feat(httpd): add support for asynchronous request handling

This commit adds support for handling multiple requests simultaneously by introducing two new functions: `httpd_req_async_handler_begin()` and `httpd_req_async_handler_complete()`. These functions allow creating an asynchronous copy of a request that can be used on a separate thread and marking the asynchronous request as completed, respectively.

Additionally, a new flag `for_async_req` has been added to the `httpd_sess_t` struct to indicate if a socket is being used for an asynchronous request and should not be purged from the LRU cache.

An example have been added to demonstrate the usage of these new functions.

Closes https://github.com/espressif/esp-idf/issues/10594

Signed-off-by: Harshit Malpani <harshit.malpani@espressif.com>

components/esp_http_server/src/httpd_txrx.c

@@ -554,6 +554,51 @@ int httpd_req_recv(httpd_req_t *r, char *buf, size_t buf_len)
     return ret;
 }
 
+esp_err_t httpd_req_async_handler_begin(httpd_req_t *r, httpd_req_t **out)
+{
+    if (r == NULL || out == NULL) {
+        return ESP_ERR_INVALID_ARG;
+    }
+
+    // alloc async req
+    httpd_req_t *async = malloc(sizeof(httpd_req_t));
+    if (async == NULL) {
+        return ESP_ERR_NO_MEM;
+    }
+    memcpy(async, r, sizeof(httpd_req_t));
+
+    // alloc async aux
+    async->aux = malloc(sizeof(struct httpd_req_aux));
+    if (async->aux == NULL) {
+        free(async);
+        return ESP_ERR_NO_MEM;
+    }
+    memcpy(async->aux, r->aux, sizeof(struct httpd_req_aux));
+
+    // mark socket as "in use"
+    struct httpd_req_aux *ra = r->aux;
+    ra->sd->for_async_req = true;
+
+    *out = async;
+
+    return ESP_OK;
+}
+
+esp_err_t httpd_req_async_handler_complete(httpd_req_t *r)
+{
+    if (r == NULL) {
+        return ESP_ERR_INVALID_ARG;
+    }
+
+    struct httpd_req_aux *ra = r->aux;
+    ra->sd->for_async_req = false;
+
+    free(r->aux);
+    free(r);
+
+    return ESP_OK;
+}
+
 int httpd_req_to_sockfd(httpd_req_t *r)
 {
     if (r == NULL) {