Re: [PATCH] osdep.h: Add doc comment for qemu_get_thread_id()

[Eric Blake <eblake@redhat.com>]

On 7/30/20 10:59 AM, Daniel P. Berrangé wrote:

>> Well, I suspect that management-layer code currently has
>> gone for "assume we're always running on Linux" and was
>> written by people who knew they were getting a Linux tid...
> 
> Yes, on the libvirt side, the functionality that relies on thread_is is
> only compiled on Linux. If someone wants to use it on other OS, they'll
> have to provide an impl using their platforms equivalent of
> sched_setaffinity and friends since none of this stuff is standardized
> across OS.
> 
> 
>>> The PID is quite unlikely to be "an OS-specific identifier of the
>>> current thread".  Shouldn't we fail instead of lie when we don't know
>>> how to compute the truth?
>>
>> Yeah, I think the default codepath is pretty bogus too. Should
>> the QMP functions have a mechanism for saying "we don't know
>> a thread-id on this platform" ?
> 
> Thread_id should be optional and thus not filled in if we
> can't provide a sensible value. Unfortunately we made it
> mandatory in QMP.

Normally, converting a mandatory output value to optional is a 
back-compatibility risk (we could break apps that depended on it being 
present).  But if the only apps that depended on it being present are 
compiled on Linux, where the member will actually be present, I think 
that changing the schema to make it optional for non-Linux platforms 
won't be a back-compatibility nightmare (but we will have to be careful 
in our documentation).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org