Re: [PATCH] Catch errors in value_get_print_value

[Daniel Jacobowitz <drow@false.org>]

On Fri, Feb 02, 2007 at 09:51:39AM +1300, Nick Roberts wrote:
> 
> Currently when common_val_print throws an error from install_new_value it gets
> caught in mi_execute_command and prints an MI error (^error,...)  without
> cleaning up.  This patch catches the error in value_get_print_value so that GDB
> can proceed normally.  It could be adapted to output something like
> "<error_reading_variable>" but I think the empty string works better.

Could you explain why?  I would be confused to see an empty value in
my locals window.  That's separate from this bug though.

> !   TRY_CATCH (except, RETURN_MASK_ERROR)
> !     {
> !       common_val_print (value, stb, format_code[(int) format], 1, 0, 0);
> !     }
> ! 

val_print, called by common_val_print, already has code to catch this
error and prettyprint it.  It doesn't work in this case, through
common_val_print, because of:

#2  0x0000000000445abd in memory_error (status=5, memaddr=4)
    at /space/fsf/commit/src/gdb/corefile.c:229
#3  0x00000000004aac7f in value_fetch_lazy (val=0x9688c0)
    at /space/fsf/commit/src/gdb/valops.c:549
#4  0x00000000004a2f2f in value_contents_all (value=0x9688c0)
    at /space/fsf/commit/src/gdb/value.c:331
#5  0x00000000004b19a4 in common_val_print (val=0x9688c0,
    stream=0x94a6b0, format=0, deref_ref=1, recurse=0,
    pretty=Val_no_prettyprint)
    at /space/fsf/commit/src/gdb/valprint.c:284

I tried handling the error in value_check_printable.  That doesn't have
quite the results that I wanted, because if install_new_value is called
first it sets value = NULL instead of leaving the unfetchable value
there.  That's what happens for a1.

That gave me an idea.  The error only arises for b1 in your testcase,
where initial && !changeable and indeed we never use var->print_value
if !changeable.  So avoiding common_val_print in that case avoids the
error.

So I checked this in instead.  Let me know if it doesn't work for you.

-- 
Daniel Jacobowitz
CodeSourcery

2007-02-08  Daniel Jacobowitz  <dan@codesourcery.com>

	* varobj.c (install_new_value): Only call value_get_print_value
	if changeable.

Index: varobj.c
===================================================================
RCS file: /cvs/src/src/gdb/varobj.c,v
retrieving revision 1.82
diff -u -p -r1.82 varobj.c
--- varobj.c	24 Jan 2007 19:54:13 -0000	1.82
+++ varobj.c	8 Feb 2007 17:38:36 -0000
@@ -953,7 +953,7 @@ install_new_value (struct varobj *var, s
   /* If the type is changeable, compare the old and the new values.
      If this is the initial assignment, we don't have any old value
      to compare with.  */
-  if (initial)
+  if (initial && changeable)
     var->print_value = value_get_print_value (value, var->format);
   else if (changeable)
     {