Though these are perhaps not the final error types, we name them the same
across COBs and have them work the same way.
All COB modules now have a `Error` enum.
In a future change, we may want the `FromHistory` trait to support
multiple error types, eg. one for `apply` and one for `validate`.
Move `merges` from `Revision` to `Patch`, and key them by actor.
This ensures that only one merge is possible per actor. We also make
sure that only one revision can be merged at a time, and signal a
conflict if more than one revision is considered merged.
Finally, we make sure that merges have the same target-branch commit
to be considered equal.
When a merge is applied to a patch, we check whether the threshold
of delegates has been reached, at which point we set the patch status
to `merged`.
This requires threading the identity document oid through the "apply"
process.
By introducing a small limitation: only allowing entries in the change
graph to be addressable, instead of individual operations; we
drastically simplify the CRDT implementation.
There are four advantages:
1. Op ids are just regular SHA-1s
2. There's no need for relative IDs, ops never refer to other ops within the
same commit
3. There's no need for a nonce, since commits can't collide, and neither can op IDs
4. `OpId` can just be an alias of `EntryId`
The disadvantage of course, is that we have to be mindful of how we
create op transactions, since each transaction creates an addressable
unit. For example, we must not include multiple patch revisions in the
same transaction.
It's fairly easy for a user to (by mistake or intentionally) create two
operations with the same OpId. This patch makes it much less likely,
and ensures that if the OpId is equal, it's because the operations
are identical.
Alexis Sellier