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

2 files changed, 4 insertions, 34 deletions

diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 5551afffc04..a3d8eacb8dc 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/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();
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 2a3da49b8fc..aa1552e0316 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -2067,34 +2067,6 @@ GetMaxSnapshotSubxidCount(void)
 }
 
 /*
- * 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
  * the fields that need to change and returns true. Otherwise it returns
@@ -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;
 }