Adrien Grand created LUCENE-7410:
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Summary: Make cache keys and closed listeners less trappy
Key: LUCENE-7410
URL: https://issues.apache.org/jira/browse/LUCENE-7410
Project: Lucene - Core
Issue Type: Bug
Reporter: Adrien Grand
IndexReader currently exposes getCoreCacheKey(),
getCombinedCoreAndDeletesKey(), addCoreClosedListener() and
addReaderClosedListener(). They are typically used to manage resources whose
lifetime needs to mimic the lifetime of segments/indexes, typically caches.
I think this is trappy for various reasons:
h3. Memory leaks
When maintaining a cache, entries are added to the cache based on the cache key
and then evicted using the cache key that is given back by the close listener,
so it is very important that both keys are the same.
But if a filter reader happens to delegate get*Key() and not
add*ClosedListener() or vice-versa then there is potential for a memory leak
since the closed listener will be called on a different key and entries will
never be evicted from the cache.
h3. Lifetime expectations
The expectation of using the core cache key is that it will not change in case
of deletions, but this is only true on SegmentReader and LeafReader impls that
delegate to it. Other implementations such as composite readers or parallel
leaf readers use the same key for "core" and "combined core and deletes".
h3. Throw-away wrappers cause cache trashing
An application might want to either expose more (with a ParrallelReader or
MultiReader) or less information (by filtering fields/docs that can be seen)
depending on the user who is logged in. In that case the application would
typically maintain a DirectoryReader and then wrap it per request depending on
the logged user and throw away the wrapper once the request is completed.
The problem is that these wrappers have their own cache keys and the
application may build something costly and put it in a cache to throw it away a
couple milliseconds later. I would rather like for such readers to have a way
to opt out from caching on order to avoid this performance trap.
h3. Type safety
The keys that are exposed are plain java.lang.Object instances, which requires
caches to look like a {{Map<Object, ?>}} which makes it very easy to either try
to get, put or remove on the wrong object since any object would be accepted.
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