HTTP Server : Return HTTPD_SOCK_ERR_ based on errno set during send / recv

This feature allows `httpd_req_recv()` and `httpd_send()` functions to return
specific `HTTPD_SOCK_ERR_` codes in case of socket errors. This is useful
in case of errors like `EAGAIN`, `EINTR`, etc. when the user may want to
retry `httpd_req_recv()` / `httpd_send()` function call.

components/http_server/include/http_server.h

@@ -348,19 +348,39 @@ esp_err_t httpd_unregister_uri(httpd_handle_t handle, const char* uri);
  * @{
  */
 
+#define HTTPD_SOCK_ERR_FAIL      -1
+#define HTTPD_SOCK_ERR_INVALID   -2
+#define HTTPD_SOCK_ERR_TIMEOUT   -3
+
 /**
  * @brief  Prototype for HTTPDs low-level send function
+ *
+ * @note   User specified send function must handle errors internally,
+ *         depending upon the set value of errno, and return specific
+ *         HTTPD_SOCK_ERR_ codes, which will eventually be conveyed as
+ *         return value of httpd_send() function
+ *
  * @return
  *  - Bytes : The number of bytes sent successfully
- *  - -VE   : In case of error
+ *  - HTTPD_SOCK_ERR_INVALID  : Invalid arguments
+ *  - HTTPD_SOCK_ERR_TIMEOUT  : Timeout/interrupted while calling socket send()
+ *  - HTTPD_SOCK_ERR_FAIL     : Unrecoverable error while calling socket send()
  */
 typedef int (*httpd_send_func_t)(int sockfd, const char *buf, size_t buf_len, int flags);
 
 /**
  * @brief  Prototype for HTTPDs low-level recv function
+ *
+ * @note   User specified recv function must handle errors internally,
+ *         depending upon the set value of errno, and return specific
+ *         HTTPD_SOCK_ERR_ codes, which will eventually be conveyed as
+ *         return value of httpd_req_recv() function
+ *
  * @return
  *  - Bytes : The number of bytes received successfully
- *  - -VE   : In case of error
+ *  - HTTPD_SOCK_ERR_INVALID  : Invalid arguments
+ *  - HTTPD_SOCK_ERR_TIMEOUT  : Timeout/interrupted while calling socket recv()
+ *  - HTTPD_SOCK_ERR_FAIL     : Unrecoverable error while calling socket recv()
  */
 typedef int (*httpd_recv_func_t)(int sockfd, char *buf, size_t buf_len, int flags);
 
@@ -462,7 +482,9 @@ int httpd_req_to_sockfd(httpd_req_t *r);
  * @return
  *  - Bytes    : Number of bytes read into the buffer successfully
  *  - Zero     : When no more data is left for read
- *  - -1       : On raw recv error / Null arguments / Request pointer is invalid
+ *  - HTTPD_SOCK_ERR_INVALID  : Invalid arguments
+ *  - HTTPD_SOCK_ERR_TIMEOUT  : Timeout/interrupted while calling socket recv()
+ *  - HTTPD_SOCK_ERR_FAIL     : Unrecoverable error while calling socket recv()
  */
 int httpd_req_recv(httpd_req_t *r, char *buf, size_t buf_len);
 
@@ -796,7 +818,9 @@ esp_err_t httpd_resp_send_404(httpd_req_t *r);
  *
  * @return
  *  - Bytes : Number of bytes that were sent successfully
- *  - -1    : Error in raw send / Invalid request / Null arguments
+ *  - HTTPD_SOCK_ERR_INVALID  : Invalid arguments
+ *  - HTTPD_SOCK_ERR_TIMEOUT  : Timeout/interrupted while calling socket send()
+ *  - HTTPD_SOCK_ERR_FAIL     : Unrecoverable error while calling socket send()
  */
 int httpd_send(httpd_req_t *r, const char *buf, size_t buf_len);