fix #2135

Handling of reserved nicknames is special-cased due to #1594, but we want to send
ERR_NICKNAMEINUSE if the nickname is actually in use, since that doesn't pose any
client compatibility problems.

irc/client_lookup_set.go

@@ -116,6 +116,8 @@ func (clients *ClientManager) SetNick(client *Client, session *Session, newNick
 		useAccountName = alwaysOn || config.Accounts.NickReservation.ForceNickEqualsAccount
 	}
 
+	nickIsReserved := false
+
 	if useAccountName {
 		if registered && newNick != accountName {
 			return "", errNickAccountMismatch, false
@@ -167,7 +169,9 @@ func (clients *ClientManager) SetNick(client *Client, session *Session, newNick
 
 		reservedAccount, method := client.server.accounts.EnforcementStatus(newCfNick, newSkeleton)
 		if method == NickEnforcementStrict && reservedAccount != "" && reservedAccount != account {
-			return "", errNicknameReserved, false
+			// see #2135: we want to enter the critical section, see if the nick is actually in use,
+			// and return errNicknameInUse in that case
+			nickIsReserved = true
 		}
 	}
 
@@ -219,6 +223,9 @@ func (clients *ClientManager) SetNick(client *Client, session *Session, newNick
 	if skeletonHolder != nil && skeletonHolder != client {
 		return "", errNicknameInUse, false
 	}
+	if nickIsReserved {
+		return "", errNicknameReserved, false
+	}
 
 	if dryRun {
 		return "", nil, false

irc/nickname.go

@@ -43,6 +43,8 @@ func performNickChange(server *Server, client *Client, target *Client, session *
 		}
 	} else if err == errNicknameReserved {
 		if !isSanick {
+			// see #1594 for context: ERR_NICKNAMEINUSE can confuse clients if the nickname is not
+			// literally in use:
 			if !client.registered {
 				rb.Add(nil, server.name, ERR_NICKNAMEINUSE, details.nick, utils.SafeErrorParam(nickname), client.t("Nickname is reserved by a different account"))
 			}