full cut: ffmpeg requires a seekable output file

Most formats like mp4 require ffmpeg to make changes at the start of the file
throughout writing.

Unfortunately, this prevents us from streaming the upload as we cut it.

Instead, we spool to a temporary file until ffmpeg exits,
then upload that all at once.

common/common/segments.py

@@ -13,6 +13,7 @@ import shutil
 import sys
 from collections import namedtuple
 from contextlib import closing
+from tempfile import TemporaryFile
 
 import gevent
 from gevent import subprocess
@@ -286,9 +287,12 @@ def ffmpeg_cut_segment(segment, cut_start=None, cut_end=None):
 	return subprocess.Popen(args, stdout=subprocess.PIPE)
 
 
-def ffmpeg_cut_stdin(cut_start, duration, encode_args):
+def ffmpeg_cut_stdin(output_file, cut_start, duration, encode_args):
 	"""Return a Popen object which is ffmpeg cutting from stdin.
-	This is used when doing a full cut."""
+	This is used when doing a full cut.
+	Note the explicit output file object instead of using a pipe,
+	because most video formats require a seekable file.
+	"""
 	args = [
 		'ffmpeg',
 		'-hide_banner', '-loglevel', 'error', # suppress noisy output
@@ -296,11 +300,18 @@ def ffmpeg_cut_stdin(cut_start, duration, encode_args):
 		'-ss', cut_start,
 		'-t', duration,
 	] + list(encode_args) + [
-		'-', # output to stdout
+		# We want ffmpeg to write to our tempfile, which is its stdout.
+		# However, it assumes that '-' means the output is not seekable.
+		# We trick it into understanding that its stdout is seekable by
+		# telling it to write to the fd via its /proc/self filename.
+		'/proc/self/fd/1',
+		# But of course, that file "already exists", so we need to give it
+		# permission to "overwrite" it.
+		'-y',
 	]
 	args = map(str, args)
 	logging.info("Running full cut with args: {}".format(" ".join(args)))
-	return subprocess.Popen(args, stdin=subprocess.PIPE, stdout=subprocess.PIPE)
+	return subprocess.Popen(args, stdin=subprocess.PIPE, stdout=output_file)
 
 
 def read_chunks(fileobj, chunk_size=16*1024):
@@ -384,20 +395,6 @@ def fast_cut_segments(segments, start, end):
 					yield chunk
 
 
-def feed_input(segments, pipe):
-	"""Write each segment's data into the given pipe in order.
-	This is used to provide input to ffmpeg in a full cut."""
-	for segment in segments:
-		with open(segment.path) as f:
-			try:
-				shutil.copyfileobj(f, pipe)
-			except OSError as e:
-				# ignore EPIPE, as this just means the end cut meant we didn't need all input
-				if e.errno != errno.EPIPE:
-					raise
-	pipe.close()
-
-
 def full_cut_segments(segments, start, end, encode_args):
 	# how far into the first segment to begin
 	cut_start = max(0, (start - segments[0].start).total_seconds())
@@ -405,21 +402,33 @@ def full_cut_segments(segments, start, end, encode_args):
 	duration = (end - start).total_seconds()
 
 	ffmpeg = None
-	input_feeder = None
 	try:
-		ffmpeg = ffmpeg_cut_stdin(cut_start, duration, encode_args)
-		input_feeder = gevent.spawn(feed_input, segments, ffmpeg.stdin)
-		# stream the output until it is closed
-		for chunk in read_chunks(ffmpeg.stdout):
-			yield chunk
+		# Most ffmpeg output formats require a seekable file.
+		# For the same reason, it's not safe to begin uploading until ffmpeg
+		# has finished. We create a temporary file for this.
+		tempfile = TemporaryFile()
+		ffmpeg = ffmpeg_cut_stdin(tempfile, cut_start, duration, encode_args)
+
+		# stream the input
+		for segment in segments:
+			with open(segment.path) as f:
+				try:
+					shutil.copyfileobj(f, ffmpeg.stdin)
+				except OSError as e:
+					# ignore EPIPE, as this just means the end cut meant we didn't need all input
+					if e.errno != errno.EPIPE:
+						raise
+		ffmpeg.stdin.close()
+
 		# check if any errors occurred in input writing, or if ffmpeg exited non-success.
 		if ffmpeg.wait() != 0:
 			raise Exception("Error while streaming cut: ffmpeg exited {}".format(ffmpeg.returncode))
-		input_feeder.get() # re-raise any errors from feed_input()
+
+		# Now actually yield the resulting file
+		for chunk in read_chunks(tempfile):
+			yield chunk
 	finally:
 		# if something goes wrong, try to clean up ignoring errors
-		if input_feeder is not None:
-			input_feeder.kill()
 		if ffmpeg is not None and ffmpeg.poll() is None:
 			for action in (ffmpeg.kill, ffmpeg.stdin.close, ffmpeg.stdout.close):
 				try: