[pulseaudio-discuss] [PATCH] echo-cancel: fix the obviously-wrong "buffer+=buffer" logic

[Arun Raghavan]

On 23 February 2015 at 22:10, Alexander E. Patrakov <patrakov at gmail.com> wrote:
> 23.02.2015 21:12, Peter Meerwald wrote:
>>
>> Hi Alexander,
>>
>>> Same bug as in module-loopback, pointed out by Georg Chini in a private
>>> email.
>>
>>
>> can you please elaborate? I fail to see the obviousness
>
>
> The recv_counter == send_counter case looks like a neutral case here
> (although I might be wrong), with no inherent discontinuity at this point.
> So, any branch of the "if" should give the same result in this corner case,
> and the whole point of the "if"/"PA_CLIP_SUB" construction, as I see it, is
> to avoid producing negative numbers by mistake.
>
> The true branch simplifies to: buffer_latency += 0
>
> The original false branch simplifies to: buffer_latency +=
> PA_CLIP_SUB(buffer_latency, 0), which, for buffer_latency > 0, means
> buffer_latency += buffer_latency - 0. I.e. doubling the latency (as opposed
> to not changing it), which looks strange even by itself.

The falce branch would only be called when recv_counter >
send_counter, so you'd never actually have the zero case. AIUI, this
all just accounting for any "in-flight" rewinds being propagated from
the sink-input to the source-output.

Also, overwriting the value as you do loses the latency values that
were added in previous lines.

-- Arun