`read_checksum()` loads the checksum component from disk and stores a
non-owning reference in the shareable components. To avoid loading the
same component twice, the function has an early return statement.
However, this does not guarantee atomicity - two fibers or threads may
load the component and update the shareable components concurrently.
This can lead to use-after-free situations when accessing the component
through the shareable components, since the reference stored there is
non-owning. This can happen when multiple compaction tasks run on the
same SSTable (e.g., regular compaction and scrub-validate).
Fix this by not updating the reference in shareable components, if a
reference is already in place. Instead, create an owning reference to
the existing component for the current fiber. This is less efficient
than using a mutex, since the component may be loaded multiple times
from disk before noticing the race, but no locks are used for any other
SSTable component either. Also, this affects uncompressed SSTables,
which are not that common.
Fixes#23728.
Signed-off-by: Nikos Dragazis <nikolaos.dragazis@scylladb.com>
Closesscylladb/scylladb#23872
eaa2ce1bb5