From mike's patch, now that these are no longer mutable (LUCENE-6317), this should be easier to do and we know it won't cause trouble.
Bulk close after 5.1 release
Commit 1663302 from Robert Muir in branch 'dev/branches/branch_5x'
[ https://svn.apache.org/r1663302 ]
LUCENE-6318: re-use immutable maps across fieldinfos when they are the same
When backporting, i only applied to lucene 5 codec. The 4.x FISReaders have a lot of scary stuff going on already.
Commit 1663299 from Robert Muir in branch 'dev/trunk'
[ https://svn.apache.org/r1663299 ]
+1, thanks Rob.