The record file was written from the stream thread. As a consequence,
any blocking I/O to write the file delayed the decoder.
For maximum performance even when recording is enabled, send
(refcounted) packets to a separate recording thread.
To packetize the H.264 raw stream, av_parser_parse2() (called by
av_read_frame()) knows that it has received a full frame only after it
has received some data for the next frame. As a consequence, the client
always waited until the next frame before sending the current frame to
the decoder!
On the device side, we know packets boundaries. To reduce latency,
make the device always transmit the "frame meta" to packetize the stream
manually (it was already implemented to send PTS, but only enabled on
recording).
On the client side, replace av_read_frame() by manual packetizing and
parsing.
<https://stackoverflow.com/questions/50682518/replacing-av-read-frame-to-reduce-delay>
<https://trac.ffmpeg.org/ticket/3354>
On socket disconnection, on Linux, recv() returns -1 and errno is set.
But on Windows, errno is 0.
In that case, AVERROR(errno) == 0, leading to the warning:
> Invalid return value 0 for stream protocol
To avoid the problem, if errno is 0, return AVERROR_EOF.
Ref: commit 2876463d39
Limit source code to 80 chars, and declare functions return type and
modifiers on a separate line.
This allows to avoid very long lines, and all function names are
aligned.
(We do this on VLC, and I like it.)
The decoder initially read from the socket, decoded the video and sent
the decoded frames to the screen:
+---------+ +----------+
socket ---> | decoder | ---> | screen |
+---------+ +----------+
The design was simple, but the decoder had several responsabilities.
Then we added the recording feature, so we added a recorder, which
reused the packets received from the socket managed by the decoder:
+----------+
---> | screen |
+---------+ / +----------+
socket ---> | decoder | ----
+---------+ \ +----------+
---> | recorder |
+----------+
This lack of separation of concerns now have concrete implications: we
could not (properly) disable the decoder/display to only record the
video.
Therefore, split the decoder to extract the stream:
+----------+ +----------+
---> | decoder | ---> | screen |
+---------+ / +----------+ +----------+
socket ---> | stream | ----
+---------+ \ +----------+
---> | recorder |
+----------+
This will allow to record the stream without decoding the video.
