Can't keep segments larger than twice +1 the list size?

[nicol]

Summary of the bug: I'm trying to keep segments using -hls_delete_threshold option, but can not keep segments larger than twice +1 the list size.

-hls_list_size 5 -hls_delete_threshold 6 keeps 5 + 6 = 11 segments. (output13.ts - output23.ts)
-hls_list_size 5 -hls_delete_threshold 7 not keeps 5 + 7 = 12 but keeps 5 + 6 = 11 segments. (output13.ts - output23.ts)

How to reproduce:

% ffmpeg -f lavfi -i color=d=240:r=25 -c:v libx264 -f hls -hls_list_size 5 -hls_flags +delete_segments -hls_delete_threshold 7 tmp/output.m3u8
ffmpeg version 2021-06-02-git-071930de72-full_build-www.gyan.dev Copyright (c) 2000-2021 the FFmpeg developers
  built with gcc 10.3.0 (Rev2, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libdav1d --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-libaom --enable-libopenjpeg --enable-libvpx --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-libglslang --enable-vulkan --enable-opencl --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint
  libavutil      57.  0.100 / 57.  0.100
  libavcodec     59.  1.100 / 59.  1.100
  libavformat    59.  2.101 / 59.  2.101
  libavdevice    59.  0.100 / 59.  0.100
  libavfilter     8.  0.101 /  8.  0.101
  libswscale      6.  0.100 /  6.  0.100
  libswresample   4.  0.100 /  4.  0.100
  libpostproc    56.  0.100 / 56.  0.100
Input #0, lavfi, from 'color=d=240:r=25':
  Duration: N/A, start: 0.000000, bitrate: N/A
  Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 320x240 [SAR 1:1 DAR 4:3], 25 tbr, 25 tbn
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> h264 (libx264))
Press [q] to stop, [?] for help
[libx264 @ 000002b983fc8740] using SAR=1/1
[libx264 @ 000002b983fc8740] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
[libx264 @ 000002b983fc8740] profile High, level 1.3, 4:2:0, 8-bit
[libx264 @ 000002b983fc8740] 264 - core 163 r3059 b684ebe - H.264/MPEG-4 AVC codec - Copyleft 2003-2021 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=7 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, hls, to 'tmp/output.m3u8':
  Metadata:
    encoder         : Lavf59.2.101
  Stream #0:0: Video: h264, yuv420p(progressive), 320x240 [SAR 1:1 DAR 4:3], q=2-31, 25 fps, 90k tbn
    Metadata:
      encoder         : Lavc59.1.100 libx264
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A
[hls @ 000002b983fc58c0] Opening 'tmp/output0.ts' for writing/A speed=N/A
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output1.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output2.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output3.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output4.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output5.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output6.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output7.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output8.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output9.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output10.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output11.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output12.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output13.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output14.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output15.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output16.ts' for writing/A speed= 338x
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output17.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output18.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output19.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output20.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output21.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output22.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output23.ts' for writing
[hls @ 000002b983fc58c0] Opening 'tmp/output.m3u8.tmp' for writing
frame= 6000 fps=0.0 q=-1.0 Lsize=N/A time=00:03:59.92 bitrate=N/A speed= 341x
video:100kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
[libx264 @ 000002b983fc8740] frame I:24    Avg QP: 6.12  size:    64
[libx264 @ 000002b983fc8740] frame P:1512  Avg QP: 9.07  size:    23
[libx264 @ 000002b983fc8740] frame B:4464  Avg QP:12.67  size:    15
[libx264 @ 000002b983fc8740] consecutive B-frames:  0.8%  0.0%  0.0% 99.2%
[libx264 @ 000002b983fc8740] mb I  I16..4: 100.0%  0.0%  0.0%
[libx264 @ 000002b983fc8740] mb P  I16..4:  0.0%  0.0%  0.0%  P16..4:  0.0%  0.0%  0.0%  0.0%  0.0%    skip:100.0%
[libx264 @ 000002b983fc8740] mb B  I16..4:  0.0%  0.0%  0.0%  B16..8:  0.0%  0.0%  0.0%  direct: 0.0%  skip:100.0%
[libx264 @ 000002b983fc8740] 8x8 transform intra:0.0%
[libx264 @ 000002b983fc8740] coded y,uvDC,uvAC intra: 0.0% 0.0% 0.0% inter: 0.0% 0.0% 0.0%
[libx264 @ 000002b983fc8740] i16 v,h,dc,p: 93%  0%  7%  0%
[libx264 @ 000002b983fc8740] i8c dc,h,v,p: 100%  0%  0%  0%
[libx264 @ 000002b983fc8740] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 000002b983fc8740] kb/s:3.39
ffmpeg version
built on ...

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