Fix wrong data table horizon computation during backend startup.

When ComputeXidHorizons() was called before MyDatabaseOid is set,
e.g. because a dead row in a shared relation is encountered during
InitPostgres(), the horizon for normal tables was computed too
aggressively, ignoring all backends connected to a database.

During subsequent pruning in a data table the too aggressive horizon
could end up still being used, possibly leading to still needed tuples
being removed. Not good.

This is a bug in dc7420c2c9, which the test added in 94bc27b576 made
visible, if run with force_parallel_mode set to regress. In that case
the bug is reliably triggered, because "pruning_query" is run in a
parallel worker and the start of that parallel worker is likely to
encounter a dead row in pg_database.

The fix is trivial: Compute a more pessimistic data table horizon if
MyDatabaseId is not yet known.

Author: Andres Freund
Discussion: https://postgr.es/m/20201029040030.p4osrmaywhqaesd4@alap3.anarazel.de

src/backend/storage/ipc/procarray.c

@@ -1757,9 +1757,16 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
 		 * the shared horizon. But in recovery we cannot compute an accurate
 		 * per-database horizon as all xids are managed via the
 		 * KnownAssignedXids machinery.
+		 *
+		 * Be careful to compute a pessimistic value when MyDatabaseId is not
+		 * set. If this is a backend in the process of starting up, we may not
+		 * use a "too aggressive" horizon (otherwise we could end up using it
+		 * to prune still needed data away). If the current backend never
+		 * connects to a database that is harmless, because
+		 * data_oldest_nonremovable will never be utilized.
 		 */
 		if (in_recovery ||
-			proc->databaseId == MyDatabaseId ||
+			MyDatabaseId == InvalidOid || proc->databaseId == MyDatabaseId ||
 			proc->databaseId == 0)	/* always include WalSender */
 		{
 			h->data_oldest_nonremovable =