Prevent GIN deleted pages from being reclaimed too early

When GIN vacuum deletes a posting tree page, it assumes that no concurrent
searchers can access it, thanks to ginStepRight() locking two pages at once.
However, since 9.4 searches can skip parts of posting trees descending from the
root.  That leads to the risk that page is deleted and reclaimed before
concurrent search can access it.

This commit prevents the risk of above by waiting for every transaction, which
might wait to reference this page, to finish.  Due to binary compatibility
we can't change GinPageOpaqueData to store corresponding transaction id.
Instead we reuse page header pd_prune_xid field, which is unused in index pages.

Discussion: https://postgr.es/m/31a702a.14dd.166c1366ac1.Coremail.chjischj%40163.com
Author: Andrey Borodin, Alexander Korotkov
Reviewed-by: Alexander Korotkov
Backpatch-through: 9.4

1 files changed, 5 insertions, 1 deletions

diff --git a/src/backend/access/gin/ginvacuum.c b/src/backend/access/gin/ginvacuum.c
index cc440d93547..aed60cb7ae5 100644
--- a/src/backend/access/gin/ginvacuum.c
+++ b/src/backend/access/gin/ginvacuum.c
@@ -261,6 +261,9 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
 	page = BufferGetPage(dBuffer);
 	rightlink = GinPageGetOpaque(page)->rightlink;
 
+	/* For deleted page remember last xid which could knew its address */
+	GinPageSetDeleteXid(page, ReadNewTransactionId());
+
 	page = BufferGetPage(lBuffer);
 	GinPageGetOpaque(page)->rightlink = rightlink;
 
@@ -300,6 +303,7 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
 		data.parentOffset = myoff;
 		data.leftBlkno = leftBlkno;
 		data.rightLink = GinPageGetOpaque(page)->rightlink;
+		data.deleteXid = GinPageGetDeleteXid(page);
 
 		/*
 		 * We can't pass buffer_std = TRUE, because we didn't set pd_lower on
@@ -777,7 +781,7 @@ ginvacuumcleanup(PG_FUNCTION_ARGS)
 		LockBuffer(buffer, GIN_SHARE);
 		page = (Page) BufferGetPage(buffer);
 
-		if (PageIsNew(page) || GinPageIsDeleted(page))
+		if (GinPageIsRecyclable(page))
 		{
 			Assert(blkno != GIN_ROOT_BLKNO);
 			RecordFreeIndexPage(index, blkno);