resolver: use select for client communication

userspace/resolver/main.cpp

@@ -11,6 +11,7 @@
 #include <netinet/in.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <sys/select.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
 #include <sys/un.h>
@@ -53,6 +54,12 @@ struct DNSEntry
 	BAN::IPv4Address	address 	{ 0 };
 };
 
+struct DNSResponse
+{
+	uint16_t id;
+	DNSEntry entry;
+};
+
 bool send_dns_query(int socket, BAN::StringView domain, uint16_t id)
 {
 	static uint8_t buffer[4096];
@@ -91,7 +98,7 @@ bool send_dns_query(int socket, BAN::StringView domain, uint16_t id)
 	return true;
 }
 
-BAN::Optional<DNSEntry> read_dns_response(int socket, uint16_t id)
+BAN::Optional<DNSResponse> read_dns_response(int socket)
 {
 	static uint8_t buffer[4096];
 
@@ -103,11 +110,6 @@ BAN::Optional<DNSEntry> read_dns_response(int socket, uint16_t id)
 	}
 
 	DNSPacket& reply = *reinterpret_cast<DNSPacket*>(buffer);
-	if (reply.identification != id)
-	{
-		dprintln("Reply to invalid packet");
-		return {};
-	}
 	if (reply.flags & 0x0F)
 	{
 		dprintln("DNS error (rcode {})", (unsigned)(reply.flags & 0xF));
@@ -134,9 +136,10 @@ BAN::Optional<DNSEntry> read_dns_response(int socket, uint16_t id)
 		return {};
 	}
 
-	DNSEntry result;
-	result.valid_until	= time(nullptr) + answer.ttl();
-	result.address		= BAN::IPv4Address(*reinterpret_cast<uint32_t*>(answer.data));
+	DNSResponse result;
+	result.id = reply.identification;
+	result.entry.valid_until = time(nullptr) + answer.ttl();
+	result.entry.address = BAN::IPv4Address(*reinterpret_cast<uint32_t*>(answer.data));
 
 	return result;
 }
@@ -207,22 +210,87 @@ int main(int, char**)
 
 	BAN::HashMap<BAN::String, DNSEntry> dns_cache;
 
+	struct Client
+	{
+		const int	socket;
+		bool		close { false };
+		uint16_t	query_id { 0 };
+		BAN::String	query;
+	};
+
+	BAN::LinkedList<Client> clients;
+
 	for (;;)
+	{
+		int max_sock = BAN::Math::max(service_socket, dns_socket);
+
+		fd_set fds;
+		FD_ZERO(&fds);
+		FD_SET(service_socket, &fds);
+		FD_SET(dns_socket, &fds);
+		for (auto& client : clients)
+		{
+			FD_SET(client.socket, &fds);
+			max_sock = BAN::Math::max(max_sock, client.socket);
+		}
+
+		int nselect = select(max_sock + 1, &fds, nullptr, nullptr, nullptr);
+		if (nselect == -1)
+		{
+			perror("select");
+			continue;
+		}
+
+		if (FD_ISSET(service_socket, &fds))
 		{
 			int client = accept(service_socket, nullptr, nullptr);
 			if (client == -1)
 			{
-			dprintln("accept: {}", strerror(errno));
+				perror("accept");
 				continue;
 			}
 
-		auto query = read_service_query(client);
-		if (!query.has_value())
+			MUST(clients.emplace_back(client));
+		}
+
+		if (FD_ISSET(dns_socket, &fds))
 		{
-			close(client);
+			auto result = read_dns_response(dns_socket);
+			if (!result.has_value())
+				continue;
+
+			for (auto& client : clients)
+			{
+				if (client.query_id != result->id)
+					continue;
+
+				(void)dns_cache.insert(client.query, result->entry);
+
+				sockaddr_storage storage;
+				storage.ss_family = AF_INET;
+				memcpy(storage.ss_storage, &result->entry.address.raw, sizeof(result->entry.address.raw));
+				if (send(client.socket, &storage, sizeof(storage), 0) == -1)
+					dprintln("send: {}", strerror(errno));
+				client.close = true;
+				break;
+			}
+		}
+
+		for (auto& client : clients)
+		{
+			if (!FD_ISSET(client.socket, &fds))
+				continue;
+
+			if (!client.query.empty())
+			{
+				dprintln("Client already has a query");
 				continue;
 			}
 
+			auto query = read_service_query(client.socket);
+			if (!query.has_value())
+				continue;
+
 			BAN::Optional<DNSEntry> result;
 
 			if (dns_cache.contains(*query))
@@ -234,28 +302,33 @@ int main(int, char**)
 					dns_cache.remove(*query);
 			}
 
-		if (!result.has_value())
-		{
-			uint16_t id = rand() % 0xFFFF;
-			if (send_dns_query(dns_socket, *query, id))
-			{
-				result = read_dns_response(dns_socket, id);
 			if (result.has_value())
-					(void)dns_cache.insert(*query, *result);
-			}
-		}
-
-		if (!result.has_value())
-			result = DNSEntry { .valid_until = 0, .address = BAN::IPv4Address(INADDR_ANY) };
-
+			{
 				sockaddr_storage storage;
 				storage.ss_family = AF_INET;
 				memcpy(storage.ss_storage, &result->address.raw, sizeof(result->address.raw));
-
-		if (send(client, &storage, sizeof(storage), 0) == -1)
+				if (send(client.socket, &storage, sizeof(storage), 0) == -1)
 					dprintln("send: {}", strerror(errno));
+				client.close = true;
+				continue;
+			}
 
-		close(client);
+			client.query = query.release_value();
+			client.query_id = rand() % 0xFFFF;
+			send_dns_query(dns_socket, client.query, client.query_id);
+		}
+
+		for (auto it = clients.begin(); it != clients.end();)
+		{
+			if (!it->close)
+			{
+				it++;
+				continue;
+			}
+
+			close(it->socket);
+			it = clients.remove(it);
+		}
 	}
 
 	return 0;