Keep already buffered data when seeking to another period

[andrew-norris]

Issue description

I am trying to play multiple media streams in a ConcatenatingMediaSource. I can get the next tracks to buffer ahead of time. However when seeking within a track or to the next track, the buffered media is re-downloaded. We tracked this issue using Charles proxy and monitoring our network requests.

Reproduction steps

• Set up Charles Proxy to monitor network traffic.
• Enter the "Clear -> Enc -> Clear -> Enc -> Enc" sample playlist.
• Press the Seek Forward Button (Second from the right)

At this point if you look at the Charles traffic there should be multiple responses with the same byte range. We have attached our Charles logs and you can see that the 2nd and 8th, as well as the 5th and the 10th are duplicate calls.
exoplayer_issue.chls.zip

Link to test content

This was reproduced using the Exoplayer demo app.

Version of ExoPlayer being used

2.9.0

Device(s) and version(s) of Android being used

Samsung Galaxy S9, Android 8.0
Google Pixel 2, Android 8.0

A full bug report captured from the device

bugreport-2018-10-30-01-07-36.zip

[tonihei]

That is probably working as intended.

Note that we can't keep arbitrary amounts of buffer in memory. That's why we only buffer a few seconds ahead as configured in the LoadControl (default is 50 seconds maximum).

• We also buffer ahead into the next playlist item if we already finished buffering the current one.
• The buffer is kept if you seek to a position within the current buffer (no matter if it's in the current or the next item). However, we do throw away the buffer of other playlist items you didn't seek into.
• As soon as you seek somewhere outside the current buffer (that could be before the current position or behind the buffered position), we throw away the already buffered data and start buffering from the new position.

I wasn't able to able to open the traffic log as I don't have access to the Charles application. But does the description above fit what you are seeing?

We should try to keep the buffer of other upcoming playlist items if we were able to keep the buffer of the target item. Marking as an enhancement to do that. But it's low-priority for now.