Re: [patch] Fix testsuite annotate-quit race (PR 544)

[Chet Ramey <chet.ramey@case.edu>]

Daniel Jacobowitz wrote:
> On Mon, Mar 24, 2008 at 09:01:30PM -0400, Chet Ramey wrote:
>> Daniel Jacobowitz wrote:
>>
>>> What I think is happening is that rl_redisplay has some optimizations
>>> which cause it not to redisplay.  And since we have a custom function
>>> installed readline isn't calling the alternate hooks that force
>>> display.  So rl_redisplay does not actually meet the interface
>>> associated with rl_redisplay_function despite being its default
>>> value.  Nasty.
>> That's interesting, if true.  Can anyone add detail confirming this?
> 
> Sure.  There's a check "rl_redisplay_function == rl_redisplay" in the
> readline source, and another in the definition of
> CUSTOM_REDISPLAY_FUNCTION.  So that function is inherently special.
> 
> From what Nick and Jan have found, stty -echo and then running GDB
> with a custom redisplay function that just calls rl_redisplay
> will not display the GDB prompt.

You were pretty close to the answer; had you looked at the source
around the check you identified, you would have found it
(readline.c:readline_internal_setup()):

   /* If we're not echoing, we still want to at least print a prompt, because
      rl_redisplay will not do it for us.  If the calling application has a
      custom redisplay function, though, let that function handle it. */
   if (readline_echoing_p == 0 && rl_redisplay_function == rl_redisplay)
     {
       if (rl_prompt && rl_already_prompted == 0)
         {
           nprompt = _rl_strip_prompt (rl_prompt);
           fprintf (_rl_out_stream, "%s", nprompt);
           fflush (_rl_out_stream);
           free (nprompt);
         }
     }

You've turned off echo, and the redisplay code won't print anything in that
case.  Readline, when using its internal redisplay function, chooses to
print a prompt to at least visually clue the user that it's around.
Having accepted responsibility for handling redisplay, gdb can do whatever
it deems appropriate.

Chet
-- 
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Chet Ramey, ITS, CWRU    chet@case.edu    http://cnswww.cns.cwru.edu/~chet/