Optimize server selection with lazy evaluation

Implement lazy server selection to avoid unnecessary lock acquisition
when queries are dropped or rejected by plugins. Instead of
calling serversInfo.getOne(), we now pass a callback function that
retrieves server info only when needed.

Author: jedisct1

dnscrypt-proxy/plugins.go

@@ -273,7 +273,7 @@ func NewPluginsState(
 func (pluginsState *PluginsState) ApplyQueryPlugins(
 	pluginsGlobals *PluginsGlobals,
 	packet []byte,
-	needsEDNS0Padding bool,
+	getServerInfo func() (*ServerInfo, bool),
 ) ([]byte, error) {
 	msg := dns.Msg{}
 	if err := msg.Unpack(packet); err != nil {
@@ -318,12 +318,18 @@ func (pluginsState *PluginsState) ApplyQueryPlugins(
 	if err != nil {
 		return packet, err
 	}
-	if needsEDNS0Padding && pluginsState.action == PluginsActionContinue {
-		padLen := 63 - ((len(packet2) + 63) & 63)
-		if paddedPacket2, _ := addEDNS0PaddingIfNoneFound(&msg, packet2, padLen); paddedPacket2 != nil {
-			return paddedPacket2, nil
+
+	// Only get server info if we're continuing and need padding
+	if pluginsState.action == PluginsActionContinue && getServerInfo != nil {
+		_, needsEDNS0Padding := getServerInfo()
+		if needsEDNS0Padding {
+			padLen := 63 - ((len(packet2) + 63) & 63)
+			if paddedPacket2, _ := addEDNS0PaddingIfNoneFound(&msg, packet2, padLen); paddedPacket2 != nil {
+				return paddedPacket2, nil
+			}
 		}
 	}
+
 	return packet2, nil
 }
 

dnscrypt-proxy/proxy.go

@@ -713,17 +713,29 @@ func (proxy *Proxy) processIncomingQuery(
 	// Initialize plugin state
 	pluginsState := NewPluginsState(proxy, clientProto, clientAddr, serverProto, start)
 
-	// Get server info and initialize parameters
-	serverName := "-"
-	needsEDNS0Padding := false
-	serverInfo := proxy.serversInfo.getOne()
-	if serverInfo != nil {
-		serverName = serverInfo.Name
-		needsEDNS0Padding = (serverInfo.Proto == stamps.StampProtoTypeDoH || serverInfo.Proto == stamps.StampProtoTypeTLS)
-	}
-
-	// Apply query plugins
-	query, _ = pluginsState.ApplyQueryPlugins(&proxy.pluginsGlobals, query, needsEDNS0Padding)
+	var serverInfo *ServerInfo
+	var serverName string = "-"
+
+	// Apply query plugins with lazy server selection
+	query, _ = pluginsState.ApplyQueryPlugins(
+		&proxy.pluginsGlobals,
+		query,
+		func() (*ServerInfo, bool) {
+			// Only get server info once when actually needed
+			if serverInfo == nil {
+				serverInfo = proxy.serversInfo.getOne()
+				if serverInfo != nil {
+					serverName = serverInfo.Name
+				}
+			}
+			if serverInfo == nil {
+				return nil, false
+			}
+			needsPadding := (serverInfo.Proto == stamps.StampProtoTypeDoH ||
+				serverInfo.Proto == stamps.StampProtoTypeTLS)
+			return serverInfo, needsPadding
+		},
+	)
 	if !validateQuery(query) {
 		return response
 	}
@@ -753,29 +765,37 @@ func (proxy *Proxy) processIncomingQuery(
 	}
 
 	// Process query with a DNS server if there's no cached response
-	if len(response) == 0 && serverInfo != nil {
-		pluginsState.serverName = serverName
+	// Note: if serverInfo is still nil here, we need to get it
+	if len(response) == 0 {
+		if serverInfo == nil {
+			serverInfo = proxy.serversInfo.getOne()
+			if serverInfo != nil {
+				serverName = serverInfo.Name
+			}
+		}
+		if serverInfo != nil {
+			pluginsState.serverName = serverName
 
-		// Exchange DNS request with the server
-		exchangeResponse, err := handleDNSExchange(proxy, serverInfo, &pluginsState, query, serverProto)
+			exchangeResponse, err := handleDNSExchange(proxy, serverInfo, &pluginsState, query, serverProto)
 
-		// Update server statistics for WP2 strategy
-		success := (err == nil && exchangeResponse != nil)
-		proxy.serversInfo.updateServerStats(serverName, success)
+			// Update server statistics for WP2 strategy
+			success := (err == nil && exchangeResponse != nil)
+			proxy.serversInfo.updateServerStats(serverName, success)
 
-		if err != nil || exchangeResponse == nil {
-			return response
-		}
+			if err != nil || exchangeResponse == nil {
+				return response
+			}
 
-		response = exchangeResponse
+			response = exchangeResponse
 
-		// Process the response through plugins
-		processedResponse, err := processPlugins(proxy, &pluginsState, query, serverInfo, response)
-		if err != nil {
-			return response
-		}
+			// Process the response through plugins
+			processedResponse, err := processPlugins(proxy, &pluginsState, query, serverInfo, response)
+			if err != nil {
+				return response
+			}
 
-		response = processedResponse
+			response = processedResponse
+		}
 	}
 
 	// Validate the response before sending