Verify FFmpeg presence before attempting to preview videos

activestorage/lib/active_storage/previewer/video_previewer.rb

@@ -2,8 +2,20 @@
 
 module ActiveStorage
   class Previewer::VideoPreviewer < Previewer
-    def self.accept?(blob)
-      blob.video?
+    class << self
+      def accept?(blob)
+        blob.video? && ffmpeg_exists?
+      end
+
+      def ffmpeg_exists?
+        return @ffmpeg_exists if defined?(@ffmpeg_exists)
+
+        @ffmpeg_exists = system(ffmpeg_path, "-version", out: File::NULL, err: File::NULL)
+      end
+
+      def ffmpeg_path
+        ActiveStorage.paths[:ffmpeg] || "ffmpeg"
+      end
     end
 
     def preview(**options)
@@ -16,11 +28,7 @@ def preview(**options)
 
     private
       def draw_relevant_frame_from(file, &block)
-        draw ffmpeg_path, "-i", file.path, "-y", "-vframes", "1", "-f", "image2", "-", &block
-      end
-
-      def ffmpeg_path
-        ActiveStorage.paths[:ffmpeg] || "ffmpeg"
+        draw self.class.ffmpeg_path, "-i", file.path, "-y", "-vframes", "1", "-f", "image2", "-", &block
       end
   end
 end