Add comments about how _xcb_conn_ret_error() works

If xcb_connect() fails, it doesn't return NULL. Instead, it always
returns an xcb_connection_t*, and the user should check for errors with
the xcb_connection_has_error() function. What this function does is
check if conn->has_error contains a non-zero error code, and returns it.

If an error did occur, xcb doesn't actually return a full
xcb_connection_t though, it just returns (xcb_connection_t *)
error_code. Since the 'has_error' field is the first, it is still
possible to check conn->has_error.

That last trick was not immediately obvious to me, so add some guiding
comments. This also ensures no one obliviously rearranges the struct.

Signed-off-by: Ran Benita <ran234@gmail.com>
Signed-off-by: Uli Schlachter <psychon@znc.in>

src/xcb_conn.c

@@ -374,6 +374,9 @@ void _xcb_conn_shutdown(xcb_connection_t *c, int err)
 /* Return connection error state.
  * To make thread-safe, I need a seperate static
  * variable for every possible error.
+ * has_error is the first field in xcb_connection_t, so just
+ * return a casted int here; checking has_error (and only
+ * has_error) will be safe.
  */
 xcb_connection_t *_xcb_conn_ret_error(int err)
 {

src/xcbint.h

@@ -192,6 +192,7 @@ void _xcb_ext_destroy(xcb_connection_t *c);
 /* xcb_conn.c */
 
 struct xcb_connection_t {
+    /* This must be the first field; see _xcb_conn_ret_error(). */
     int has_error;
 
     /* constant data */