[MOD-3535] snapshots capture references to modal objects which become

[thundergolfer]

Describe your changes

• MOD-3535
• I've also added an integration test in the other repo (#15223).

In cases where a Modal object is looked up in global scope or in an @enter lifecycle function, memory snapshotting will freeze that object's ID and reuse it in perpetuity, not invalidating the reference if it creates a runtime error after restore.

f = modal.Function.lookup(...)
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╰───> FunctionGet ──> self.object_id = fu-123  # can go stale!

This PR changes it so that upon restore these lookups must be redone. This is a small performance loss but maintains correctness.

Backward/forward compatibility checks

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Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself

Note on protobuf: protobuf message changes in one place may have impact to
multiple entities (client, server, worker, database). See points above.

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[mwaskom]

Random thing this just made me wonder about — what happens if you snapshot/restore across client version boundaries?

[thundergolfer]

what happens if you snapshot/restore across client version boundaries?

@mwaskom the client version will be fixed across snapshot/restore because it's the same container with the same client mount. If the Function's client mount changes the the snapshot would be invalidated.