ffmpeg built with schannel freezes when outputting HLS with http_persistent

[r1ch]

When outputting HLS to a HTTPS URL using http_persistent, if ffmpeg is built with the schannel TLS backend (Windows), it will freeze as it writes the first segment. Switching the TLS backend to gnutls allows this to work as expected.

How to reproduce (Windows only):

Build ffmpeg with --enable-schannel. Stream some input file to Youtube with the HLS ingest endpoint (or any HLS server of your choice). You must use -http_persistent 1 to reproduce the issue.

% ffmpeg -i "bbb_sunflower_1080p_60fps_normal.mp4" -acodec aac -vcodec libx264 -vb 2.5M -f hls -hls_time 4 -http_persistent 1 "https://a.upload.youtube.com/http_upload_hls?cid=VALID_YOUTUBE_STREAM_KEY&copy=0&file=01.ts"

ffmpeg version N-100057-g76a99b2da0-ffmpeg-windows-build-helpers Copyright (c) 2000-2020 the FFmpeg developers
  built with gcc 10.1.0 (GCC)
  configuration: --pkg-config=pkg-config --pkg-config-flags=--static --extra-version=ffmpeg-windows-build-helpers --enable-version3 --disable-debug --disable-w32threads --arch=x86_64 --target-os=mingw32 --cross-prefix=/home/r1ch/ffmpeg-windows-build-helpers/sandbox/cross_compilers/mingw-w64-x86_64/bin/x86_64-w64-mingw32- --enable-libcaca --enable-gray --enable-libtesseract --enable-fontconfig --enable-gmp --enable-schannel --enable-libass --enable-libbluray --enable-libbs2b --enable-libflite --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopus --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libvorbis --enable-libwebp --enable-libzimg --enable-libzvbi --enable-libmysofa --enable-libopenjpeg --enable-libopenh264 --enable-liblensfun --enable-libvmaf --enable-libsrt --enable-libaribb24 --enable-demuxer=dash --enable-libxml2 --enable-opengl --enable-libdav1d --enable-cuda-llvm --enable-libaom --enable-libvpx --enable-nvenc --enable-nvdec --extra-libs=-lharfbuzz --extra-libs=-lm --extra-libs=-lpthread --extra-cflags=-DLIBTWOLAME_STATIC --extra-cflags=-DMODPLUG_STATIC --extra-cflags=-DCACA_STATIC --enable-amf --enable-libmfx --enable-gpl --enable-frei0r --enable-filter=frei0r --enable-librubberband --enable-libvidstab --enable-libx264 --enable-libx265 --enable-libxvid --enable-libdavs2 --enable-libxavs2 --enable-libxavs --enable-avresample --extra-cflags='-mtune=generic' --extra-cflags=-O3 --enable-shared --disable-static --prefix=/home/r1ch/ffmpeg-windows-build-helpers/sandbox/win64/ffmpeg_git_shared
  libavutil      56. 61.100 / 56. 61.100
  libavcodec     58.113.100 / 58.113.100
  libavformat    58. 64.100 / 58. 64.100
  libavdevice    58. 11.103 / 58. 11.103
  libavfilter     7. 90.100 /  7. 90.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  8.100 /  5.  8.100
  libswresample   3.  8.100 /  3.  8.100
  libpostproc    55.  8.100 / 55.  8.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'bbb_sunflower_1080p_60fps_normal.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 1
    compatible_brands: isomavc1
    creation_time   : 2013-12-16T17:59:32.000000Z
    title           : Big Buck Bunny, Sunflower version
    artist          : Blender Foundation 2008, Janus Bager Kristensen 2013
    comment         : Creative Commons Attribution 3.0 - http://bbb3d.renderfarming.net
    genre           : Animation
    composer        : Sacha Goedegebure
  Duration: 00:10:34.53, start: 0.000000, bitrate: 4486 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], 4001 kb/s, 60 fps, 60 tbr, 60k tbn, 120 tbc (default)
    Metadata:
      creation_time   : 2013-12-16T17:59:32.000000Z
      handler_name    : GPAC ISO Video Handler
    Stream #0:1(und): Audio: mp3 (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 160 kb/s (default)
    Metadata:
      creation_time   : 2013-12-16T17:59:37.000000Z
      handler_name    : GPAC ISO Audio Handler
    Stream #0:2(und): Audio: ac3 (ac-3 / 0x332D6361), 48000 Hz, 5.1(side), fltp, 320 kb/s (default)
    Metadata:
      creation_time   : 2013-12-16T17:59:37.000000Z
      handler_name    : GPAC ISO Audio Handler
    Side data:
      audio service type: main
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))
  Stream #0:2 -> #0:1 (ac3 (native) -> aac (native))
Press [q] to stop, [?] for help
[libx264 @ 000001c1ed118680] using SAR=1/1
[libx264 @ 000001c1ed118680] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
[libx264 @ 000001c1ed118680] profile High, level 4.2, 4:2:0, 8-bit
[libx264 @ 000001c1ed118680] 264 - core 161 r3020M d198931 - H.264/MPEG-4 AVC codec - Copyleft 2003-2020 - 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=34 lookahead_threads=5 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=abr mbtree=1 bitrate=2500 ratetol=1.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
[aac @ 000001c1edec98c0] Using a PCE to encode channel layout "5.1(side)"
[hls muxer @ 000001c1ed116000] No HTTP method set, hls muxer defaulting to method PUT.
Output #0, hls, to 'https://a.upload.youtube.com/http_upload_hls?cid=xxxx&copy=0&file=01.ts':
  Metadata:
    major_brand     : isom
    minor_version   : 1
    compatible_brands: isomavc1
    composer        : Sacha Goedegebure
    title           : Big Buck Bunny, Sunflower version
    artist          : Blender Foundation 2008, Janus Bager Kristensen 2013
    comment         : Creative Commons Attribution 3.0 - http://bbb3d.renderfarming.net
    genre           : Animation
    encoder         : Lavf58.64.100
    Stream #0:0(und): Video: h264 (libx264), yuv420p(progressive), 1920x1080 [SAR 1:1 DAR 16:9], q=-1--1, 2500 kb/s, 60 fps, 90k tbn, 60 tbc (default)
    Metadata:
      creation_time   : 2013-12-16T17:59:32.000000Z
      handler_name    : GPAC ISO Video Handler
      encoder         : Lavc58.113.100 libx264
    Side data:
      cpb: bitrate max/min/avg: 0/0/2500000 buffer size: 0 vbv_delay: N/A
    Stream #0:1(und): Audio: aac (LC), 48000 Hz, 5.1(side), fltp, 394 kb/s (default)
    Metadata:
      creation_time   : 2013-12-16T17:59:37.000000Z
      handler_name    : GPAC ISO Audio Handler
      encoder         : Lavc58.113.100 aac
    Side data:
      audio service type: main
[hls @ 000001c1ed985800] Opening 'https://a.upload.youtube.com/http_upload_hls?cid=xxxx&copy=0&file=010.ts' for writing

(ffmpeg becomes frozen at this point)

[OBSJim]

I've bisected the bug to commit f267a2ac9c740c77cee2f2c5fd2bd9bc048768b1 in the FFmpeg repository, related to #7975.