esp_http_server: support dynamic payload len for ws server

Closes https://github.com/espressif/esp-idf/issues/6433

examples/protocols/http_server/ws_echo_server/README.md

@@ -16,6 +16,20 @@ Each outgoing frame has the FIN flag set by default.
 In case an application wants to send fragmented data, it must be done manually by setting the
 `fragmented` option and using the `final` flag as described in [RFC6455, section 5.4](https://tools.ietf.org/html/rfc6455#section-5.4).
 
+`httpd_ws_recv_frame` support two ways to get frame payload.
+* Static buffer -- Allocate maximum expected packet length (either statically or dynamically) and call `httpd_ws_recv_frame()` referencing this buffer and it's size. (Unnecessarily large buffers might cause memory waste)
+
+```
+#define MAX_PAYLOAD_LEN 128
+uint8_t buf[MAX_PAYLOAD_LEN] = { 0 };
+httpd_ws_frame_t ws_pkt;
+ws_pkt.payload = buf;
+httpd_ws_recv_frame(req, &ws_pkt, MAX_PAYLOAD_LEN);
+```
+* Dynamic buffer -- Refer to the examples, which receive websocket data in these three steps:
+   1) Call `httpd_ws_recv_frame()` with zero buffer size
+   2) Allocate the size based on the received packet length
+   3) Call `httpd_ws_recv_frame()` with the allocated buffer
 
 ### Hardware Required
 

examples/protocols/http_server/ws_echo_server/main/ws_echo_server.c

@@ -67,12 +67,25 @@ static esp_err_t trigger_async_send(httpd_handle_t handle, httpd_req_t *req)
  */
 static esp_err_t echo_handler(httpd_req_t *req)
 {
-    uint8_t buf[128] = { 0 };
     httpd_ws_frame_t ws_pkt;
     memset(&ws_pkt, 0, sizeof(httpd_ws_frame_t));
-    ws_pkt.payload = buf;
     ws_pkt.type = HTTPD_WS_TYPE_TEXT;
-    esp_err_t ret = httpd_ws_recv_frame(req, &ws_pkt, 128);
+    /* Set max_len = 0 to get the frame len */
+    esp_err_t ret = httpd_ws_recv_frame(req, &ws_pkt, 0);
+    if (ret != ESP_OK) {
+        ESP_LOGE(TAG, "httpd_ws_recv_frame failed to get frame len with %d", ret);
+        return ret;
+    }
+    ESP_LOGI(TAG, "frame len is %d", ws_pkt.len);
+    /* ws_pkt.len + 1 is for NULL termination as we are expecting a string */
+    uint8_t *buf = calloc(1, ws_pkt.len + 1);
+    if (buf == NULL) {
+        ESP_LOGE(TAG, "Failed to calloc memory for buf");
+        return ESP_ERR_NO_MEM;
+    }
+    ws_pkt.payload = buf;
+    /* Set max_len = ws_pkt.len to get the frame payload */
+    ret = httpd_ws_recv_frame(req, &ws_pkt, ws_pkt.len);
     if (ret != ESP_OK) {
         ESP_LOGE(TAG, "httpd_ws_recv_frame failed with %d", ret);
         return ret;
@@ -81,6 +94,8 @@ static esp_err_t echo_handler(httpd_req_t *req)
     ESP_LOGI(TAG, "Packet type: %d", ws_pkt.type);
     if (ws_pkt.type == HTTPD_WS_TYPE_TEXT &&
         strcmp((char*)ws_pkt.payload,"Trigger async") == 0) {
+        free(buf);
+        buf = NULL;
         return trigger_async_send(req->handle, req);
     }
 
@@ -88,6 +103,8 @@ static esp_err_t echo_handler(httpd_req_t *req)
     if (ret != ESP_OK) {
         ESP_LOGE(TAG, "httpd_ws_send_frame failed with %d", ret);
     }
+    free(buf);
+    buf = NULL;
     return ret;
 }