[pulseaudio-discuss] [PATCH] stream: Return error in case a client peeks to early

[Tanu Kaskinen]

On Sat, 2012-11-03 at 19:51 +0100, David Henningsson wrote:
> 2012-11-03 17:19, Colin Guthrie skrev:
> > 'Twas brillig, and Tanu Kaskinen at 05/10/12 13:58 did gyre and gimble:
> >> On Wed, 2012-10-03 at 08:50 +0200, David Henningsson wrote:
> >>> On 10/02/2012 10:38 PM, Tanu Kaskinen wrote:
> >>>> On Mon, 2012-10-01 at 17:06 +0200, David Henningsson wrote:
> >>>>> If there is no silence memblock and no data, pa_memblockq_peek can
> >>>>> return NULL. In this case, do not crash on an assertion in
> >>>>> pa_memblock_acquire, but instead return a proper error to the client.
> >>>> If there is no data in the buffer, pa_stream_peek() is supposed to
> >>>> return NULL according to the documentation. And it does that: if there's
> >>>> no data, pa_memblock_peek() will return a negative value, causing
> >>>> pa_stream_peek() to return NULL.
> >>>>
> >>>> The problem is the case where the buffer does contain data, but not at
> >>>> the read index. That is, there is a hole in the buffer. The client
> >>>> documentation doesn't have any warnings about holes, so the only safe
> >>>> way to handle holes is to return silence. Fixing this should be a simple
> >>>> matter of giving a silence memchunk when creating record_memblockq.
> >>> I'm not so sure. Silence, as in all zeroes, might work for S16 audio
> >>> data, but what about other formats? Compressed audio? Peak audio (which
> >>> I think is the case here)? Etc.
> >> Good point. Regarding PCM, if pa_memchunk_silence() is used, the
> >> function will take care of filling the memory with appropriate content.
> >> But that doesn't work with compressed audio.
> >>
> >>> Also maybe it could also be valuable for the client to distinguish
> >>> between no data available, and valid zero data.
> >>>
> >>> How about returning NULL and adding to the documentation something like:
> >>>
> >>> -If no data is available this will return a NULL pointer.
> >>> +If no data is available (at the current read position), this will
> >>> return a NULL pointer.
> >> An addition: the client probably wants to know how large the hole is. It
> >> might be possible to figure that out somehow from the read index, but I
> >> think it would make sense to return the hole size in the length
> >> parameter.
> > This discussion seemed to stagnate. Is this worth fixing/documenting for
> > the 3.0 release?
> >
> > Col
> >
> >
> Returning NULL seems to be the right thing to do here, even if 
> gnome-control-center does not handle that very well IIRC. So we might 
> need an additional patch in g-c-c.
> So assuming I commit a patch doing that. If somebody else wants to add 
> logic to figure out how large the hole is, that could be discussed 
> separately.
> Any objections?

It's not clear what you meant by "add logic to figure out how large the
hole is". Add to where? pa_stream_peek() or gnome-control-center?

To me, reporting the hole length in the "nbytes" parameter of
pa_stream_peek() seems like the right thing to do, so I hope your patch
will do this.

-- 
Tanu