[petsc-dev] PetscOptionsGetViewer() problems with multiple usage on the same object but different options databases

[Jed Brown]

Can we have a -xxx_view none or null to unset the view? Could even be a viewer type that does nothing. I don't like the out-of-band -xxx_view_cancel.

Barry Smith <bsmith at petsc.dev> writes:

>   ALL the other options are sticky.  Calling setFromOptions() twice does not change the value of rtol etc etc etc that was set the last time and even -ksp_monitor is sticky, why are the viewers so special in that they are the only ones that are not sticky? Is not consistent, doesn't make sense, when -ksp_monitor is sticky. It is just a freak feature of the implementation of PetscOptionsGetViewer() that no one realized was there when they wrote it (probably me :-)).
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>> On Jan 22, 2023, at 1:16 PM, Jed Brown <jed at jedbrown.org> wrote:
>> 
>> Matthew Knepley <knepley at gmail.com> writes:
>> 
>>> On Sun, Jan 22, 2023 at 1:06 PM Barry Smith <bsmith at petsc.dev> wrote:
>>> 
>>>> 
>>>>  That makes incrementally adding new options to an existing object
>>>> difficult, since each call to setFromOptions() screws up previous calls.
>>>> 
>>>> We already have
>>>> 
>>>> -ksp_monitor_cancel
>>>> -ksp_converged_reason_view_cancel
>>>> 
>>> 
>>> Let me address it another way. If I understand correctly, what happens
>>> currently is that at each ViewFromOptions() call, the database is inspected
>>> and if a *_view is found, then the view is executed. So when you replace
>>> the database, if this option is not found then nothing happens. This seems
>>> to satisfy a kind of options locality. If instead we make some options
>>> sticky, like -ksp_view, so that once given they never expire, this would
>>> make the code exceedingly hard to reason about, especially if options are
>>> set deep inside something, before a user is knowingly doing things. This
>>> does not sound like the right model to me.
>> 
>> We could have something like PetscOptionsMerge() if one wants to preserve previously set options while adding some new ones (without tainting the global database). I don't like stickiness in general (a key reason why CMake is impossible to reason about and best practice is to run rm -rf * && cmake .. -DALL_THE_OPTIONS).