Fix #3615: Unsupported video codecs.

Don't allow uploading videos with unsupported video codecs.

The only video codecs we allow for MP4 files are H.264 and VP9. Other
codecs, including H.265 (aka HEVC), MPEG-4 part 2, and AV1, are
disallowed because they're not universally supported by browsers.
Firefox doesn't support H.265 or MPEG-4 part 2, and Safari doesn't
support AV1.

Additionally, don't allow videos with multiple video tracks, multiple
audio tracks, or no video tracks. Multiple video and audio tracks are
disallowed because they're rare and for moderation purposes, we don't
want people hiding content in extra tracks.

These restrictions really only apply to MP4 videos, since WebM files
don't support multiple video or audio tracks and only support a limited
number of codecs (VP8 and VP9 for videos, Vorbis and Opus for audio).

There are currently 22 posts with unsupported video codecs:

* https://danbooru.donmai.us/posts?tags=video+is:mp4+-exif:Track1:CompressorID=avc1+-exif:Track2:CompressorID=avc1+-exif:Track1:CompressorID=vp09+-exif:Track2:CompressorID=vp09 # AVC1 is H.264

There is one post that has multiple audio tracks:

* https://danbooru.donmai.us/posts/2382057

app/logical/ffmpeg.rb

@@ -46,11 +46,11 @@ class FFmpeg
   end
 
   def width
-    video_streams.first[:width]
+    video_stream[:width]
   end
 
   def height
-    video_streams.first[:height]
+    video_stream[:height]
   end
 
   # @see https://trac.ffmpeg.org/wiki/FFprobeTips#Duration
@@ -68,8 +68,8 @@ class FFmpeg
 
   # @return [Integer, nil] The number of frames in the video or animation, or nil if unknown.
   def frame_count
-    if video_streams.first&.has_key?(:nb_frames)
-      video_streams.first[:nb_frames].to_i
+    if video_stream.has_key?(:nb_frames)
+      video_stream[:nb_frames].to_i
     elsif playback_info.has_key?(:frame)
       playback_info[:frame].to_i
     else
@@ -83,6 +83,14 @@ class FFmpeg
     frame_count / duration
   end
 
+  def video_codec
+    video_stream[:codec_name]
+  end
+
+  def video_stream
+    video_streams.first || {}
+  end
+
   def video_streams
     metadata[:streams].to_a.select { |stream| stream[:codec_type] == "video" }
   end

app/logical/media_file.rb

@@ -156,6 +156,16 @@ class MediaFile
     file_ext.in?([:webm, :mp4])
   end
 
+  # @return [Boolean] True if the file is a MP4.
+  def is_mp4?
+    file_ext == :mp4
+  end
+
+  # @return [Boolean] True if the file is a WebM.
+  def is_webm?
+    file_ext == :webm
+  end
+
   # @return [Boolean] true if the file is a Pixiv ugoira
   def is_ugoira?
     file_ext == :zip

app/logical/media_file/video.rb

@@ -5,7 +5,7 @@
 #
 # @see https://github.com/streamio/streamio-ffmpeg
 class MediaFile::Video < MediaFile
-  delegate :duration, :frame_count, :frame_rate, :has_audio?, to: :video
+  delegate :duration, :frame_count, :frame_rate, :has_audio?, :video_codec, :video_stream, :video_streams, :audio_streams, to: :video
 
   def dimensions
     [video.width, video.height]
@@ -16,14 +16,12 @@ class MediaFile::Video < MediaFile
   end
 
   def is_supported?
-    case file_ext
-    when :webm
-      metadata["Matroska:DocType"] == "webm"
-    when :mp4
-      true
-    else
-      false
-    end
+    return false if video_streams.size != 1
+    return false if audio_streams.size > 1
+    return false if is_webm? && metadata["Matroska:DocType"] != "webm"
+    return false if is_mp4? && !video_codec.in?(["h264", "vp9"])
+
+    true
   end
 
   # True if decoding the video fails.