Remove the "snapshot too old" feature.

Remove the old_snapshot_threshold setting and mechanism for producing
the error "snapshot too old", originally added by commit 848ef42b.
Unfortunately it had a number of known problems in terms of correctness
and performance, mostly reported by Andres in the course of his work on
snapshot scalability.  We agreed to remove it, after a long period
without an active plan to fix it.

This is certainly a desirable feature, and someone might propose a new
or improved implementation in the future.

Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CACG%3DezYV%2BEvO135fLRdVn-ZusfVsTY6cH1OZqWtezuEYH6ciQA%40mail.gmail.com
Discussion: https://postgr.es/m/20200401064008.qob7bfnnbu4w5cw4%40alap3.anarazel.de
Discussion: https://postgr.es/m/CA%2BTgmoY%3Daqf0zjTD%2B3dUWYkgMiNDegDLFjo%2B6ze%3DWtpik%2B3XqA%40mail.gmail.com

src/backend/storage/buffer/bufmgr.c

@@ -5575,20 +5575,3 @@ IssuePendingWritebacks(WritebackContext *wb_context, IOContext io_context)
 
 	wb_context->nr_pending = 0;
 }
-
-
-/*
- * Implement slower/larger portions of TestForOldSnapshot
- *
- * Smaller/faster portions are put inline, but the entire set of logic is too
- * big for that.
- */
-void
-TestForOldSnapshot_impl(Snapshot snapshot, Relation relation)
-{
-	if (RelationAllowsEarlyPruning(relation)
-		&& (snapshot)->whenTaken < GetOldSnapshotThresholdTimestamp())
-		ereport(ERROR,
-				(errcode(ERRCODE_SNAPSHOT_TOO_OLD),
-				 errmsg("snapshot too old")));
-}

src/backend/storage/ipc/ipci.c

@@ -138,7 +138,6 @@ CalculateShmemSize(int *num_semaphores)
 	size = add_size(size, WalRcvShmemSize());
 	size = add_size(size, PgArchShmemSize());
 	size = add_size(size, ApplyLauncherShmemSize());
-	size = add_size(size, SnapMgrShmemSize());
 	size = add_size(size, BTreeShmemSize());
 	size = add_size(size, SyncScanShmemSize());
 	size = add_size(size, AsyncShmemSize());
@@ -298,7 +297,6 @@ CreateSharedMemoryAndSemaphores(void)
 	/*
 	 * Set up other modules that need some shared memory space
 	 */
-	SnapMgrInit();
 	BTreeShmemInit();
 	SyncScanShmemInit();
 	AsyncShmemInit();

src/backend/storage/ipc/procarray.c

@@ -2066,34 +2066,6 @@ GetMaxSnapshotSubxidCount(void)
 	return TOTAL_MAX_CACHED_SUBXIDS;
 }
 
-/*
- * Initialize old_snapshot_threshold specific parts of a newly build snapshot.
- */
-static void
-GetSnapshotDataInitOldSnapshot(Snapshot snapshot)
-{
-	if (!OldSnapshotThresholdActive())
-	{
-		/*
-		 * If not using "snapshot too old" feature, fill related fields with
-		 * dummy values that don't require any locking.
-		 */
-		snapshot->lsn = InvalidXLogRecPtr;
-		snapshot->whenTaken = 0;
-	}
-	else
-	{
-		/*
-		 * Capture the current time and WAL stream location in case this
-		 * snapshot becomes old enough to need to fall back on the special
-		 * "old snapshot" logic.
-		 */
-		snapshot->lsn = GetXLogInsertRecPtr();
-		snapshot->whenTaken = GetSnapshotCurrentTimestamp();
-		MaintainOldSnapshotTimeMapping(snapshot->whenTaken, snapshot->xmin);
-	}
-}
-
 /*
  * Helper function for GetSnapshotData() that checks if the bulk of the
  * visibility information in the snapshot is still valid. If so, it updates
@@ -2147,8 +2119,8 @@ GetSnapshotDataReuse(Snapshot snapshot)
 	snapshot->active_count = 0;
 	snapshot->regd_count = 0;
 	snapshot->copied = false;
-
-	GetSnapshotDataInitOldSnapshot(snapshot);
+	snapshot->lsn = InvalidXLogRecPtr;
+	snapshot->whenTaken = 0;
 
 	return true;
 }
@@ -2529,8 +2501,8 @@ GetSnapshotData(Snapshot snapshot)
 	snapshot->active_count = 0;
 	snapshot->regd_count = 0;
 	snapshot->copied = false;
-
-	GetSnapshotDataInitOldSnapshot(snapshot);
+	snapshot->lsn = InvalidXLogRecPtr;
+	snapshot->whenTaken = 0;
 
 	return snapshot;
 }

src/backend/storage/lmgr/lwlocknames.txt

@@ -47,7 +47,7 @@ CommitTsSLRULock					38
 CommitTsLock						39
 ReplicationOriginLock				40
 MultiXactTruncationLock				41
-OldSnapshotTimeMapLock				42
+# 42 was OldSnapshotTimeMapLock
 LogicalRepWorkerLock				43
 XactTruncationLock					44
 # 45 was XactTruncationLock until removal of BackendRandomLock