Tomasz Grabiec e6afc89735 row_cache: Record upgraded schema in memtable entries during update ...

Cache update may defer in the middle of moving of partition entry
from a flushed memtable to the cache. If the schema was changed since
the entry was written, it upgrades the schema of the partition_entry
first but doesn't update the schema_ptr in memtable_entry. The entry
is removed from the memtable afterward. If a memtable reader
encounters such an entry, it will try to upgrade it assuming it's
still at the old schema.

That is undefined behavior in general, which may include:

 - read failures due to bad_alloc, if fixed-size cells are interpreted
   as variable-sized cells, and we misinterpret a value for a huge
   size

 - wrong read results

 - node crash

This doesn't result in a permanent corruption, restarting the node
should help.

It's the more likely to happen the more rows there are in a
partition. It's unlikely to happen with single-row partitions.

Introduced in 70c7277.

Fixes #5128.

2019-10-03 22:03:29 +02:00

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