Major Description
Spinoff from LUCENE-5248, where Shai discovered this ...
SegmentCoreReaders has a SegmentReader owner member, that points to the
first SegmentReader that was opened. When that SR is reopened to SR2,
e.g. because new deletes or NDV updates happened, the same SCR is
shared.
But, even if you close SR1, any thing it points to cannot be GCd
because SCR is pointing to it.
I think the big things are liveDocs and the NDV update maps; Shai is
going to fix the latter in LUCENE-5248, so this issue should fix
liveDocs.
The simplest fix is to make liveDocs not final and null it out in
doClose ... but that's sort of fragile (what if we add other members
in the future and forget to null them on close?). I think it'd be better to
eliminate the owner reference; it's only used so we can evict
FieldCache entry once the core is closed. Maybe we can just store the
coreCacheKey instead?