Botond Dénes 627054c3d7 mutation_reader: shard_reader: fix fast-forwarding with read-ahead ...

The current `fast_forward_to(const dht::partition_range&)`
implementation has two problems:
* If the reader was not created yet, but there is an ongoing read-ahead
  (which is going to create it), the function bails out. This will
  result in this shard reader not being fast-forwarded to the new range
  at all.
* If the reader was already created and there is an ongoing read-ahead,
  the function will wait for this to complete, then fast-forward the
  reader, as it should. However, the buffer is cleared *before* the
  read-ahead is waited for. So if the read-ahead brings in new data,
  this will land in the buffer. This data will be outside of the
  fast-forwarded-to range and worse, as we just cleared the buffer, it
  might violate mutation fragment stream monotonicity requirements.

This patch fixes both of these bugs. Targeted reproducer unit tests are
coming in the next patches.

2020-07-01 09:51:02 +03:00

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