Naming cleanup, funcitonal skip_first_frames

Some testing has been done at using BufferedWriter to limit overhead of
synchronization logic, but buffering over write size causes breakage
with PyAV calls. Might investigate time based batching in the future.

videohelpersuite/server.py

@@ -31,10 +31,10 @@ async def pyav_resp(request):
         resp.headers["Content-Disposition"] = f"filename=\"{filename}\""
         await resp.prepare(request)
         loop = asyncio.get_event_loop()
-        class Writable:
-            def __init__(self, async_writable):
+        class BlockingFile:
+            def __init__(self, async_file):
                 self.lock = threading.Lock()
-                self._writable = async_writable
+                self.async_file = async_file
                 self.closed = False
                 self.error = None
             def write(self, b):
@@ -43,19 +43,26 @@ async def pyav_resp(request):
                     raise self.error
                 #NOTE assignment purely to stop premature garbage collection
                 self.ctask = loop.call_soon_threadsafe(asyncio.create_task, self.do_write(b))
+            def writable(self):
+                return True
+            def seekable(self):
+                return False
             def close(self):
                 if not self.closed:
                     self.lock.acquire()
                     self.closed = True
             async def do_write(self, b):
                 try:
-                    await self._writable.write(b)
+                    await self.async_file.write(b)
                 except Exception as e:
                     self.error = e
                     raise
                 finally:
                     self.lock.release()
-        await asyncio.to_thread(pyav_transcode, query, file, Writable(resp))
+        ret = await asyncio.to_thread(pyav_transcode, query, file, BlockingFile(resp))
+        if isinstance(ret, BaseException):
+            breakpoint()
+            raise ret
         return resp
     except (ConnectionResetError, ConnectionError) as e:
         #TODO Verify this doesn't create zombie processes
@@ -65,10 +72,14 @@ def pyav_transcode(query, ifile, ofile):
     with av.open(ifile, 'r') as icont, av.open(ofile, 'w', format='matroska') as ocont:
         istreams = {}
         processors = {}
+        start_pts = 0
+        if 'start_time' in query:
+            #TODO Verify correctness of time base
+            start_pts = int(float(query['start_time']), av.time_base)
+            icont.seek(start_pts)
         if icont.streams.video:
             target_rate = query.get('frame_rate') or icont.streams.video[0].average_rate
             frame_load_cap = int(query.get('frame_load_cap')) or float('inf')
-            #TODO Time base? :(
             ostream = ocont.add_stream('libvpx-vp9', rate=target_rate)
             ostream.options['deadline'] = 'realtime'#Maybe placebo, doesn't seem to function
             istream = icont.streams.video[0]
@@ -87,6 +98,10 @@ def pyav_transcode(query, ifile, ofile):
             fg.link_nodes(fg.add_buffer(template=icont.streams.video[0]),
                           *filters,
                           fg.add('buffersink')).configure()
+            if 'skip_first_frames' in query:
+                start_pts += int((int(query['skip_first_frames'])+1) /
+                                 (istream.average_rate * istream.time_base))
+                icont.seek(start_pts, stream=istream)
             cc = vpxr if istream.codec_context.name == 'vp9' else istream
             def process_video(packet):
                 nonlocal frame_load_cap
@@ -94,29 +109,18 @@ def pyav_transcode(query, ifile, ofile):
                     frame_load_cap -= 1
                     if frame_load_cap <= 0:
                         return
+                    if frame.pts < start_pts:
+                        continue
                     fg.push(frame)
                     yield from ostream.encode(fg.pull())
             processors[icont.streams.video[0]] = process_video
-
         if icont.streams.audio:
             #TODO skip transcode if already desired codec
             astream = ocont.add_stream('libvorbis')
             istreams['audio'] = 0
             processors[icont.streams.audio[0]] = lambda x: astream.encode(x.decode)
-        if 'start_time' in query:
-            start_time = int(float(query['start_time']) * time_base )#FIXME time base should be aquirable from container?
-        elif 'skip_first_frames' in query:
-            start_time = int(int(query['skip_first_frames']) / istream.average_rate / istream.time_base)
-        else:
-            start_time = 0
-        #TODO Time base
-        icont.seek(start_time)
         for packet in icont.demux(istreams):
-            if packet.pts is None or packet.pts < start_time:
-                continue
-            #packet.pts -= start_time
             ocont.mux(processors[packet.stream](packet))
-
         #TODO: collate?
         for stream in ocont.streams.get():
             for packet in stream.encode(None):