Tomasz Grabiec e81fc1f095 row_cache: Fix undefined behavior on key linearization ...

This is relevant only when using partition or clustering keys which
have a representation in memory which is larger than 12.8 KB (10% of
LSA segment size).

There are several places in code (cache, background garbage
collection) which may need to linearize keys because of performing key
comparison, but it's not done safely:

 1) the code does not run with the LSA region locked, so pointers may
get invalidated on linearization if it needs to reclaim memory. This
is fixed by running the code inside an allocating section.

 2) LSA region is locked, but the scope of
with_linearized_managed_bytes() encloses the allocating section. If
allocating section needs to reclaim, linearization context will
contain invalidated pointers. The fix is to reorder the scopes so
that linearization context lives within an allocating section.

Example of 1 can be found in
range_populating_reader::handle_end_of_stream() where it performs a
lookup:

  auto prev = std::prev(it);
  if (prev->key().equal(*_cache._schema, *_last_key->_key)) {
     it->set_continuous(true);

but handle_end_of_stream() is not invoked under allocating section.

Example of 2 can be found in mutation_cleaner_impl::merge_some() where
it does:

  return with_linearized_managed_bytes([&] {
  ...
    return _worker_state->alloc_section(region, [&] {

Fixes #6637.
Refs #6108.

Tests:

  - unit (all)

Message-Id: <1592218544-9435-1-git-send-email-tgrabiec@scylladb.com>

2020-06-15 16:03:33 +03:00

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