"Assertion best_input >= 0 failed" IFF upsampling AND complex filtergraph

[Hunter Hogan]

Summary of the bug:
If resampler is invoked at the start of the filtergraph to upsample, an error will be thrown at the very end despite the output file being successfully written. Assertion best_input >= 0 failed at src/fftools/ffmpeg_filter.c:2075

How to reproduce:
I reproduced it with a few different wav files, including a white noise file. All were stereo, however. Batch file and 22 logs attached. Only 4 logs have the error. The other logs are to compare changing only one variable but not getting an error.

These quick tests all have filtergraphs loaded from a textfile, and I don't think I tested it without a text file. These tests all use -/complex_filter syntax, but I got the same error with the old syntax.

I did not test downsampling at the start of the filtergraph.

I first encountered this a couple of weeks ago after building version 7. Prior to that, however, I don't think I ever had a filtergraph that included upsampling and multiple input and/or output pads, so I am not claiming the error was recently introduced.

If I recall correctly, originally, FFmpeg would crash, throw the error, and not write a usable file. IIRC, the output file was the size I expected (in bytes), but nothing could read it: ffprobe, ffmpeg, Goldwave all said the file was garbage. I interpreted it to mean that the "headers" were malformed. (By "headers", I probably mean metadata, but I don't know the precise term here.)

I worked around it by scripting:

1. upsample my input file to a temp output file
2. filter as I want, output to a new temp
3. downsample

IIRC, it was necessary for me to downsample in a different filtergraph. But, it now looks like downsampling doesn't throw an error.

Today, I rebuilt, and noticed the differences.

% FFmpeg -hide_banner -loglevel warning -stats 
-i test_input_44100_2ch.wav -/filter_complex test_filter.txt 
-map "[Z0Z_out]" -codec:a pcm_f32le 
"output\\test_input_44100_2ch.wav_filtergraph_192000.wav"

ffmpeg version: media autobuilder suite, built today on Windows

ffmpeg version N-115035-g192f5b58b7-gb7debef3cc+4 Copyright (c) 2000-2024 the FFmpeg developers
built with gcc 13.2.0 (Rev6, Built by MSYS2 project)
configuration:  --pkg-config=pkgconf --cc='ccache gcc' --cxx='ccache g++' --ld='ccache g++' --extra-cxxflags=-fpermissive --extra-cflags=-Wno-int-conversion --disable-autodetect --enable-amf --enable-bzlib --enable-d3d11va --enable-dxva2 --enable-iconv --enable-lzma --enable-zlib --enable-sdl2 --enable-libmp3lame --enable-libopus --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-libdav1d --enable-libaom --disable-debug --enable-libfdk-aac --enable-fontconfig --enable-libass --enable-libfreetype --enable-libmysofa --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-amrwbenc --enable-libwebp --enable-libxml2 --enable-libzimg --enable-libshine --enable-gpl --enable-avisynth --enable-libxvid --enable-libopenmpt --enable-version3 --enable-librav1e --enable-libgsm --enable-libvmaf --enable-libsvtav1 --enable-chromaprint --enable-frei0r --enable-libaribb24 --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libflite --enable-libfribidi --enable-libgme --enable-libilbc --enable-libsvthevc --enable-libsvtvp9 --enable-libkvazaar --enable-libmodplug --enable-librtmp --enable-librubberband --enable-libtesseract --enable-libxavs --enable-libzmq --enable-libzvbi --enable-openal --enable-libcodec2 --enable-ladspa --enable-libglslang --enable-vulkan --enable-libdavs2 --enable-libxavs2 --enable-libuavs3d --enable-opengl --enable-libopenh264 --enable-libharfbuzz --enable-libfontconfig --enable-openssl --extra-cflags=-DLIBTWOLAME_STATIC --extra-cflags=-DCACA_STATIC --extra-cflags=-DMODPLUG_STATIC --extra-cflags=-DCHROMAPRINT_NODLL --extra-cflags=-DZMQ_STATIC --extra-libs=-lpsapi --extra-cflags=-DLIBXML_STATIC --extra-libs=-liconv --disable-w32threads --extra-cflags=-DKVZ_STATIC_LIB --enable-nonfree --extra-cflags=-DAL_LIBTYPE_STATIC --extra-cflags='-IC:/apps/media-autobuild_suite/local64/include' --extra-cflags='-IC:/apps/media-autobuild_suite/local64/include/AL'
libavutil      59. 16.101 / 59. 16.101
libavcodec     61.  5.103 / 61.  5.103
libavformat    61.  3.103 / 61.  3.103
libavdevice    61.  2.100 / 61.  2.100
libavfilter    10.  2.101 / 10.  2.101
libswscale      8.  2.100 /  8.  2.100
libswresample   5.  2.100 /  5.  2.100
libpostproc    58.  2.100 / 58.  2.100

[Hunter Hogan]

reports, loglevel=56, showing error, and reports without error if one variable is changed