media file: fix calculation of video/animation duration.

Fix how the duration of videos and animated GIFs / PNGs is calculated.
If we can't determine the duration from the file metadata, then play the
entire video or animation back using FFmpeg and scrape the duration and
frame count.

This is necessary for things like WebM files where the duration metadata
is optional, or animated GIFs and PNGs that don't have a duration field
in the metadata, only a frame count and a sequence of frame delays.

app/logical/ffmpeg.rb

@@ -47,30 +47,34 @@ class FFmpeg
     video_streams.first[:height]
   end
 
+  # @see https://trac.ffmpeg.org/wiki/FFprobeTips#Duration
+  # @return [Float, nil] The duration of the video or animation in seconds, or nil if unknown.
   def duration
-    metadata.dig(:format, :duration).to_f
+    if metadata.dig(:format, :duration).present?
+      metadata.dig(:format, :duration).to_f
+    elsif playback_info.has_key?(:time)
+      hours, minutes, seconds = playback_info[:time].split(/:/)
+      hours.to_f*60*60 + minutes.to_f*60 + seconds.to_f
+    else
+      nil
+    end
   end
 
+  # @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
+    elsif playback_info.has_key?(:frame)
+      playback_info[:frame].to_i
     else
-      (duration * frame_rate).to_i
+      nil
     end
   end
 
-  # @return [Float, nil] The frame rate of the video or animation, or nil if
-  # unknown. The frame rate can be unknown for animated PNGs that have zero
-  # delay between frames.
+  # @return [Float, nil] The average frame rate of the video or animation, or nil if unknown.
   def frame_rate
-    rate = video_streams.first[:avg_frame_rate] # "100/57"
-    numerator, denominator = rate.split("/")
-
-    if numerator.to_f == 0 || denominator.to_f == 0
-      nil
-    else
-      (numerator.to_f / denominator.to_f)
-    end
+    return nil if frame_count.nil? || duration.nil? || duration == 0
+    frame_count / duration
   end
 
   def video_streams
@@ -85,6 +89,19 @@ class FFmpeg
     audio_streams.present?
   end
 
+  # Decode the full video and return a hash containing the frame count, fps, and runtime.
+  def playback_info
+    output = shell!("ffmpeg -i #{file.path.shellescape} -f null /dev/null")
+    status_line = output.lines.grep(/\Aframe=/).first.chomp
+
+    # status_line = "frame=   10 fps=0.0 q=-0.0 Lsize=N/A time=00:00:00.48 bitrate=N/A speed= 179x"
+    # info = {"frame"=>"10", "fps"=>"0.0", "q"=>"-0.0", "Lsize"=>"N/A", "time"=>"00:00:00.48", "bitrate"=>"N/A", "speed"=>"188x"}
+    info = status_line.scan(/\S+=\s*\S+/).map { |pair| pair.split(/=\s*/) }.to_h
+    info.with_indifferent_access
+  rescue Error => e
+    {}
+  end
+
   def shell!(command)
     program = command.shellsplit.first
     output, status = Open3.capture2e(command)
@@ -92,5 +109,5 @@ class FFmpeg
     output
   end
 
-  memoize :metadata
+  memoize :metadata, :playback_info, :frame_count, :duration
 end