Added a workaround option called fallback_mkv that, when merging separately downloaded video & audio that can't fit in the same container (e.g. webm video & m4a audio), tries instead to output to an mkv container

youtube_dl/__init__.py

@@ -324,6 +324,7 @@ def _real_main(argv=None):
         'encoding': opts.encoding,
         'exec_cmd': opts.exec_cmd,
         'extract_flat': opts.extract_flat,
+        'fallback_mkv': opts.fallback_mkv,
         'postprocessors': postprocessors,
     }
 

youtube_dl/options.py

@@ -385,6 +385,11 @@ def parseOpts(overrideArguments=None):
         '--bidi-workaround',
         dest='bidi_workaround', action='store_true',
         help='Work around terminals that lack bidirectional text support. Requires bidiv or fribidi executable in PATH')
+    workarounds.add_option(
+    	'--fallback-mkv',
+    	dest='fallback_mkv', action='store_true',
+    	help='If downloading separate vieo and audio (e.g. -f bestvideo+bestaudio) and merging fails, try to use an mkv container'
+    )
 
     verbosity = optparse.OptionGroup(parser, 'Verbosity / Simulation Options')
     verbosity.add_option(

youtube_dl/postprocessor/ffmpeg.py

@@ -516,13 +516,30 @@ class FFmpegMetadataPP(FFmpegPostProcessor):
         os.rename(encodeFilename(temp_filename), encodeFilename(filename))
         return True, info
 
-
 class FFmpegMergerPP(FFmpegPostProcessor):
     def run(self, info):
         filename = info['filepath']
         args = ['-c', 'copy', '-map', '0:v:0', '-map', '1:a:0', '-shortest']
         self._downloader.to_screen('[ffmpeg] Merging formats into "%s"' % filename)
-        self.run_ffmpeg_multiple_files(info['__files_to_merge'], filename, args)
+        #first attempt to merge them as normal but if a merge error happens attempt to eat it and try again with mkv output
+        #attempt to merge the files into the filename that the upstream methods have determined
+	#there's little point to try to guess from the start which video format will be compatible with which audio
+	#if the standard merge fails, fallback to mkv instead
+        try:
+        	self.run_ffmpeg_multiple_files(info['__files_to_merge'], filename, args)
+        except FFmpegPostProcessorError as fpe:
+        	if self._downloader.params.get('fallback_mkv', False) and "incorrect codec parameters" in fpe.msg.lower():
+        		warning = "Could not merge to %s format, ffmpeg said: '%s'\nAttempting to merge to mkv instead" % (info['ext'], fpe.msg)
+        		self._downloader.report_warning(warning)
+        		fname, ext = os.path.splitext(filename)
+        		mkv_filename = fname + ".mkv"
+        		self._downloader.to_screen('[ffmpeg] Merging formats into "%s"' % mkv_filename)
+        		self.run_ffmpeg_multiple_files(info['__files_to_merge'], mkv_filename, args)
+        		os.remove(encodeFilename(filename))
+        		info['filepath'] = mkv_filename
+        		info['ext'] = 'mkv'
+        	else:
+        		raise fpe	
         return True, info