes := formatMemberPrefix(*memberships, dc)
+
+					// Channel membership prefixes are listed after away status ('G'/'H')
+					// and optional server operator indicator ('*')
+					i := strings.IndexFunc(info.Flags, func(f rune) bool {
+						return f != 'G' && f != 'H' && f != '*'
+					})
+
+					if i == -1 {
+						info.Flags = prefixes + info.Flags
+					} else {
+						info.Flags = info.Flags[:i] + prefixes + info.Flags[i:]
+					}
 				}
 				dc.SendMessage(ctx, xirc.GenerateWHOXReply(fields, &info))
 			}

irc.go

@@ -194,6 +194,19 @@ func formatMemberPrefix(ms xirc.MembershipSet, dc *downstreamConn) string {
 	return string(prefixes)
 }
 
+// Remove channel membership prefixes from flags
+func stripMemberPrefixes(flags string, uc *upstreamConn) string {
+	return strings.Map(func(r rune) rune {
+		for _, v := range uc.availableMemberships {
+			if byte(r) == v.Prefix {
+				return -1
+			}
+		}
+
+		return r
+	}, flags)
+}
+
 func parseMessageParams(msg *irc.Message, out ...*string) error {
 	if len(msg.Params) < len(out) {
 		return newNeedMoreParamsError(msg.Command)

upstream.go

@@ -1541,7 +1541,7 @@ func (uc *upstreamConn) handleMessage(ctx context.Context, msg *irc.Message) err
 				Hostname: host,
 				Server:   server,
 				Nickname: nick,
-				Flags:    flags,
+				Flags:    stripMemberPrefixes(flags, uc),
 				Realname: realname,
 			})
 		}
@@ -1564,13 +1564,14 @@ func (uc *upstreamConn) handleMessage(ctx context.Context, msg *irc.Message) err
 			if err != nil {
 				return err
 			}
+
 			if uc.shouldCacheUserInfo(info.Nickname) {
 				uc.cacheUserInfo(info.Nickname, &upstreamUser{
 					Nickname: info.Nickname,
 					Username: info.Username,
 					Hostname: info.Hostname,
 					Server:   info.Server,
-					Flags:    info.Flags,
+					Flags:    stripMemberPrefixes(info.Flags, uc),
 					Account:  info.Account,
 					Realname: info.Realname,
 				})