crash evaluating bogus exception condition expression (sparc-solaris)

crash evaluating bogus exception condition expression (sparc-solaris)

With a program raising an exception, trying to debug that program
in GDB/MI mode can yield a crash:

    % gdb -i=mi foo
    (gdb)
    -catch-exception -e "Program_Error"
    ^done,bkptno="2",bkpt={number="2",type="breakpoint",[...]
    (gdb)
    -exec-continue
    ^running
    *running,thread-id="all"
    (gdb)
    =library-loaded,id=[...]
    &"warning: failed to reevaluate internal exception condition for catchpoint 2: Error in expression, near `'.\n"
    zsh: 22956 bus error (core dumped)  gdb -q -i=mi foo

The problem is triggered by a problem in the compiler which causes
EXP in the following TRY_CATCH block to change unexpectedly when
parse_exp_1 throws an error :

   |      TRY_CATCH (e, RETURN_MASK_ERROR)
   |        {
   |          exp = parse_exp_1 (&s, bl->address,
   |                             block_for_pc (bl->address), 0);
   |        }

In ada-lang.c:create_excep_cond_exprs, EXP is initialized to NULL,
and is expected to remain NULL if parse_exp_1 throws.  Instead,
its value gets changed to something invalid.  This later crashes
the debugger, when trying to evaluate the bogus expression.

This patch works around the issue by simply forcing EXP back to NULL
when an exception was thrown. A comment explaining why, and the sort
of timeline we're looking at for a fix, is also added.

gdb/ChangeLog:

        * ada-lang.c (create_excep_cond_exprs): Force EXP to NULL
        when parse_exp_1 threw an error.  Add comment.