[__init__] Fix missing subtitles if --add-metadata is used (#12423)

The previous fix for #5594 is incorrect

ChangeLog

@@ -1,3 +1,9 @@
+version <unreleased>
+
+Core
+* Fix missing subtitles if --add-metadata is used (#12423)
+
+
 version 2017.03.10
 
 Extractors

youtube_dl/__init__.py

@@ -259,6 +259,16 @@ def _real_main(argv=None):
             'key': 'FFmpegVideoConvertor',
             'preferedformat': opts.recodevideo,
         })
+    # FFmpegMetadataPP should be run after FFmpegVideoConvertorPP and
+    # FFmpegExtractAudioPP as containers before conversion may not support
+    # metadata (3gp, webm, etc.)
+    # And this post-processor should be placed before other metadata
+    # manipulating post-processors (FFmpegEmbedSubtitle) to prevent loss of
+    # extra metadata. By default ffmpeg preserves metadata applicable for both
+    # source and target containers. From this point the container won't change,
+    # so metadata can be added here.
+    if opts.addmetadata:
+        postprocessors.append({'key': 'FFmpegMetadata'})
     if opts.convertsubtitles:
         postprocessors.append({
             'key': 'FFmpegSubtitlesConvertor',
@@ -276,11 +286,6 @@ def _real_main(argv=None):
         })
         if not already_have_thumbnail:
             opts.writethumbnail = True
-    # FFmpegMetadataPP should be run after FFmpegVideoConvertorPP and
-    # FFmpegExtractAudioPP as containers before conversion may not support
-    # metadata (3gp, webm, etc.)
-    if opts.addmetadata:
-        postprocessors.append({'key': 'FFmpegMetadata'})
     # XAttrMetadataPP should be run after post-processors that may change file
     # contents
     if opts.xattrs: