This series enforces a minimum size of the unprivileged section when
performing `shrink()` operation.
When the cache is shrunk, we still drop entries first from unprivileged
section (as before this commit), however, if this section is already small
(smaller than `max_size / 2`), we will drop entries from the privileged
section.
This is necessary, as before this change the unprivileged section could
be starved. For example if the cache could store at most 50 entries and
there are 49 entries in privileged section, after adding 5 entries (that would
go to unprivileged section) 4 of them would get evicted and only the 5th one
would stay. This caused problems with BATCH statements where all
prepared statements in the batch have to stay in cache at the same time
for the batch to correctly execute.
To correctly check if the unprivileged section might get too small after
dropping an entry, `_current_size` variable, which tracked the overall size
of cache, is changed to two variables: `_unprivileged_section_size` and
`_privileged_section_size`, tracking section sizes separately.
New tests are added to check this new behavior and bookkeeping of the section
sizes. A test is added, that sets up a CQL environment with a very small
prepared statement cache, reproduces issue in #10440 and stresses the cache.
Fixes#10440.
Closes#10456
* github.com:scylladb/scylla:
loading_cache_test: test prepared stmts cache
loading_cache: force minimum size of unprivileged
loading_cache: extract dropping entries to lambdas
loading_cache: separately track size of sections
loading_cache: fix typo in 'privileged'
5169ce40ef