Revised tcp_server example.

Fixed error caused by unnecessary reopening of listening socket.
Extracted packet retransmit logic to separate function.
Added robust implementation for sending data

Closes https://github.com/espressif/esp-idf/pull/3597

examples/protocols/sockets/tcp_server/main/tcp_server.c

@@ -28,15 +28,41 @@
 
 static const char *TAG = "example";
 
+static void do_retransmit(const int sock)
+{
+    int len;
+    char rx_buffer[128];
+
+    do {
+        len = recv(sock, rx_buffer, sizeof(rx_buffer) - 1, 0);
+        if (len < 0) {
+            ESP_LOGE(TAG, "Error occurred during receiving: errno %d", errno);
+        } else if (len == 0) {
+            ESP_LOGW(TAG, "Connection closed");
+        } else {
+            rx_buffer[len] = 0; // Null-terminate whatever is received and treat it like a string
+            ESP_LOGI(TAG, "Received %d bytes: %s", len, rx_buffer);
+
+            // send() can return less bytes than supplied length.
+            // Walk-around for robust implementation. 
+            int to_write = len;
+            while (to_write > 0) {
+                int written = send(sock, rx_buffer + (len - to_write), to_write, 0);
+                if (written < 0) {
+                    ESP_LOGE(TAG, "Error occurred during sending: errno %d", errno);
+                }
+                to_write -= written;
+            }
+        }
+    } while (len > 0);
+}
 
 static void tcp_server_task(void *pvParameters)
 {
-    char rx_buffer[128];
     char addr_str[128];
     int addr_family;
     int ip_protocol;
 
-    while (1) {
 
 #ifdef CONFIG_EXAMPLE_IPV4
     struct sockaddr_in dest_addr;
@@ -59,22 +85,26 @@ static void tcp_server_task(void *pvParameters)
     int listen_sock = socket(addr_family, SOCK_STREAM, ip_protocol);
     if (listen_sock < 0) {
         ESP_LOGE(TAG, "Unable to create socket: errno %d", errno);
-            break;
+        vTaskDelete(NULL);
+        return;
     }
     ESP_LOGI(TAG, "Socket created");
 
     int err = bind(listen_sock, (struct sockaddr *)&dest_addr, sizeof(dest_addr));
     if (err != 0) {
         ESP_LOGE(TAG, "Socket unable to bind: errno %d", errno);
-            break;
+        goto CLEAN_UP;
     }
     ESP_LOGI(TAG, "Socket bound, port %d", PORT);
 
     err = listen(listen_sock, 1);
     if (err != 0) {
         ESP_LOGE(TAG, "Error occurred during listen: errno %d", errno);
-            break;
+        goto CLEAN_UP;
     }
+
+    while (1) {
+
         ESP_LOGI(TAG, "Socket listening");
 
         struct sockaddr_in6 source_addr; // Large enough for both IPv4 or IPv6
@@ -84,47 +114,23 @@ static void tcp_server_task(void *pvParameters)
             ESP_LOGE(TAG, "Unable to accept connection: errno %d", errno);
             break;
         }
-        ESP_LOGI(TAG, "Socket accepted");
 
-        while (1) {
+        // Convert ip address to string
-            int len = recv(sock, rx_buffer, sizeof(rx_buffer) - 1, 0);
-            // Error occurred during receiving
-            if (len < 0) {
-                ESP_LOGE(TAG, "recv failed: errno %d", errno);
-                break;
-            }
-            // Connection closed
-            else if (len == 0) {
-                ESP_LOGI(TAG, "Connection closed");
-                break;
-            }
-            // Data received
-            else {
-                // Get the sender's ip address as string
         if (source_addr.sin6_family == PF_INET) {
             inet_ntoa_r(((struct sockaddr_in *)&source_addr)->sin_addr.s_addr, addr_str, sizeof(addr_str) - 1);
         } else if (source_addr.sin6_family == PF_INET6) {
             inet6_ntoa_r(source_addr.sin6_addr, addr_str, sizeof(addr_str) - 1);
         }
+        ESP_LOGI(TAG, "Socket accepted ip address: %s", addr_str);
 
-                rx_buffer[len] = 0; // Null-terminate whatever we received and treat like a string
+        do_retransmit(sock);
-                ESP_LOGI(TAG, "Received %d bytes from %s:", len, addr_str);
-                ESP_LOGI(TAG, "%s", rx_buffer);
 
-                int err = send(sock, rx_buffer, len, 0);
-                if (err < 0) {
-                    ESP_LOGE(TAG, "Error occurred during sending: errno %d", errno);
-                    break;
-                }
-            }
-        }
-
-        if (sock != -1) {
-            ESP_LOGE(TAG, "Shutting down socket and restarting...");
         shutdown(sock, 0);
         close(sock);
     }
-    }
+
+CLEAN_UP:
+    close(listen_sock);
     vTaskDelete(NULL);
 }