[ffmpeg] Clarify rationale for pipe(-) exclusion in _ffmpeg_filename_argument

diff --git a/youtube_dl/postprocessor/ffmpeg.py b/youtube_dl/postprocessor/ffmpeg.py
index 30af6eb5a5ee16774858f3da37b4d91bf647dd30..1793a878cb57b60ab7e59cb55f17eb4127c32f92 100644 (file)
--- a/youtube_dl/postprocessor/ffmpeg.py
+++ b/youtube_dl/postprocessor/ffmpeg.py
@@ -175,6 +175,7 @@ class FFmpegPostProcessor(PostProcessor):
         # Always use 'file:' because the filename may contain ':' (ffmpeg
         # interprets that as a protocol) or can start with '-' (-- is broken in
         # ffmpeg, see https://ffmpeg.org/trac/ffmpeg/ticket/2127 for details)
+        # Also leave '-' intact in order not to break streaming to stdout.
         return 'file:' + fn if fn != '-' else fn