New debugging facility: backtraces from errors in Lisp called from redisplay

Setting backtrace-on-redisplay-error to non-nil enables the generation of a
Lisp backtrace in buffer *Redisplay-trace* following an error in Lisp called
from redisplay.

* doc/lispref/debugging.texi (Debugging Redisplay): New subsection.
(Error Debugging): Reference to the new subsection.

* etc/NEWS: New entry for the new facility.

* src/eval.c (redisplay_deep_handler): New variable.
(init_eval): Initialize redisplay_deep_handler.
(call_debugger): Don't throw to top-level after calling debug-early
(internal_condition_case_n): "Bind" redisplay_deep_handler to the current
handler.
(backtrace_yet): New boolean variable.
(signal_or_quit): New code section to handle Lisp errors occurring in
redisplay.
(syms_of_eval): New DEFVAR_BOOL backtrace-on-redisplay-error.

* src/keyboard.c (command_loop_1): Set backtrace_yet to false each time around
the loop.
(safe_run_hooks_error): Allow args to be up to four Lisp_Objects long.
(safe_run_hooks_2): New function.

* src/lisp.h (top level): declare as externs backtrace_yet and
safe_run_hooks_2.

* src/xdisp.c (run_window_scroll_functions): Replace a call to
run_hook_with_args_2 with one to safe_run_hooks_2.

1 files changed, 7 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 8d54ccc0dc8..2747cec18c5 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1339,6 +1339,13 @@ When invoked with a non-zero prefix argument, as in 'C-u C-x C-e',
 this command will pop up a new buffer and show the full pretty-printed
 value there.
 
++++
+*** You can now generate a backtrace from Lisp errors in redisplay.
+To do this, set the new variable 'backtrace-on-redisplay-error' to a
+non-nil value.  The backtrace will be written to buffer
+*Redisplay-trace*.  This buffer will not be automatically displayed in
+a window.
+
 ** Compile
 
 +++