esp_http_server improvements to allow adding transport layer encryption

Changes:
- renamed `httpd_free_sess_ctx_fn_t` to `httpd_free_ctx_fn_t`
- added a `httpd_handle_t` argument to `httpd_send_func_t` and `httpd_recv_func_t`
- internal function `httpd_sess_get()` is no longer static, as it's used in other
  files besides httpd_sess.c

Bug fixes:
- removed a trailing semicolon from `HTTPD_DEFAULT_CONFIG()`
- fixed issue with failed `select()`, now it automatically closes invalid sockets
  instead of shutting down the entire server

New features:
- `httpd_resp_send()` and `httpd_resp_send_chunk()` now accept -1 as length to use
  `strlen()` internally
- added `httpd_sess_set_ctx()` to accompany `httpd_sess_get_ctx()`
- added a "transport context" to the session structure (next to user context)
- added `httpd_sess_{get,set}_transport_ctx()` to work with this transport context
- added "global user context" and "global transport context" stored in the server
  config (and then the handle); supports a user-provided free_fn
- added a "pending func" to e.g. check for data in the transport layer receive
  buffer
- added functions `httpd_set_sess_{send,recv,pending}_override()` that target
  a session by ID (i.e. not using a request object)
- added `httpd_set_pending_override()`
- added a "open_fn" and "close_fn" - functions called when creating and closing
  a session. These may be used to set up transport layer encryption or some other
  session-wide feature

components/esp_http_server/src/esp_httpd_priv.h

@@ -118,10 +118,13 @@ typedef enum {
 struct sock_db {
     int fd;                                 /*!< The file descriptor for this socket */
     void *ctx;                              /*!< A custom context for this socket */
+    void *transport_ctx;                    /*!< A custom 'transport' context for this socket, to be used by send/recv/pending */
     httpd_handle_t handle;                  /*!< Server handle */
-    httpd_free_sess_ctx_fn_t free_ctx;      /*!< Function for freeing the context */
+    httpd_free_ctx_fn_t free_ctx;      /*!< Function for freeing the context */
+    httpd_free_ctx_fn_t free_transport_ctx; /*!< Function for freeing the 'transport' context */
     httpd_send_func_t send_fn;              /*!< Send function for this socket */
-    httpd_recv_func_t recv_fn;              /*!< Send function for this socket */
+    httpd_recv_func_t recv_fn;              /*!< Receive function for this socket */
+    httpd_pending_func_t pending_fn;        /*!< Pending function for this socket */
     int64_t timestamp;                      /*!< Timestamp indicating when the socket was last used */
     char pending_data[PARSER_BLOCK_SIZE];   /*!< Buffer for pending data to be received */
     size_t pending_len;                     /*!< Length of pending data to be received */
@@ -169,6 +172,23 @@ struct httpd_data {
  * @{
  */
 
+/**
+ * @brief Retrieve a session by its descriptor
+ *
+ * @param[in] hd     Server instance data
+ * @param[in] sockfd Socket FD
+ * @return pointer into the socket DB, or NULL if not found
+ */
+struct sock_db *httpd_sess_get(struct httpd_data *hd, int sockfd);
+
+/**
+ * @brief Delete sessions whose FDs have became invalid.
+ *        This is a recovery strategy e.g. after select() fails.
+ *
+ * @param[in] hd    Server instance data
+ */
+void httpd_sess_delete_invalid(struct httpd_data *hd);
+
 /**
  * @brief   Initializes an http session by resetting the sockets database.
  *
@@ -454,6 +474,7 @@ size_t httpd_unrecv(struct httpd_req *r, const char *buf, size_t buf_len);
  *          NEVER be called directly. The semantics of this is exactly similar to
  *          send() of the BSD socket API.
  *
+ * @param[in] hd      Server instance data
  * @param[in] sockfd  Socket descriptor for sending data
  * @param[in] buf     Pointer to the buffer from where the body of the response is taken
  * @param[in] buf_len Length of the buffer
@@ -463,13 +484,14 @@ size_t httpd_unrecv(struct httpd_req *r, const char *buf, size_t buf_len);
  *  - Length of data : if successful
  *  - -1             : if failed (appropriate errno is set)
  */
-int httpd_default_send(int sockfd, const char *buf, size_t buf_len, int flags);
+int httpd_default_send(httpd_handle_t hd, int sockfd, const char *buf, size_t buf_len, int flags);
 
 /**
  * @brief   This is the low level default recv function of the HTTPD. This should
  *          NEVER be called directly. The semantics of this is exactly similar to
  *          recv() of the BSD socket API.
  *
+ * @param[in] hd      Server instance data
  * @param[in] sockfd  Socket descriptor for sending data
  * @param[out] buf    Pointer to the buffer which will be filled with the received data
  * @param[in] buf_len Length of the buffer
@@ -479,7 +501,7 @@ int httpd_default_send(int sockfd, const char *buf, size_t buf_len, int flags);
  *  - Length of data : if successful
  *  - -1             : if failed (appropriate errno is set)
  */
-int httpd_default_recv(int sockfd, char *buf, size_t buf_len, int flags);
+int httpd_default_recv(httpd_handle_t hd, int sockfd, char *buf, size_t buf_len, int flags);
 
 /** End of Group : Send and Receive
  * @}