[config] server.listen-backlog option (fixes #1825, #2116)

See doc/config/lighttpd.conf for explanation of listen() backlog queue

Additionally, mod_fastcgi and mod_scgi backend servers can now also be
configured with separate listen-backlog settings per server

x-ref:
  "add server.listen-backlog option instead of hard-coded value (128 * 8) for listen()"
  https://redmine.lighttpd.net/issues/2116
  "Don't disable backend when overloaded"
  https://redmine.lighttpd.net/issues/1825

github:
Closes #50

NEWS

@@ -79,6 +79,7 @@ NEWS
   * [mod_redirect,mod_rewrite] short-circuit if blank replacement (fixes #2085)
   * [mod_indexfile] save physical path to env (fixes #448, #892)
   * [core] open fd when appending file to cq (fixes #2655)
+  * [config] server.listen-backlog option (fixes #1825, #2116)
 
 - 1.4.39 - 2016-01-02
   * [core] fix memset_s call (fixes #2698)

doc/config/lighttpd.conf

@@ -206,6 +206,35 @@ server.network-backend = "sendfile"
 ##
 server.max-fds = 2048
 
+##
+## listen-backlog is the size of the listen() backlog queue requested when
+## the lighttpd server ask the kernel to listen() on the provided network
+## address.  Clients attempting to connect() to the server enter the listen()
+## backlog queue and wait for the lighttpd server to accept() the connection.
+##
+## The out-of-box default on many operating systems is 128 and is identified
+## as SOMAXCONN.  This can be tuned on many operating systems.  (On Linux,
+## cat /proc/sys/net/core/somaxconn)  Requesting a size larger than operating
+## system limit will be silently reduced to the limit by the operating system.
+##
+## When there are too many connection attempts waiting for the server to
+## accept() new connections, the listen backlog queue fills and the kernel
+## rejects additional connection attempts.  This can be useful as an
+## indication to an upstream load balancer that the server is busy, and
+## possibly overloaded.  In that case, configure a smaller limit for
+## server.listen-backlog.  On the other hand, configure a larger limit to be
+## able to handle bursts of new connections, but only do so up to an amount
+## that the server can keep up with responding in a reasonable amount of
+## time.  Otherwise, clients may abandon the connection attempts and the
+## server will waste resources servicing abandoned connections.
+##
+## It is best to leave this setting at its default unless you have modelled
+## your traffic and tested that changing this benefits your traffic patterns.
+##
+## Default: 1024
+##
+#server.listen-backlog = 128
+
 ##
 ## Stat() call caching.
 ##

src/base.h

@@ -294,6 +294,7 @@ typedef struct {
 	unsigned short etag_use_size;
 	unsigned short force_lowercase_filenames; /* if the FS is case-insensitive, force all files to lower-case */
 	unsigned int max_request_size;
+	int listen_backlog;
 
 	unsigned short kbytes_per_second; /* connection kb/s limit */
 

src/configfile.c

@@ -111,6 +111,7 @@ static int config_insert(server *srv) {
 		{ "ssl.empty-fragments",               NULL, T_CONFIG_BOOLEAN, T_CONFIG_SCOPE_CONNECTION }, /* 67 */
 		{ "server.upload-temp-file-size",      NULL, T_CONFIG_INT,     T_CONFIG_SCOPE_SERVER     }, /* 68 */
 		{ "mimetype.xattr-name",               NULL, T_CONFIG_STRING,  T_CONFIG_SCOPE_SERVER     }, /* 69 */
+		{ "server.listen-backlog",             NULL, T_CONFIG_INT,     T_CONFIG_SCOPE_CONNECTION }, /* 70 */
 
 		{ "server.host",
 			"use server.bind instead",
@@ -229,6 +230,7 @@ static int config_insert(server *srv) {
 		s->ssl_verifyclient_depth = 9;
 		s->ssl_verifyclient_export_cert = 0;
 		s->ssl_disable_client_renegotiation = 1;
+		s->listen_backlog = (0 == i ? 1024 : srv->config_storage[0]->listen_backlog);
 
 		/* all T_CONFIG_SCOPE_CONNECTION options */
 		cv[2].destination = s->errorfile_prefix;
@@ -285,6 +287,7 @@ static int config_insert(server *srv) {
 		cv[65].destination = &(s->ssl_disable_client_renegotiation);
 		cv[66].destination = &(s->ssl_honor_cipher_order);
 		cv[67].destination = &(s->ssl_empty_fragments);
+		cv[70].destination = &(s->listen_backlog);
 
 		srv->config_storage[i] = s;
 
@@ -355,6 +358,7 @@ int config_setup_connection(server *srv, connection *con) {
 
 	PATCH(range_requests);
 	PATCH(force_lowercase_filenames);
+	/*PATCH(listen_backlog);*//*(not necessary; used only at startup)*/
 	PATCH(ssl_enabled);
 
 	PATCH(ssl_pemfile);
@@ -483,6 +487,10 @@ int config_patch_connection(server *srv, connection *con) {
 				PATCH(allow_http11);
 			} else if (buffer_is_equal_string(du->key, CONST_STR_LEN("server.force-lowercase-filenames"))) {
 				PATCH(force_lowercase_filenames);
+		      #if 0 /*(not necessary; used only at startup)*/
+			} else if (buffer_is_equal_string(du->key, CONST_STR_LEN("server.listen-backlog"))) {
+				PATCH(listen_backlog);
+		      #endif
 			} else if (buffer_is_equal_string(du->key, CONST_STR_LEN("server.kbytes-per-second"))) {
 				PATCH(global_kbytes_per_second);
 				PATCH(global_bytes_per_second_cnt);

src/mod_fastcgi.c

@@ -250,6 +250,8 @@ typedef struct {
 				       applications prefer SIGUSR1 while the
 				       rest of the world would use SIGTERM
 				       *sigh* */
+
+	int listen_backlog;
 } fcgi_extension_host;
 
