Re: Watchpoint on an unloaded shared library(2)

[Joel Brobecker <brobecker@adacore.com>]

> The cause of a crash is that print_one_breakpoint_location in breakpoint.c
> doesn't care about whether the expression for the watchpoint is valid:
> 
>       case bp_watchpoint:
>       case bp_hardware_watchpoint:
>       case bp_read_watchpoint:
>       case bp_access_watchpoint:
>         /* Field 4, the address, is omitted (which makes the columns
>            not line up too nicely with the headers, but the effect
>            is relatively readable).  */
>         if (opts.addressprint)
>           ui_out_field_skip (uiout, "addr");
>         annotate_field (5);
>         print_expression (b->exp, stb->stream);
>         ui_out_field_stream (uiout, "what", stb);
>         break;
> 
> Here, b->exp for the watchpoints set on an unloaded shared library can
> be NULL, because breakpoint_re_set_one has done it.  However, what
> should we do instead?
> 
> I have considered two solutions:
> 
>  a) Print b->exp_string and b->cond_string. 
>     We might make some effort to display it like as its expression is
>     valid for annotations... I have no idea whether it is worthwhile
>     to try.  

I think that this is the best we can do (print exp_string).
I personally wouldn't worry about trying to massage the string
into something that would look like we're printing an expression.
I'm not even sure why we use "print_expression (b->exp) rather
than printing exp_string directly - perhaps someone does?

>  b) Don't set b->exp to NULL in update_watchpoint (called by
>     breakpoint_re_set_one), do_enable_breakpoint and so on.  
>     Maybe we should add some flags to `struct expression' to avoid
>     passing invalid symtabs to some interacting functions.  

I don't think we can do that. Otherwise, the expression could
reference some symbols that no longer exist.

> "Skip printing" is another possibility, but I'd ignore it: skipping
> means that the user can't refer to the information about what they
> were.  

Right, as long as the watchpoint is defined, albeit disabled,
we need to show it in the list. Another alternative of your suggestion
is to not print the "what".  But I would find this confusing, since
you would no longer know what the watchpoint entry is watching.

-- 
Joel