esp_http_server : Only accept new connections if server has capacity to handle more

This fix prevents HTTP server from accepting new connections when the total count of connected
sockets has reached the max_open_sockets limit set during configuration. The pending connections
are kept in backlog until atleast one of the connected sockets is closed. The maximum number of
connection requests that can kept in backlog is specified as backlog_conn configuration option.
Note that this modification has no effect when LRU purge is enabled.

Also added sanity check on setting for max_open_sockets during configuration.

Solution suggested by jimparis https://github.com/espressif/esp-idf/issues/3183#issue-421234265

Closes https://github.com/espressif/esp-idf/issues/3183

diff --git a/components/esp_http_server/src/httpd_main.c b/components/esp_http_server/src/httpd_main.c
index 52228a97c4573e91334991787287116604569aad..d361bfee634b585f9c2f5b68327d249c9ca73358 100644 (file)
--- a/components/esp_http_server/src/httpd_main.c
+++ b/components/esp_http_server/src/httpd_main.c
@@ -156,7 +156,12 @@ static esp_err_t httpd_server(struct httpd_data *hd)
 {
     fd_set read_set;
     FD_ZERO(&read_set);
-    FD_SET(hd->listen_fd, &read_set);
+    if (hd->config.lru_purge_enable || httpd_is_sess_available(hd)) {
+        /* Only listen for new connections if server has capacity to
+         * handle more (or when LRU purge is enabled, in which case
+         * older connections will be closed) */
+        FD_SET(hd->listen_fd, &read_set);
+    }
     FD_SET(hd->ctrl_fd, &read_set);
 
     int tmp_max_fd;
@@ -348,6 +353,23 @@ esp_err_t httpd_start(httpd_handle_t *handle, const httpd_config_t *config)
         return ESP_ERR_INVALID_ARG;
     }
 
+    /* Sanity check about whether LWIP is configured for providing the
+     * maximum number of open sockets sufficient for the server. Though,
+     * this check doesn't guarantee that many sockets will actually be
+     * available at runtime as other processes may use up some sockets.
+     * Note that server also uses 3 sockets for its internal use :
+     *     1) listening for new TCP connections
+     *     2) for sending control messages over UDP
+     *     3) for receiving control messages over UDP
+     * So the total number of required sockets is max_open_sockets + 3
+     */
+    if (CONFIG_LWIP_MAX_SOCKETS < config->max_open_sockets + 3) {
+        ESP_LOGE(TAG, "Configuration option max_open_sockets is too large (max allowed %d)\n\t"
+                      "Either decrease this or configure LWIP_MAX_SOCKETS to a larger value",
+                      CONFIG_LWIP_MAX_SOCKETS - 3);
+        return ESP_ERR_INVALID_ARG;
+    }
+
     struct httpd_data *hd = httpd_create(config);
     if (hd == NULL) {
         /* Failed to allocate memory */