This commit introduces code that is intended to replace the current
implicit state machine used by PartitionData. The existing code is still
in use, the new code is NOT used in this commit. A follow-up commit will
switch over to minimise the diff.
This change has two main goals;
* encapsulation & simplification for callers
* robust implementation so developing correct additions is easier
This is a significant refactor of the partition buffering logic to
encapsulate the various states of data (buffering, snapshot, persisting
and the mixed states between them) within the Partition. This alleviates
the rest of the system from having to be concerned with the differences
between "buffering" data, and "unpersisted data", "snapshot data",
"persisting data", "persisting with snapshots" etc - callers now invoke
a method called get_query_data() and they are provided with all the
relevant data for a partition. This abstraction change alone
significantly reduces code and test complexity in the rest of the
ingester.
For the second goal, the new implementation leverages an explicit state
machine, encoded using typestates. Typestate ensures compile-time
correctness of transitions and method calls, and the explicit FSM itself
helps ensure the system progresses in the desired manner - this fixes
and helps prevent bugs caused by implicit states such as:
https://github.com/influxdata/influxdb_iox/issues/5805
This state machine makes the system states explicit and
self-descriptive, helping to reduce the cost of developer on-boarding
(no prior knowledge of "how this bit works") and reduces ongoing
developer burden. This explicit nature also de-risks adding new
functionality - it should be relatively easy to add concurrent snapshot
generation or incremental compaction without introducing bugs. The state
transition logic is abstracted away from callers, minimising the
overhead of this strategy.
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