[PATCH] Replace deprecated_target_wait_hook by an observer

[Simon Marchi]

On 2021-08-25 12:19 p.m., Andrew Burgess wrote:
> * Patrick Monnerat <patrick@monnerat.net> [2021-08-25 15:30:28 +0200]:
> 
>>
>> On 8/24/21 6:14 PM, Andrew Burgess wrote:
>>> * Patrick Monnerat via Gdb-patches <gdb-patches@sourceware.org> [2021-08-22 18:42:56 +0200]:
>>>
>>>> +DEFINE_OBSERVABLE (waiting_for_target);
>>> Given we already have events 'target_changed' and 'target_resumed', I
>>> wonder if it would be more consistent to name this event 'target_wait'?
>>>
>> Yes, it's possible. Here are the two reasons why I did not name this
>> observer 'target_wait':
>>
>> 1) Because the observer is not supposed to wait by itself, I fear it will be
>> a source of confusion.
>>
>> 2) As a good old C programmer, I still have some reluctance naming an object
>> as a global procedure. Not being declared in the same namespace though!
>>
>> Comments are welcome!
> 
> How about 'target_waiting' then?  This seems more inline with the
> existing naming, seems to indicate that the target _is_ waiting, not
> the that observer _should_ wait (so avoiding #1), and is a new name
> (so avoiding #2).

Oh, I have always seen it as the caller waiting for the target to
produce some event (in a context where everything was blocking / not
async... nowadays, with async, I always find using the term "wait" for
"fetch an event" confusing).  So the event would be
"target_waited_on".  Another way (it's not our current style so I
wouldn't choose that) would be to name all events "on something".  So
"on_target_wait".

Regardless, since we are debating this, I would suggest splitting the
observer in two actuall:

 - target_pre_wait (or target_wait_pre or whatever)
 - target_post_wait (or target_wait_post or whatever)

I think it's better to keep 1 observable == 1 event, rather than
having two events in one with a boolean to differentiate.

>> That would make sense, but in case we have an exception, event_ptid is not
>> known. How would you handle it? pass it as null_ptid?
> 
> Yeah I guess that would make sense.

I think that's ok, I don't think target_ops::wait implementations ever
return null_ptid (although I haven't checked, it's just from memory).

Simon