Re: [PATCH 2/3] qapi, audio: respect build time conditions in audio schema

[Markus Armbruster]

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Wed, Mar 03, 2021 at 08:00:59AM +0100, Gerd Hoffmann wrote:
>> On Tue, Mar 02, 2021 at 05:55:23PM +0000, Daniel P. Berrangé wrote:
>> > Currently the -audiodev accepts any audiodev type regardless of what is
>> > built in to QEMU. An error only occurs later at runtime when a sound
>> > device tries to use the audio backend.
>> > 
>> > With this change QEMU will immediately reject -audiodev args that are
>> > not compiled into the binary. The QMP schema will also be introspectable
>> > to identify what is compiled in.
>> 
>> Note that audio backends are modularized, so "compiled with
>> CONFIG_AUDIO_ALSA" doesn't imply "alsa support is available".
>
> AFAIK, there's no way to handle this with QAPI schema reporting. We
> can only conditionalize based on what's available at compile time,
> not what's installed at runtime.

Correct.  The schema is fixed at compile-time.  query-qmp-schema
reflects what we compiled into the binary or modules we built along with
the binary.  It cannot tell which of the modules the binary can load.

I'd like the commit message to discuss how the patch interacts with
modules, because my own memory of the details is rather uncertain :)

I guess we can configure which audio backends to build, and whether to
build them as modules.

When not building them as modules, the patch compiles out some useless
code.  Correct?

When building them as modules, the patch compiles out code for modules
we don't build.  Correct?

Such code is useless, unless you somehow manage to supply the resulting
binary with working modules from another build.  Which is probably a bad
idea.  Compiling out the code stops this (questionable) usage cold.  No
objection, but the commit message should at least hint at it.

> To get runtime info, we would have to introduce an explicit
> "query-audiodev-types" command where just report the backends
> that have been installed.

TTOCTOU.  Harmless one.  Still, the robust check for "can module M be
loaded?" is to try loading it.