Don't duplicate parallel seqscan shmem sizing logic in nbtree.

This is architecturally mildly problematic, which becomes more
pronounced with the upcoming introduction of pluggable storage.

To fix, teach heap_parallelscan_estimate() to deal with SnapshotAny
snapshots, and then use it from _bt_parallel_estimate_shared().

Author: Andres Freund
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

src/backend/access/heap/heapam.c

@@ -1615,8 +1615,14 @@ heap_endscan(HeapScanDesc scan)
 Size
 heap_parallelscan_estimate(Snapshot snapshot)
 {
-	return add_size(offsetof(ParallelHeapScanDescData, phs_snapshot_data),
-					EstimateSnapshotSpace(snapshot));
+	Size		sz = offsetof(ParallelHeapScanDescData, phs_snapshot_data);
+
+	if (IsMVCCSnapshot(snapshot))
+		sz = add_size(sz, EstimateSnapshotSpace(snapshot));
+	else
+		Assert(snapshot == SnapshotAny);
+
+	return sz;
 }
 
 /* ----------------

src/backend/access/nbtree/nbtsort.c

@@ -158,7 +158,7 @@ typedef struct BTShared
 	/*
 	 * This variable-sized field must come last.
 	 *
-	 * See _bt_parallel_estimate_shared().
+	 * See _bt_parallel_estimate_shared() and heap_parallelscan_estimate().
 	 */
 	ParallelHeapScanDescData heapdesc;
 } BTShared;
@@ -1405,15 +1405,8 @@ _bt_end_parallel(BTLeader *btleader)
 static Size
 _bt_parallel_estimate_shared(Snapshot snapshot)
 {
-	if (!IsMVCCSnapshot(snapshot))
-	{
-		Assert(snapshot == SnapshotAny);
-		return sizeof(BTShared);
-	}
-
-	return add_size(offsetof(BTShared, heapdesc) +
-					offsetof(ParallelHeapScanDescData, phs_snapshot_data),
-					EstimateSnapshotSpace(snapshot));
+	return add_size(offsetof(BTShared, heapdesc),
+					heap_parallelscan_estimate(snapshot));
 }
 
 /*