Block data for flat storage head may be GC-d about nearcore HOT 4 CLOSED

I would propose the following:

• Introduce an invariant that TAIL <= flat head. This can be enforced by not allowing garbage collection to go beyond the flat head.
• Lock the flat head for the duration of state snapshot (this is already done, no changes here)
• Move the flat head to the final block, unless it is locked. This means that it should also be moved on empty chunks.
• Add a warning if the tail is more than GC_PERIOD + 2 behind head.

from nearcore.

Move the flat head to the final block, unless it is locked

Strictly speaking, flat head can be moved too far before snapshotting is triggered. This is because arbitrary number of chunks in the end of epoch can be missing.

from nearcore.

Strictly speaking, flat head can be moved too far before snapshotting is triggered. This is because arbitrary number of chunks in the end of epoch can be missing.

Ah I see. What do we do about it today? Is this the reason why we don't move the flat head on empty chunks? Would it make sense to always move the flat head to the latest new chunk (barring the other invariants)?

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fixed in #11599

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