Class LogFileProducer

What's the plan?

the LogFile has just one FD, used for both reading and writing. Each time you add an entry, fd.seek to the end and then write.

Each reader (i.e. Producer) keeps track of their own offset. The reader starts by seeking to the start of the logfile, and reading forwards. Between each hunk of file they yield chunks, so they must remember their offset before yielding and re-seek back to that offset before reading more data. When their read() returns EOF, they're finished with the first phase of the reading (everything that's already been written to disk).

After EOF, the remaining data is entirely in the current entries list. These entries are all of the same channel, so we can do one "".join and obtain a single chunk to be sent to the listener. But since that involves a yield, and more data might arrive after we give up control, we have to subscribe them before yielding. We can't subscribe them any earlier, otherwise they'd get data out of order.

We're using a generator in the first place so that the listener can throttle us, which means they're pulling. But the subscription means we're pushing. Really we're a Producer. In the first phase we can be either a PullProducer or a PushProducer. In the second phase we're only a PushProducer.