Report sort phase progress in parallel btree build

We were already reporting it, but only after the parallel workers were
finished, which is visibly much later than what happens in a serial
build.

With this change we report it when the leader starts its own sort phase
when participating in the build (the normal case).  Now this might
happen a little later than when the workers start their sorting phases,
but a) communicating the actual phase start from workers is likely to be
a hassle, and b) the sort phase start is pretty fuzzy anyway, since
sorting per se is actually initiated by tuplesort.c internally earlier
than tuplesort_performsort() is called.

Backpatch to pg12, where the progress reporting code for CREATE INDEX
went in.

Reported-by: Tomas Vondra <tomas.vondra@enterprisedb.com>
Author: Matthias van de Meent <boekewurm+postgres@gmail.com>
Reviewed-by: Greg Nancarrow <gregn4422@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/1128176d-1eee-55d4-37ca-e63644422adb

Author: alvherre

src/backend/access/nbtree/nbtsort.c

@@ -562,6 +562,7 @@ _bt_leafbuild(BTSpool *btspool, BTSpool *btspool2)
 	}
 #endif							/* BTREE_BUILD_STATS */
 
+	/* Execute the sort */
 	pgstat_progress_update_param(PROGRESS_CREATEIDX_SUBPHASE,
 								 PROGRESS_BTREE_PHASE_PERFORMSORT_1);
 	tuplesort_performsort(btspool->sortstate);
@@ -1833,16 +1834,18 @@ _bt_parallel_scan_and_sort(BTSpool *btspool, BTSpool *btspool2,
 									   true, progress, _bt_build_callback,
 									   (void *) &buildstate, scan);
 
-	/*
-	 * Execute this worker's part of the sort.
-	 *
-	 * Unlike leader and serial cases, we cannot avoid calling
-	 * tuplesort_performsort() for spool2 if it ends up containing no dead
-	 * tuples (this is disallowed for workers by tuplesort).
-	 */
+	/* Execute this worker's part of the sort */
+	if (progress)
+		pgstat_progress_update_param(PROGRESS_CREATEIDX_SUBPHASE,
+									 PROGRESS_BTREE_PHASE_PERFORMSORT_1);
 	tuplesort_performsort(btspool->sortstate);
 	if (btspool2)
+	{
+		if (progress)
+			pgstat_progress_update_param(PROGRESS_CREATEIDX_SUBPHASE,
+										 PROGRESS_BTREE_PHASE_PERFORMSORT_2);
 		tuplesort_performsort(btspool2->sortstate);
+	}
 
 	/*
 	 * Done.  Record ambuild statistics, and whether we encountered a broken