馃悰 Avoid mutating milestone snapshots when building from ops #651
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ericyhwang
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May 14, 2024
| next(); | ||
| }, | ||
| function(next) { | ||
| connection.fetchSnapshot('books', 'mocking-bird', 3, next); |
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Let's add a comment indicating that this test case is making sure the client-facing connection.fetchSnapshot doesn't cause an in-place mutation to the stored milestone snapshot returned by the internal milestoneDb.getMilestoneSnapshot.
It took me a bit to realize where the mutation would formerly have occurred.
At the moment, we directly pass our milestone snapshot to `ot.applyOps()`, which [mutates the snapshot][1]. In most Production-grade implementations of the `MilestoneDb`, this is fine, since mutating the in-memory `Snapshot` will have no effect on the underlying database representation. However, this does break the `MemoryMilestoneDb` (eg in tests). In order to fix this, and generally be defensive (eg if db adapters decide to implement an in-memory cache), this change clones the snapshot before calling `ot.applyOps()` on it. [1]: https://github.com/share/sharedb/blob/a5c19b986d65c93abbc9ed11f7a25c0dbaf0788c/lib/ot.js#L172
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At the moment, we directly pass our milestone snapshot to
ot.applyOps(), which mutates the snapshot.In most Production-grade implementations of the
MilestoneDb, this is fine, since mutating the in-memorySnapshotwill have no effect on the underlying database representation.However, this does break the
MemoryMilestoneDb(eg in tests).In order to fix this, and generally be defensive (eg if db adapters decide to implement an in-memory cache), this change clones the snapshot before calling
ot.applyOps()on it.