Add Remaining and Size to perf/ring Record

[brycekahle]

This allows tracking utilization of the underlying buffers. Processing the values to make sense in a user's monitoring system is left to the user.

Using the post-read remaining bytes allows the value to reach zero, whereas a pre-read value would never reach zero.

[lmb]

Looks good, can you add test coverage please?

[lmb]

Size is fixed for the reader, why not expose this as Reader.Size() int instead? You can move the call to perfBufferSize from newPerfEventRing to NewReaderWithOptions. Same idea applies to ringbuf.Reader.

[brycekahle]

The size we are interested in, is the data portion, without the metadata page. Moving the call to perfBufferSize would've required changes to a bunch of other places that expect newPerfEventRing to handle the size manipulation. I did something different, let me know what you think.

[brycekahle]

Looks good, can you add test coverage please?

Added. For the backward/overwriteable buffer it isn't possible to get a precise remaining count once it has been overwritten.

[lmb]

Looks good, thanks for adding the tests! Final question: seems like the semantics for overwritable aren't clear. Should we just set remaining to -1 for now in that case and document that this means the data remaining can't be determined?

[brycekahle]

Should we just set remaining to -1 for now in that case and document that this means the data remaining can't be determined?

We could. It does work correctly until any kind of overwrite happens, and then it becomes inaccurate by reporting more data remaining than their actually is (because of the partial leftover records).

[lmb]

Pushed a doc fix. Please squash then we are good to go.

[brycekahle]

squashed

[brycekahle]

@lmb ready to go.