semi-async `RaftLogReader::try_get_log_entries`

[drmingdrmer]

openraft/src/core/sm/worker.rs line 166 at r1 (raw file):

        let n_entries = end - since;

        let apply_results = self.state_machine.apply(entries).await?;

Now that this is in the same method, it would make sense to somehow overlap the processing. E.g., change the interface of getting log entries to a (semi-async) iterator (which can do prefetching as it sees fit for on-disk data) and feed this iterator to apply() directly? The result could be another (semi-async) iterator, which you can then use to get rid of applying_entries.

Originally posted by @schreter in #1154 (review)

Upgrade RaftLogReader::try_get_log_entries() -> Result<Vec<Entry>, StorageError> to return a pollable result, a Stream or an iterator of Future.
So that prefetch can be done before the caller consuming log entries.

trait RaftLogReader {
  // Return a Stream:
  async fn try_get_log_entries(&mut self, range) -> impl Stream<Item=Result<Entry, StorageError>>;

  // Or return an iterator of Future
  type Fu: Future<Item=Result<Entry, StorageError>>;
  async fn try_get_log_entries(&mut self, range) -> impl Iterator<Item=Fu>;
}

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[drmingdrmer]

@schreter
Does these two forms of return type look good to you?

Usually I prefer Stream because it is more generic than a iterator of Future, because itself is poll-able, but an iterator might block a task when fetching the next Future item.

[schreter]

Usually I prefer Stream because it is more generic than a iterator of Future

I am open to what to use. Stream is probably more "modern" and allows a non-blocking implementation, so it's most likely preferable, indeed.