In bms_add_member(), use repalloc() if the bms needs to be enlarged.

Previously bms_add_member() would palloc a whole-new copy of the existing
set, copy the words, and pfree the old one. repalloc() is potentially much
faster, and more importantly, this is less surprising if CurrentMemoryContext
is not the same as the context the old set is in. bms_add_member() still
allocates a new bitmapset in CurrentMemoryContext if NULL is passed as
argument, but that is a lot less likely to induce bugs.

Nicholas White.

src/backend/nodes/bitmapset.c

@@ -632,21 +632,20 @@ bms_add_member(Bitmapset *a, int x)
 		return bms_make_singleton(x);
 	wordnum = WORDNUM(x);
 	bitnum = BITNUM(x);
+
+	/* enlarge the set if necessary */
 	if (wordnum >= a->nwords)
 	{
-		/* Slow path: make a larger set and union the input set into it */
-		Bitmapset  *result;
-		int			nwords;
+		int			oldnwords = a->nwords;
 		int			i;
 
-		result = bms_make_singleton(x);
-		nwords = a->nwords;
-		for (i = 0; i < nwords; i++)
-			result->words[i] |= a->words[i];
-		pfree(a);
-		return result;
+		a = (Bitmapset *) repalloc(a, BITMAPSET_SIZE(wordnum + 1));
+		a->nwords = wordnum + 1;
+		/* zero out the enlarged portion */
+		for (i = oldnwords; i < a->nwords; i++)
+			a->words[i] = 0;
 	}
-	/* Fast path: x fits in existing set */
+
 	a->words[wordnum] |= ((bitmapword) 1 << bitnum);
 	return a;
 }