This commit
- adds a configuration in which no step holds, replication cursors, etc. are created
- removes the send.step_holds.disable_incremental setting
- creates a new config option `replication` for active-side jobs
- adds the replication.protection.{initial,incremental} settings, each
of which can have values
- `guarantee_resumability`
- `guarantee_incremental`
- `guarantee_nothing`
(refer to docs/configuration/replication.rst for semantics)
The `replication` config from an active side is sent to both endpoint.Sender and endpoint.Receiver
for each replication step. Sender and Receiver then act accordingly.
For `guarantee_incremental`, we add the new `tentative-replication-cursor` abstraction.
The necessity for that abstraction is outlined in https://github.com/zrepl/zrepl/issues/340.
fixes https://github.com/zrepl/zrepl/issues/340
This is a stop-gap solution until we re-write the pruner to support
rules for removing step holds.
Note that disabling step holds for incremental sends does not affect
zrepl's guarantee that incremental replication is always possible:
Suppose you yank the external drive during an incremental @from -> @to step:
* restarting that step or future incrementals @from -> @to_later` will be possible
because the replication cursor bookmark points to @from until the step is complete
* resuming @from -> @to will work as long as the pruner on your internal pool doesn't come around to destroy @to.
* in that case, the replication algorithm should determine that the resumable state
on the receiving side isuseless because @to no longer exists on the sending side,
and consequently clear it, and restart an incremental step @from -> @to_later
refs #288
