This commit adds logic for cache pinning to handle subtxn. It also makes
it easier to find cache pinning leaks. Finally, it fixes handling of
cross-commit operations like VACUUM and CLUSTER. Previously, such
operations incorrectly released that cache pin on the first commit
even though the object was used after that.
Currently, if a transaction ends (normally or abnormally) while a
cache is pinned, that cache will never be released, leading to a
memory leak. This change ensures that all taken cache pins are
released when a transaction ends by registering all pins in a list and
cleaning it up in a transaction end callback. This makes the use of
cache_release() optional. However, timely calls to cache_release() is
still encouraged since it is better to release memory as soon as
possible during long-running transactions.
Previously, the extension could end up in a bad state if it
was dropped as part of a cascade. This PR fixes that by
checking explicitly for the presence of the proxy table
to make sure we are not in the middle of an extension
drop. Fixes#73.
In case new allocations are made during a cache entry update, we
should use the cache's internal memory context to ensure that new data
is allocated and freed in a way consistent with the rest of the cache.
