Re: [Qemu-devel] Missing mon in monitor_cur_is_qmp() and qerror_report()

[Markus Armbruster]

Jan Kiszka <address@hidden> writes:

> Markus Armbruster wrote:
>> Jan Kiszka <address@hidden> writes:
>> 
>>> Luiz,
>>>
>>> I missed this when the API was first proposed:
>>>
>>> cur_mon is scheduled for removal (one day...). It's just an intermediate
>>> step to convert all users to explicit 'mon' passing. Thus, new APIs
>>> should not rely it.
>>>
>>> I just realized that monitor_cur_is_qmp() does so. It should be
>>> refactored to monitor_is_qmp(Monitor *mon). And qerror should be enhance
>>> by a 'mon' argument as well. Callers that aren't passed a 'mon'
>>> themselves should either be fixed at this chance or could fall back to
>>> cur_mon for the time being.
>>>
>>> So far for the theory - do you see any pitfalls in the existing usage?
>> 
>> I put in the new uses of cur_mon, Luiz "only" ACKed them.
>> 
>> At any point in the program execution, we have one current monitor, or
>> none.  Passing around the current monitor within monitor code is
>> workable, if somewhat tedious.  But we need it not just in monitor code,
>> we need it anywhere where we report errors.  In other words, pretty much
>> everywhere.  Including places that do not and should not know about the
>> monitor.  Handing a monitor parameter down pretty much every call chain
>> is beyond tedious, it's impractical.
>
> It's a process, but I don't think it's impractical per se.
>
>> 
>> The code reporting an error generally does not and should not know
>> anything about *how* the error gets communicated to the user.
>> Insulating it from that detail is proper separation of concerns, and
>> global variable cur_mon is my tool to get it.  Good software
>> engineering.  Like many powerful tools, global variables should be used
>> sparingly and with care.  I feel this use is well justified.
>> 
>> Instead of eliminating cur_mon, I'd like it to be hidden within
>> monitor.c.  There are a few uses left outside it.
>
> If we start to allow cur_mon for error reporting, there is no reason not
> to convert monitor_printf back to where it came from. Back then we
> agreed on the current path. If we now decide to roll back, then let's
> make it consistently.

Makes sense.

>                       But we already refactored quite a lot of code for
> explicit monitor passing...
>
> Jan
>
> PS: A patch for establishing monitor_is_qmp is in my queue. Holding it
> back for now until we agreed how to proceed.

monitor_is_qmp() is used only in a few places.  The real troublemakers
are error_report() & friends, and qerror_report().  These are all over
the place, with more to come.