Failing Test SoundSourceProxyTest.seekBoundaries with fdk-aac-free with ffmpeg-free

[daschuer]

Bug Description

We plan to use fdk-aac-free for STEM support on macOS. However we heva a failing test:
SoundSourceProxyTest.seekBoundaries
https://github.com/daschuer/mixxx/actions/runs/14022353777/job/39255927374

Version

2.6-alpha

OS

macOS 13

[daschuer]

I can confirm the issue with a fdk-aac enabled ffmpeg build on Ubuntu as well.
So we can assume this is an OS independent issue.

[daschuer]

The yellow line is the fdk-aac decoder and the red line is the internal aac decoder when decoding over-test-ffmpeg-aac.m4a form the very beginning.

We see that the fdk-aac decoder outputs garbage from an uninitialized buffer. The aac decoder has plain zero in that case.
We see that later real samples are similar and without an offset.

fdk-aac reports these garbage as priming samples which are forced to zero here:

mixxx/src/sources/soundsourceffmpeg.cpp

Line 1195 in 8c28797

streamFrameIndex = 0;

as a bug fix for:
#10464

When removing this hack, the garbage is gone and the SoundSourceProxyTest.firstSoundTest passes. However there are still failing tests when seeking.

[daschuer]

This is unfortunately promoted to a blocker. On windows the fdk-aac-free with the pending bug is used.
This way user will collect faulty metadata and once we have if fixed will have they will no longer match.

Currently I see this with windows:

13:25:57.172 Warning [CachingReaderWorker 1] SoundSourceSTEM - Internal aac decoder not found in your FFmpeg build. To enable AAC support, please install an FFmpeg version with the internal aac decoder enabled.Note 1: The libfdk_aac decoder is no working properly with Mixxx, FFmpeg's internal AAC decoder does. Note 2: AAC decoding may be subject to patent restrictions, depending on your country.
13:25:57.172 Debug [CachingReaderWorker 1] SoundSourceSTEM - using FFmpeg decoder: Fraunhofer FDK AAC

[daschuer]

My work on this stalls over:
#14658

Can one have a look that we get it merged and continue here?

[daschuer]

This fixes the issue finally:
#16440