Add the "snapshot too old" feature

This feature is controlled by a new old_snapshot_threshold GUC.  A
value of -1 disables the feature, and that is the default.  The
value of 0 is just intended for testing.  Above that it is the
number of minutes a snapshot can reach before pruning and vacuum
are allowed to remove dead tuples which the snapshot would
otherwise protect.  The xmin associated with a transaction ID does
still protect dead tuples.  A connection which is using an "old"
snapshot does not get an error unless it accesses a page modified
recently enough that it might not be able to produce accurate
results.

This is similar to the Oracle feature, and we use the same SQLSTATE
and error message for compatibility.

1 files changed, 20 insertions, 11 deletions

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 66b23540fe2..29fd31a819d 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -394,7 +394,8 @@ heapgetpage(HeapScanDesc scan, BlockNumber page)
 	 */
 	LockBuffer(buffer, BUFFER_LOCK_SHARE);
 
-	dp = BufferGetPage(buffer, NULL, NULL, BGP_NO_SNAPSHOT_TEST);
+	dp = BufferGetPage(buffer, snapshot, scan->rs_rd,
+					   BGP_TEST_FOR_OLD_SNAPSHOT);
 	lines = PageGetMaxOffsetNumber(dp);
 	ntup = 0;
 
@@ -537,7 +538,7 @@ heapgettup(HeapScanDesc scan,
 
 		LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);
 
-		dp = BufferGetPage(scan->rs_cbuf, NULL, NULL, BGP_NO_SNAPSHOT_TEST);
+		dp = BufferGetPage(scan->rs_cbuf, snapshot, scan->rs_rd, BGP_TEST_FOR_OLD_SNAPSHOT);
 		lines = PageGetMaxOffsetNumber(dp);
 		/* page and lineoff now reference the physically next tid */
 
@@ -582,7 +583,8 @@ heapgettup(HeapScanDesc scan,
 
 		LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);
 
-		dp = BufferGetPage(scan->rs_cbuf, NULL, NULL, BGP_NO_SNAPSHOT_TEST);
+		dp = BufferGetPage(scan->rs_cbuf, snapshot, scan->rs_rd,
+						   BGP_TEST_FOR_OLD_SNAPSHOT);
 		lines = PageGetMaxOffsetNumber(dp);
 
 		if (!scan->rs_inited)
@@ -616,7 +618,8 @@ heapgettup(HeapScanDesc scan,
 			heapgetpage(scan, page);
 
 		/* Since the tuple was previously fetched, needn't lock page here */
-		dp = BufferGetPage(scan->rs_cbuf, NULL, NULL, BGP_NO_SNAPSHOT_TEST);
+		dp = BufferGetPage(scan->rs_cbuf, snapshot, scan->rs_rd,
+						   BGP_TEST_FOR_OLD_SNAPSHOT);
 		lineoff = ItemPointerGetOffsetNumber(&(tuple->t_self));
 		lpp = PageGetItemId(dp, lineoff);
 		Assert(ItemIdIsNormal(lpp));
@@ -745,7 +748,8 @@ heapgettup(HeapScanDesc scan,
 
 		LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);
 
-		dp = BufferGetPage(scan->rs_cbuf, NULL, NULL, BGP_NO_SNAPSHOT_TEST);
+		dp = BufferGetPage(scan->rs_cbuf, snapshot, scan->rs_rd,
+						   BGP_TEST_FOR_OLD_SNAPSHOT);
 		lines = PageGetMaxOffsetNumber((Page) dp);
 		linesleft = lines;
 		if (backward)
@@ -832,7 +836,8 @@ heapgettup_pagemode(HeapScanDesc scan,
 			lineindex = scan->rs_cindex + 1;
 		}
 
-		dp = BufferGetPage(scan->rs_cbuf, NULL, NULL, BGP_NO_SNAPSHOT_TEST);
+		dp = BufferGetPage(scan->rs_cbuf, scan->rs_snapshot, scan->rs_rd,
+						   BGP_TEST_FOR_OLD_SNAPSHOT);
 		lines = scan->rs_ntuples;
 		/* page and lineindex now reference the next visible tid */
 
@@ -875,7 +880,8 @@ heapgettup_pagemode(HeapScanDesc scan,
 			page = scan->rs_cblock;		/* current page */
 		}
 
-		dp = BufferGetPage(scan->rs_cbuf, NULL, NULL, BGP_NO_SNAPSHOT_TEST);
+		dp = BufferGetPage(scan->rs_cbuf, scan->rs_snapshot, scan->rs_rd,
+						   BGP_TEST_FOR_OLD_SNAPSHOT);
 		lines = scan->rs_ntuples;
 
 		if (!scan->rs_inited)
@@ -908,7 +914,8 @@ heapgettup_pagemode(HeapScanDesc scan,
 			heapgetpage(scan, page);
 
 		/* Since the tuple was previously fetched, needn't lock page here */
-		dp = BufferGetPage(scan->rs_cbuf, NULL, NULL, BGP_NO_SNAPSHOT_TEST);
+		dp = BufferGetPage(scan->rs_cbuf, scan->rs_snapshot, scan->rs_rd,
+						   BGP_TEST_FOR_OLD_SNAPSHOT);
 		lineoff = ItemPointerGetOffsetNumber(&(tuple->t_self));
 		lpp = PageGetItemId(dp, lineoff);
 		Assert(ItemIdIsNormal(lpp));
@@ -1027,7 +1034,8 @@ heapgettup_pagemode(HeapScanDesc scan,
 
 		heapgetpage(scan, page);
 
-		dp = BufferGetPage(scan->rs_cbuf, NULL, NULL, BGP_NO_SNAPSHOT_TEST);
+		dp = BufferGetPage(scan->rs_cbuf, scan->rs_snapshot, scan->rs_rd,
+						   BGP_TEST_FOR_OLD_SNAPSHOT);
 		lines = scan->rs_ntuples;
 		linesleft = lines;
 		if (backward)
@@ -1871,7 +1879,7 @@ heap_fetch(Relation relation,
 	 * Need share lock on buffer to examine tuple commit status.
 	 */
 	LockBuffer(buffer, BUFFER_LOCK_SHARE);
-	page = BufferGetPage(buffer, NULL, NULL, BGP_NO_SNAPSHOT_TEST);
+	page = BufferGetPage(buffer, snapshot, relation, BGP_TEST_FOR_OLD_SNAPSHOT);
 
 	/*
 	 * We'd better check for out-of-range offnum in case of VACUUM since the
@@ -2200,7 +2208,8 @@ heap_get_latest_tid(Relation relation,
 		 */
 		buffer = ReadBuffer(relation, ItemPointerGetBlockNumber(&ctid));
 		LockBuffer(buffer, BUFFER_LOCK_SHARE);
-		page = BufferGetPage(buffer, NULL, NULL, BGP_NO_SNAPSHOT_TEST);
+		page = BufferGetPage(buffer, snapshot, relation,
+							 BGP_TEST_FOR_OLD_SNAPSHOT);
 
 		/*
 		 * Check for bogus item number.  This is not treated as an error