Fix limit block handling in pg_wal_summary_contents().

Previously, pg_wal_summary_contents() had two issues,
causing discrepancies between pg_wal_summary_contents()
and the pg_walsummary command on the same WAL summary file:

(1) It did not emit the limit block when that's the only data for
     a particular relation fork.
(2) It emitted the same limit block multiple times if the list of
     block numbers was long enough.

This commit fixes these issues.

Backpatch to v17 where pg_wal_summary_contents() was added.

Author: Fujii Masao
Reviewed-by: Robert Haas
Discussion: https://postgr.es/m/90980ee6-2da6-42f6-a7b0-b7bae62ae279@oss.nttdata.com

Author: MasaoFujii

src/backend/backup/walsummaryfuncs.c

@@ -118,6 +118,23 @@ pg_wal_summary_contents(PG_FUNCTION_ARGS)
 		values[2] = ObjectIdGetDatum(rlocator.dbOid);
 		values[3] = Int16GetDatum((int16) forknum);
 
+		/*
+		 * If the limit block is not InvalidBlockNumber, emit an extra row
+		 * with that block number and limit_block = true.
+		 *
+		 * There is no point in doing this when the limit_block is
+		 * InvalidBlockNumber, because no block with that number or any higher
+		 * number can ever exist.
+		 */
+		if (BlockNumberIsValid(limit_block))
+		{
+			values[4] = Int64GetDatum((int64) limit_block);
+			values[5] = BoolGetDatum(true);
+
+			tuple = heap_form_tuple(rsi->setDesc, values, nulls);
+			tuplestore_puttuple(rsi->setResult, tuple);
+		}
+
 		/* Loop over blocks within the current relation fork. */
 		while (1)
 		{
@@ -143,23 +160,6 @@ pg_wal_summary_contents(PG_FUNCTION_ARGS)
 				tuple = heap_form_tuple(rsi->setDesc, values, nulls);
 				tuplestore_puttuple(rsi->setResult, tuple);
 			}
-
-			/*
-			 * If the limit block is not InvalidBlockNumber, emit an extra row
-			 * with that block number and limit_block = true.
-			 *
-			 * There is no point in doing this when the limit_block is
-			 * InvalidBlockNumber, because no block with that number or any
-			 * higher number can ever exist.
-			 */
-			if (BlockNumberIsValid(limit_block))
-			{
-				values[4] = Int64GetDatum((int64) limit_block);
-				values[5] = BoolGetDatum(true);
-
-				tuple = heap_form_tuple(rsi->setDesc, values, nulls);
-				tuplestore_puttuple(rsi->setResult, tuple);
-			}
 		}
 	}