Extent fails to open after unclean shutdown

[nblumhardt]

Via: https://docs.datalust.co/discuss/5d642417456566002e5057c7

A bug in the storage engine's recovery logic can cause a start-up failure after an unclean shutdown, in some (rare) circumstances.

The error message is:

System.Exception: Flare native storage failed (InternalError), internal panic with 'assertion failed: self.remaining() >= dst.len()'

This is because the storage engine fails to recognize a write as incomplete if metadata is truncated at a specific point in the frame (an eight-byte window). The fix requires an additional length check before proceeding to copy from the read buffer.

The bug should not cause data corruption/loss, but manual recovery will be needed if this appears again before the fix is in the wild.