Avoid unnecessary GinFormTuple() calls for incompressible posting lists.

Previously, we attempted to form a posting list tuple even when
ginCompressPostingList() failed to compress the posting list due to
its size. While there was no functional failure, it always wasted one
GinFormTuple() call when item pointers didn't fit in a posting list
tuple.

This commit ensures that a GIN index tuple is formed only when all
item pointers in the posting list are successfully compressed.

Author: Arseniy Mukhin <arseniy.mukhin.dev@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/CAE7r3M+C=jcpTD93f_RBHrQp3C+=TAXFs+k4tTuZuuxboK8AvA@mail.gmail.com

src/backend/access/gin/gininsert.c

@@ -218,7 +218,8 @@ addItemPointersToLeafTuple(GinState *ginstate,
 	ItemPointerData *newItems,
 			   *oldItems;
 	int			oldNPosting,
-				newNPosting;
+				newNPosting,
+				nwritten;
 	GinPostingList *compressedList;
 
 	Assert(!GinIsPostingTree(old));
@@ -235,18 +236,19 @@ addItemPointersToLeafTuple(GinState *ginstate,
 
 	/* Compress the posting list, and try to a build tuple with room for it */
 	res = NULL;
-	compressedList = ginCompressPostingList(newItems, newNPosting, GinMaxItemSize,
-											NULL);
-	pfree(newItems);
-	if (compressedList)
+	compressedList = ginCompressPostingList(newItems, newNPosting, GinMaxItemSize, &nwritten);
+	if (nwritten == newNPosting)
 	{
 		res = GinFormTuple(ginstate, attnum, key, category,
 						   (char *) compressedList,
 						   SizeOfGinPostingList(compressedList),
 						   newNPosting,
 						   false);
-		pfree(compressedList);
 	}
+
+	pfree(newItems);
+	pfree(compressedList);
+
 	if (!res)
 	{
 		/* posting list would be too big, convert to posting tree */
@@ -293,17 +295,19 @@ buildFreshLeafTuple(GinState *ginstate,
 {
 	IndexTuple	res = NULL;
 	GinPostingList *compressedList;
+	int			nwritten;
 
 	/* try to build a posting list tuple with all the items */
-	compressedList = ginCompressPostingList(items, nitem, GinMaxItemSize, NULL);
-	if (compressedList)
+	compressedList = ginCompressPostingList(items, nitem, GinMaxItemSize, &nwritten);
+	if (nwritten == nitem)
 	{
 		res = GinFormTuple(ginstate, attnum, key, category,
 						   (char *) compressedList,
 						   SizeOfGinPostingList(compressedList),
 						   nitem, false);
-		pfree(compressedList);
 	}
+	pfree(compressedList);
+
 	if (!res)
 	{
 		/* posting list would be too big, build posting tree */