refactor(migration-canister): cache last event for every migration #7948
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Co-authored-by: michael-weigelt <[email protected]>
| HISTORY.with_borrow(|h| { | ||
| let event = h | ||
| .get(&idx) | ||
| .expect("Missing event in history (this is a bug!)"); |
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Since the method returns Option anyway, why not log a critical error and resume? Even if the panic is currently not reachable on critical paths (I have not checked), this may not stay that way.
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What do you mean by a "critical error"? Is there some monitoring/alerting set up that looks for some string patterns?
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No. But logging a bug and continuing (with None) may be better than crashing for a state machine where we haven't isolated all individual tasks.
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This PR refactors the stable memory representation of events in the migration canister to
Alternatives considered:
BTreeMap<CanisterMigrationArgs, Vec<Event>, Memory>- this representation is inefficient if there are (maliciously) many events for a given pair of source and target canisters;BTreeMap<Event, (), Memory>with events sorted by their pairs of source and target canisters - relying on range queries is fragile and introduces substantial complexity for testing.