Performance optimization for determining friends in WHO <mask>.

Construct a hash set of the user's channels and check that rather than
querying channel membership, to reduce the number of locks that need to be
acquired.

irc/client.go

@@ -1022,21 +1022,6 @@ func (client *Client) ModeString() (str string) {
 	return "+" + client.modes.String()
 }
 
-// IsFriend() returns true if the given otherClient shares a channel with this client, or if they are the same user.
-func (client *Client) IsFriend(otherClient *Client) bool {
-	if client == otherClient {
-		return true
-	}
-
-	for _, channel := range client.Channels() {
-		if channel.hasClient(otherClient) {
-			return true
-		}
-	}
-
-	return false
-}
-
 // Friends refers to clients that share a channel with this client.
 func (client *Client) Friends(capabs ...caps.Capability) (result map[*Session]bool) {
 	result = make(map[*Session]bool)

irc/handlers.go

@@ -2636,8 +2636,28 @@ func whoHandler(server *Server, client *Client, msg ircmsg.IrcMessage, rb *Respo
 			}
 		}
 	} else {
+		// Construct set of channels the client is in.
+		userChannels := make(map[*Channel]bool)
+		for _, channel := range client.Channels() {
+			userChannels[channel] = true
+		}
+
+		// Another client is a friend if they share at least one channel, or they are the same client.
+		isFriend := func(otherClient *Client) bool {
+			if client == otherClient {
+				return true
+			}
+
+			for _, channel := range otherClient.Channels() {
+				if userChannels[channel] {
+					return true
+				}
+			}
+			return false
+		}
+
 		for mclient := range server.clients.FindAll(mask) {
-			if isOper || !mclient.HasMode(modes.Invisible) || mclient.IsFriend(client) {
+			if isOper || !mclient.HasMode(modes.Invisible) || isFriend(mclient) {
 				client.rplWhoReply(nil, mclient, rb)
 			}
 		}