 /*
@@ -991,7 +993,7 @@ static int fcgi_spawn_connection(server *srv,
 			return -1;
 		}
 
-		if (-1 == listen(fcgi_fd, 1024)) {
+		if (-1 == listen(fcgi_fd, host->listen_backlog)) {
 			log_error_write(srv, __FILE__, __LINE__, "ss",
 				"listen failed:", strerror(errno));
 			close(fcgi_fd);
@@ -1288,6 +1290,7 @@ SETDEFAULTS_FUNC(mod_fastcgi_set_defaults) {
 						{ "strip-request-uri",  NULL, T_CONFIG_STRING, T_CONFIG_SCOPE_CONNECTION },      /* 13 */
 						{ "kill-signal",        NULL, T_CONFIG_SHORT, T_CONFIG_SCOPE_CONNECTION },       /* 14 */
 						{ "fix-root-scriptname",   NULL, T_CONFIG_BOOLEAN, T_CONFIG_SCOPE_CONNECTION },  /* 15 */
+						{ "listen-backlog",    NULL, T_CONFIG_INT,   T_CONFIG_SCOPE_CONNECTION },        /* 16 */
 
 						{ NULL,                NULL, T_CONFIG_UNSET, T_CONFIG_SCOPE_UNSET }
 					};
@@ -1314,6 +1317,7 @@ SETDEFAULTS_FUNC(mod_fastcgi_set_defaults) {
 					host->allow_xsendfile = 0; /* handle X-LIGHTTPD-send-file */
 					host->kill_signal = SIGTERM;
 					host->fix_root_path_name = 0;
+					host->listen_backlog = 1024;
 
 					fcv[0].destination = host->host;
 					fcv[1].destination = host->docroot;
@@ -1333,6 +1337,7 @@ SETDEFAULTS_FUNC(mod_fastcgi_set_defaults) {
 					fcv[13].destination = host->strip_request_uri;
 					fcv[14].destination = &(host->kill_signal);
 					fcv[15].destination = &(host->fix_root_path_name);
+					fcv[16].destination = &(host->listen_backlog);
 
 					if (0 != config_insert_values_internal(srv, da_host->value, fcv, T_CONFIG_SCOPE_CONNECTION)) {
 						goto error;

src/mod_scgi.c

@@ -223,6 +223,8 @@ typedef struct {
 
 	only if a process is killed max_id waits for the process itself
 	to die and decrements its afterwards */
+
+	int listen_backlog;
 } scgi_extension_host;
 
 /*
@@ -786,7 +788,7 @@ static int scgi_spawn_connection(server *srv,
 			return -1;
 		}
 
-		if (-1 == listen(scgi_fd, 1024)) {
+		if (-1 == listen(scgi_fd, host->listen_backlog)) {
 			log_error_write(srv, __FILE__, __LINE__, "ss",
 				"listen failed:", strerror(errno));
 			close(scgi_fd);
@@ -1053,6 +1055,7 @@ SETDEFAULTS_FUNC(mod_scgi_set_defaults) {
 						{ "bin-environment",   NULL, T_CONFIG_ARRAY, T_CONFIG_SCOPE_CONNECTION },        /* 11 */
 						{ "bin-copy-environment", NULL, T_CONFIG_ARRAY, T_CONFIG_SCOPE_CONNECTION },     /* 12 */
 						{ "fix-root-scriptname",  NULL, T_CONFIG_BOOLEAN, T_CONFIG_SCOPE_CONNECTION },   /* 13 */
+						{ "listen-backlog",    NULL, T_CONFIG_INT,   T_CONFIG_SCOPE_CONNECTION },        /* 14 */
 
 
 						{ NULL,                NULL, T_CONFIG_UNSET, T_CONFIG_SCOPE_UNSET }
@@ -1076,6 +1079,7 @@ SETDEFAULTS_FUNC(mod_scgi_set_defaults) {
 					df->idle_timeout = 60;
 					df->disable_time = 60;
 					df->fix_root_path_name = 0;
+					df->listen_backlog = 1024;
 
 					fcv[0].destination = df->host;
 					fcv[1].destination = df->docroot;
@@ -1093,6 +1097,7 @@ SETDEFAULTS_FUNC(mod_scgi_set_defaults) {
 					fcv[11].destination = df->bin_env;
 					fcv[12].destination = df->bin_env_copy;
 					fcv[13].destination = &(df->fix_root_path_name);
+					fcv[14].destination = &(df->listen_backlog);
 
 
 					if (0 != config_insert_values_internal(srv, da_host->value, fcv, T_CONFIG_SCOPE_CONNECTION)) {

src/network.c

@@ -436,7 +436,7 @@ static int network_server_init(server *srv, buffer *host_token, specific_config
 		goto error_free_socket;
 	}
 
-	if (-1 == listen(srv_socket->fd, 128 * 8)) {
+	if (-1 == listen(srv_socket->fd, s->listen_backlog)) {
 		log_error_write(srv, __FILE__, __LINE__, "ss", "listen failed: ", strerror(errno));
 		goto error_free_socket;
 	}