Re: GIN improvements part 1: additional information

I noticed that the gin vacuum redo routine is dead code, except for the data-leaf page handling, because we never remove entries or internal nodes (page deletion is a separate wal record type). And the data-leaf case is functionally equivalent to heap newpage records. I removed the dead code and made it more clear that it resembles heap newpage.

Attached is a yet another version, with more bugs fixed and more comments added and updated. I would appreciate some heavy-testing of this patch now. If you could re-run the tests you've been using, that could be great. I've tested the WAL replay by replicating GIN operations over streaming replication. That doesn't guarantee it's correct, but it's a good smoke test.

I tried my test-suite but it hangs on index scan with infinite loop. I re-tried it on my laptop with -O0. I found it to crash on update and vacuum in some random places like:
Assert(GinPageIsData(page)); in xlogVacuumPage
Assert(ndecoded == totalpacked); in ginCompressPostingList

Trying to debug it.