Replace ffmpeg_cut_stdin() with ffmpeg_cut_many()

This is a more featureful wrapper around ffmpeg with notable differences:
- It's used as a context manager, and so can manage its own cleanup
- It takes care of input feeding
- It can handle multiple inputs (via pipes), instead of one (via stdin)

This drastically reduces the setup and cleanup code required for most basic usage,
and the multi-input support will be used in followup changes.

common/common/segments.py

@@ -11,12 +11,13 @@ import logging
 import os
 import shutil
 from collections import namedtuple
-from contextlib import closing
+from contextlib import closing, contextmanager
 from tempfile import TemporaryFile
 from uuid import uuid4
 
 import gevent
 from gevent import subprocess
+from gevent.fileobject import FileObject
 
 from .cached_iterator import CachedIterator
 from .stats import timed
@@ -390,24 +391,30 @@ def ffmpeg_cut_segment(segment, cut_start=None, cut_end=None):
 	return subprocess.Popen(args, stdout=subprocess.PIPE)
 
 
-def ffmpeg_cut_stdin(output_file, encode_args):
+@contextmanager
-	"""Return a Popen object which is ffmpeg cutting from stdin.
+def ffmpeg_cut_many(inputs, output_file, encode_args):
-	This is used when doing a full cut, plus various functions for transforming segment video without doing a multi-range cut.
-	If output_file is not subprocess.PIPE,
-	uses explicit output file object instead of using a pipe,
-	because some output formats require a seekable file.
 	"""
-	args = [
+	Context manager that produces a Popen object which is ffmpeg cutting the given inputs.
+
+	INPUTS is a list of (segments, input_args). The list of segments will be fed as input data,
+	preceeded by the given input args.
+	OUTPUT_FILE may be a writable file object (with fileno) or subprocess.PIPE.
+	If subprocess.PIPE, then output can be read from the Popen object's stdout.
+	Using a stdout pipe is preferred but a file can be useful if the output needs to be seekable.
+
+	Upon successful context exit, we block until ffmpeg finishes and raise if anything errored.
+	Upon unsuccessful exit, ffmpeg will be killed if still running.
+	In either case all files will be closed and everything cleaned up.
+	"""
+	BASE_ARGS = [
 		'ffmpeg',
 		'-hide_banner', '-loglevel', 'error', # suppress noisy output
-		'-i', '-',
 	]
-	args += list(encode_args)
 
 	if output_file is subprocess.PIPE:
-		args.append('-') # output to stdout
+		output_args = ['-'] # output to stdout
 	else:
-		args += [
+		output_args = [
 			# 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
@@ -417,9 +424,63 @@ def ffmpeg_cut_stdin(output_file, encode_args):
 			# permission to "overwrite" it.
 			'-y',
 		]
+
+	input_pipes = []
+	input_feeders = []
+	ffmpeg = None
+	try:
+
+		# Create pipes and feeders, and prepare input args
+		all_input_args = []
+		for segments, input_args in inputs:
+			# prepare the input pipe
+			read_fd, write_fd = os.pipe()
+			input_pipes.append(read_fd)
+			# set up the writer to fill the pipe
+			write_file = FileObject(write_fd, 'wb')
+			input_feeder = gevent.spawn(feed_input, segments, write_file)
+			input_feeders.append(input_feeder)
+			# add input file to ffmpeg args
+			all_input_args += input_args + ["-i", "/proc/self/fd/{}".format(read_fd)]
+
+		# Prepare final arg list and spawn the process
+		args = BASE_ARGS + all_input_args + encode_args + output_args
 		args = list(map(str, args))
-	logging.info("Cutting from stdin with args: {}".format(" ".join(args)))
+		logging.info("Running ffmpeg with args: {}".format(" ".join(args)))
-	return subprocess.Popen(args, stdin=subprocess.PIPE, stdout=output_file)
+		ffmpeg = subprocess.Popen(args, stdout=output_file, pass_fds=input_pipes)
+
+		# Close input fds now that the child is holding them.
+		# Note we remove them from the list one at a time so any failure in a close()
+		# call will still close the rest of them during cleanup.
+		while input_pipes:
+			fd = input_pipes.pop()
+			os.close(fd)
+
+		# produce context manager result, everything after this only applies if
+		# the context block succeeds
+		yield ffmpeg
+
+		# check if any errors occurred in input writing, or if ffmpeg exited non-success.
+		if ffmpeg.wait() != 0:
+			raise Exception("Error while cutting: ffmpeg exited {}".format(ffmpeg.returncode))
+		for input_feeder in input_feeders:
+			input_feeder.get() # re-raise any errors from feed_input() calls
+
+	finally:
+		# if something goes wrong, try to clean up ignoring errors
+		for input_feeder in input_feeders:
+			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:
+					action()
+				except (OSError, IOError):
+					pass
+		for fd in input_pipes:
+			try:
+				os.close(fd)
+			except (OSError, IOError):
+				pass
 
 
 def read_chunks(fileobj, chunk_size=16*1024):
@@ -622,10 +683,6 @@ def full_cut_segments(segments, start, end, encode_args, stream=False):
 	# duration
 	duration = (end - start).total_seconds()
 
