Update comments in X error handling code

* src/xterm.c (struct x_error_message_stack, x_error_message)
(x_error_catcher, x_catch_errors, x_had_errors_p)
(x_clear_errors): Long-overdue comment update.  Describe the
fields of the error message stack, and the fact that
`x_error_message' is a stack, not a pointer to a string (which
has been true since 2006).  Also describe the updated conditions
used to find an applicable error handler and to call
x_uncatch_errors_after_check.

diff --git a/src/xterm.c b/src/xterm.c
index 27ee01dbce124cedbde34d6d4025f0526b21900f..fa7e285425cb12cef9ec68002fc4b125ea85281f 100644 (file)
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -21783,18 +21783,22 @@ x_text_icon (struct frame *f, const char *icon_name)
 \f
 #define X_ERROR_MESSAGE_SIZE 200
 
-/* If non-nil, this should be a string.  It means catch X errors and
-   store the error message in this string.
-
-   The reason we use a stack is that x_catch_error/x_uncatch_error can
-   be called from a signal handler.  */
-
 struct x_error_message_stack
 {
+  /* Buffer containing the error message of any error that was
+     generated.  */
   char string[X_ERROR_MESSAGE_SIZE];
+
+  /* The display this error handler applies to.  */
   Display *dpy;
+
+  /* A function to call upon an error if non-NULL.  */
   x_special_error_handler handler;
+
+  /* Some data to pass to that handler function.  */
   void *handler_data;
+
+  /* The previous handler in this stack.  */
   struct x_error_message_stack *prev;
 
   /* The first request that this error handler applies to.  Keeping
@@ -21803,6 +21807,14 @@ struct x_error_message_stack
   unsigned long first_request;
 };
 
+/* Stack of X error message handlers.  Whenever an error is generated
+   on a display, look in this stack for an appropriate error handler,
+   set its `string' to the error message and call its `handler' with
+   `handler_data'.  If no handler applies to the error, don't catch
+   it, and let it crash Emacs instead.
+
+   This used to be a pointer to a string in which any error would be
+   placed before 2006.  */
 static struct x_error_message_stack *x_error_message;
 
 static struct x_error_message_stack *
@@ -21825,9 +21837,10 @@ x_find_error_handler (Display *dpy, XErrorEvent *event)
   return NULL;
 }
 
-/* An X error handler which stores the error message in
-   *x_error_message.  This is called from x_error_handler if
-   x_catch_errors is in effect.  */
+/* An X error handler which stores the error message in the first
+   applicable handler in the x_error_message stack.  This is called
+   from *x_error_handler if an x_catch_errors for DISPLAY is in
+   effect.  */
 
 static void
 x_error_catcher (Display *display, XErrorEvent *event,
@@ -21844,8 +21857,8 @@ x_error_catcher (Display *display, XErrorEvent *event,
 /* Begin trapping X errors for display DPY.
 
    After calling this function, X protocol errors generated on DPY no
-   longer cause Emacs to exit; instead, they are recorded in the
-   string stored in *x_error_message.
+   longer cause Emacs to exit; instead, they are recorded in an error
+   handler pushed onto the stack `x_error_message'.
 
    Calling x_check_errors signals an Emacs error if an X error has
    occurred since the last call to x_catch_errors or x_check_errors.
@@ -21853,8 +21866,10 @@ x_error_catcher (Display *display, XErrorEvent *event,
    Calling x_uncatch_errors resumes the normal error handling,
    skipping an XSync if the last request made is known to have been
    processed.  Calling x_uncatch_errors_after_check is similar, but
-   skips an XSync to the server, and should be used only immediately
-   after x_had_errors_p or x_check_errors.  */
+   always skips an XSync to the server, and should be used only
+   immediately after x_had_errors_p or x_check_errors, or when it is
+   known that no requests have been made since the last x_catch_errors
+   call for DPY.  */
 
 void
 x_catch_errors_with_handler (Display *dpy, x_special_error_handler handler,
@@ -21962,8 +21977,8 @@ x_check_errors (Display *dpy, const char *format)
     }
 }
 
-/* Nonzero if we had any X protocol errors
-   since we did x_catch_errors on DPY.  */
+/* Nonzero if any X protocol errors were generated since the last call
+   to x_catch_errors on DPY.  */
 
 bool
 x_had_errors_p (Display *dpy)
@@ -21983,7 +21998,8 @@ x_had_errors_p (Display *dpy)
   return x_error_message->string[0] != 0;
 }
 
-/* Forget about any errors we have had, since we did x_catch_errors on DPY.  */
+/* Forget about any errors we have had, since we did x_catch_errors on
+   DPY.  */
 
 void
 x_clear_errors (Display *dpy)