ben-manes commented on code in PR #1155:
URL: https://github.com/apache/solr/pull/1155#discussion_r1043745746
##########
solr/core/src/java/org/apache/solr/core/TransientSolrCoreCacheDefault.java:
##########
@@ -70,10 +80,15 @@ public TransientSolrCoreCacheDefault(CoreContainer
coreContainer) {
Caffeine<String, SolrCore> transientCoresCacheBuilder =
Caffeine.newBuilder()
.initialCapacity(initialCapacity)
- // Use the current thread to queue evicted cores for closing. This
ensures the
- // cache max size is respected (with a different thread the max
size would be
- // respected asynchronously only eventually).
- .executor(Runnable::run)
+ // Do NOT use the current thread to queue evicted cores for
closing. Although this
+ // ensures the cache max size is respected only eventually, it
should be a very
+ // short period of time until the cache maintenance task kicks in.
+ // The onEvict method needs the writeLock from SolrCores to
operate safely.
+ // However, the current thread most probably has acquired a
read-lock already
+ // somewhere up the call stack and would deadlock.
+ // Note that Caffeine cache has an internal maintenance task
rescheduling logic which
+ // explicitly checks on the common pool.
+ .executor(ForkJoinPool.commonPool())
Review Comment:
I am still reluctant to introduce an eviction approver as I believe it is an
easy but wrong answer. It is perfectly pragmatic for a private api where the
caveats are known, but for a library it is a poor api with footguns that will
have a large blast radius.
I would use a secondary cache with weak referenced values for automatic
clean up. That looks similar to the overflow cache here, except that by using
reference counting it is discarded by the GC. This way there are no memory
leaks and the same instance is retrievable if any other thread holds a strong
reference to it to avoid duplications due to races. If simpler then the second
cache can be inclusive since it is based on what is GC reachable rather than a
size/time bound, so juggling the exclusivity might be useless logic that does
not save any memory. By using Map computations the interactions between caches
can be atomic, e.g. to load from the weak cache if absent from the primary one.
A caveat is that a weak value means that a listener on that cache would have
a null value (as GC'd), so it may not be immediately compatible with
`SolrCores.queueCoreToClose(core)`. It may be that refactoring to coordinate
the close based on reachability could solve that and remove the complexity that
`SolrCores` must go through to safely shutdown an instance.
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