-	ffmpeg = None
-	input_feeder = None
-	try:
-
 	if stream:
 		# When streaming, we can just use a pipe
 		tempfile = subprocess.PIPE
@@ -641,33 +698,18 @@ def full_cut_segments(segments, start, end, encode_args, stream=False):
 	if duration is not None:
 		args += ['-t', duration]
 	args += list(encode_args)
-		ffmpeg = ffmpeg_cut_stdin(tempfile, cut_start, duration, args)
-		input_feeder = gevent.spawn(feed_input, segments, ffmpeg.stdin)
 
+	with ffmpeg_cut_many([segments, ()], tempfile, args) as ffmpeg:
 		# When streaming, we can return data as it is available
+		# Otherwise, just exit the context manager so tempfile is fully written.
 		if stream:
 			for chunk in read_chunks(ffmpeg.stdout):
 				yield chunk
 
-		# 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()
-
 	# When not streaming, we can only return the data once ffmpeg has exited
 	if not stream:
 		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:
-					action()
-				except (OSError, IOError):
-					pass
 
 
 @timed('cut', cut_type='archive', normalize=lambda ret, sr, ranges: range_total(ranges))
@@ -692,32 +734,15 @@ def archive_cut_segments(segment_ranges, ranges, tempdir):
 	for segments in segment_ranges:
 		contiguous_ranges += list(split_contiguous(segments))
 	for segments in contiguous_ranges:
-		ffmpeg = None
-		input_feeder = None
 		tempfile_name = os.path.join(tempdir, "archive-temp-{}.mkv".format(uuid4()))
 		try:
-			tempfile = open(tempfile_name, "wb")
+			with open(tempfile_name, "wb") as tempfile:
-
+				with ffmpeg_cut_many([(segments, [])], tempfile, encode_args):
-			ffmpeg = ffmpeg_cut_stdin(tempfile, encode_args)
+					# We just want ffmpeg to run to completion, which ffmpeg_cut_many()
-			input_feeder = gevent.spawn(feed_input, segments, ffmpeg.stdin)
+					# will do on exit for us.
-
-			# since we've now handed off the tempfile fd to ffmpeg, close ours
-			tempfile.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()
-		except:
-			# 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:
-						action()
-					except (OSError, IOError):
 					pass
+		except:
+			# if something goes wrong, try to delete the tempfile
 			try:
 				os.remove(tempfile_name)
 			except (OSError, IOError):
@@ -742,9 +767,6 @@ def render_segments_waveform(segments, size=(1024, 128), scale='sqrt', color='#0
 	# Remove holes
 	segments = [segment for segment in segments if segment is not None]
 
-	ffmpeg = None
-	input_feeder = None
-	try:
 	args = [
 		# create waveform from input audio
 		'-filter_complex',
@@ -754,27 +776,10 @@ def render_segments_waveform(segments, size=(1024, 128), scale='sqrt', color='#0
 		# output as png
 		'-f', 'image2', '-c', 'png',
 	]
-		ffmpeg = ffmpeg_cut_stdin(subprocess.PIPE, args)
+	with ffmpeg_cut_many([(segments, [])], subprocess.PIPE, args) as ffmpeg:
-		input_feeder = gevent.spawn(feed_input, segments, ffmpeg.stdin)
-
 		for chunk in read_chunks(ffmpeg.stdout):
 			yield chunk
 
-		# check if any errors occurred in input writing, or if ffmpeg exited non-success.
-		if ffmpeg.wait() != 0:
-			raise Exception("Error while rendering waveform: ffmpeg exited {}".format(ffmpeg.returncode))
-		input_feeder.get() # re-raise any errors from feed_input()
-	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:
-					action()
-				except (OSError, IOError):
-					pass
-
 
 @timed('extract_frame')
 def extract_frame(segments, timestamp):
@@ -790,39 +795,18 @@ def extract_frame(segments, timestamp):
 
 	# "cut" input so that first frame is our target frame
 	cut_start = (timestamp - segments[0].start).total_seconds()
+	input_args = ["-ss", cut_start]
 
-	ffmpeg = None
-	input_feeder = None
-	try:
 	args = [
-			# cut to correct start frame
-			"-ss", cut_start,
 		# get a single frame
 		'-vframes', '1',
 		# output as png
 		'-f', 'image2', '-c', 'png',
 	]
-		ffmpeg = ffmpeg_cut_stdin(subprocess.PIPE, args)
+	with ffmpeg_cut_many([(segments, input_args)], subprocess.PIPE, args) as ffmpeg:
-		input_feeder = gevent.spawn(feed_input, segments, ffmpeg.stdin)
-
 		for chunk in read_chunks(ffmpeg.stdout):
 			yield chunk
 
-		# check if any errors occurred in input writing, or if ffmpeg exited non-success.
-		if ffmpeg.wait() != 0:
-			raise Exception("Error while extracting frame: ffmpeg exited {}".format(ffmpeg.returncode))
-		input_feeder.get() # re-raise any errors from feed_input()
-	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:
-					action()
-				except (OSError, IOError):
-					pass
-
 
 def split_contiguous(segments):
 	"""For a list of segments, return a list of contiguous ranges of segments.