Fix excessive CPU usage when the number of sockets exceeds the system limit.

Closes #948. Thanks to wiebeytec.

ChangeLog.txt

@@ -4,9 +4,11 @@
 Broker:
 - Fix build when using WITH_ADNS=yes.
 - Fix incorrect call to setsockopt() for TCP_NODELAY. Closes #941.
+- Fix excessive CPU usage when the number of sockets exceeds the system limit.
+  Closes #948.
 
 Build:
-- Make it easier to build with bundled uthash.h using "WITH_BUNDLED_DEPS=no".
+- Make it easier to build without bundled uthash.h using "WITH_BUNDLED_DEPS=no".
 
 
 1.5.1 - 20180816

src/mosquitto.c

@@ -239,6 +239,7 @@ int main(int argc, char *argv[])
 	memset(&int_db, 0, sizeof(struct mosquitto_db));
 
 	net__init();
+	int_db.spare_sock = socket(AF_INET, SOCK_STREAM, 0);
 
 	config__init(&int_db, &config);
 	rc = config__parse_args(&int_db, &config, argc, argv);

src/mosquitto_broker_internal.h

@@ -392,6 +392,7 @@ struct mosquitto_db{
 #ifdef WITH_EPOLL
 	int epollfd;
 #endif
+	mosq_sock_t spare_sock;
 };
 
 enum mosquitto__bridge_direction{

src/net.c

@@ -97,7 +97,31 @@ int net__socket_accept(struct mosquitto_db *db, mosq_sock_t listensock)
 #endif
 
 	new_sock = accept(listensock, NULL, 0);
-	if(new_sock == INVALID_SOCKET) return -1;
+	if(new_sock == INVALID_SOCKET){
+#ifdef WIN32
+		errno = WSAGetLastError();
+		if(errno == WSAEMFILE){
+#else
+		if(errno == EMFILE || errno == ENFILE){
+#endif
+			/* Close the spare socket, which means we should be able to accept
+			 * this connection. Accept it, then close it immediately and create
+			 * a new spare_sock. This prevents the situation of ever properly
+			 * running out of sockets.
+			 * It would be nice to send a "server not available" connack here,
+			 * but there are lots of reasons why this would be tricky (TLS
+			 * being the big one). */
+			COMPAT_CLOSE(db->spare_sock);
+			new_sock = accept(listensock, NULL, 0);
+			if(new_sock != INVALID_SOCKET){
+				COMPAT_CLOSE(new_sock);
+			}
+			db->spare_sock = socket(AF_INET, SOCK_STREAM, 0);
+			log__printf(NULL, MOSQ_LOG_NOTICE,
+					"Unable to accept new connection, system socket count has been exceeded. Try increasing \"ulimit -n\" or equivalent.");
+		}
+		return -1;
+	}
 
 	G_SOCKET_CONNECTIONS_INC();