Fix code for re-finding scan position in a multicolumn GIN index.

collectMatchBitmap() needs to re-find the index tuple it was previously
looking at, after transiently dropping lock on the index page it's on.
The tuple should still exist and be at its prior position or somewhere
to the right of that, since ginvacuum never removes tuples but
concurrent insertions could add one.  However, there was a thinko in
that logic, to the effect of expecting any inserted tuples to have the
same index "attnum" as what we'd been scanning.  Since there's no
physical separation of tuples with different attnums, it's not terribly
hard to devise scenarios where this fails, leading to transient "lost
saved point in index" errors.  (While I've duplicated this with manual
testing, it seems impossible to make a reproducible test case with our
available testing technology.)

Fix by just continuing the scan when the attnum doesn't match.

While here, improve the error message used if we do fail, so that it
matches the wording used in btree for a similar case.

collectMatchBitmap()'s posting-tree code path was previously not
exercised at all by our regression tests.  While I can't make
a regression test that exhibits the bug, I can at least improve
the code coverage here, so do that.  The test case I made for this
is an extension of one added by 4b754d6c1, so it only works in
HEAD and v13; didn't seem worth trying hard to back-patch it.

Per bug #16595 from Jesse Kinkead.  This has been broken since
multicolumn capability was added to GIN (commit 27cb66fdf),
so back-patch to all supported branches.

Discussion: https://postgr.es/m/16595-633118be8eef9ce2@postgresql.org

src/backend/access/gin/ginget.c

@@ -264,24 +264,28 @@ collectMatchBitmap(GinBtreeData *btree, GinBtreeStack *stack,
 			/* Search forward to re-find idatum */
 			for (;;)
 			{
-				Datum		newDatum;
-				GinNullCategory newCategory;
-
 				if (moveRightIfItNeeded(btree, stack, snapshot) == false)
-					elog(ERROR, "lost saved point in index");	/* must not happen !!! */
+					ereport(ERROR,
+							(errcode(ERRCODE_INTERNAL_ERROR),
+							 errmsg("failed to re-find tuple within index \"%s\"",
+									RelationGetRelationName(btree->index))));
 
 				page = BufferGetPage(stack->buffer);
 				itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, stack->off));
 
-				if (gintuple_get_attrnum(btree->ginstate, itup) != attnum)
-					elog(ERROR, "lost saved point in index");	/* must not happen !!! */
-				newDatum = gintuple_get_key(btree->ginstate, itup,
-											&newCategory);
+				if (gintuple_get_attrnum(btree->ginstate, itup) == attnum)
+				{
+					Datum		newDatum;
+					GinNullCategory newCategory;
 
-				if (ginCompareEntries(btree->ginstate, attnum,
-									  newDatum, newCategory,
-									  idatum, icategory) == 0)
-					break;		/* Found! */
+					newDatum = gintuple_get_key(btree->ginstate, itup,
+												&newCategory);
+
+					if (ginCompareEntries(btree->ginstate, attnum,
+										  newDatum, newCategory,
+										  idatum, icategory) == 0)
+						break;	/* Found! */
+				}
 
 				stack->off++;
 			}

src/test/regress/expected/gin.out

@@ -199,6 +199,71 @@ from
   i @> '{1}' and j @> '{10}'               | 2               | 0                  | t
 (10 rows)
 
+reset enable_seqscan;
+reset enable_bitmapscan;
+-- re-purpose t_gin_test_tbl to test scans involving posting trees
+insert into t_gin_test_tbl select array[1, g, g/10], array[2, g, g/10]
+  from generate_series(1, 20000) g;
+select gin_clean_pending_list('t_gin_test_tbl_i_j_idx') is not null;
+ ?column? 
+----------
+ t
+(1 row)
+
+analyze t_gin_test_tbl;
+set enable_seqscan = off;
+set enable_bitmapscan = on;
+explain (costs off)
+select count(*) from t_gin_test_tbl where j @> array[50];
+                       QUERY PLAN                        
+---------------------------------------------------------
+ Aggregate
+   ->  Bitmap Heap Scan on t_gin_test_tbl
+         Recheck Cond: (j @> '{50}'::integer[])
+         ->  Bitmap Index Scan on t_gin_test_tbl_i_j_idx
+               Index Cond: (j @> '{50}'::integer[])
+(5 rows)
+
+select count(*) from t_gin_test_tbl where j @> array[50];
+ count 
+-------
+    11
+(1 row)
+
+explain (costs off)
+select count(*) from t_gin_test_tbl where j @> array[2];
+                       QUERY PLAN                        
+---------------------------------------------------------
+ Aggregate
+   ->  Bitmap Heap Scan on t_gin_test_tbl
+         Recheck Cond: (j @> '{2}'::integer[])
+         ->  Bitmap Index Scan on t_gin_test_tbl_i_j_idx
+               Index Cond: (j @> '{2}'::integer[])
+(5 rows)
+
+select count(*) from t_gin_test_tbl where j @> array[2];
+ count 
+-------
+ 20000
+(1 row)
+
+explain (costs off)
+select count(*) from t_gin_test_tbl where j @> '{}'::int[];
+                       QUERY PLAN                        
+---------------------------------------------------------
+ Aggregate
+   ->  Bitmap Heap Scan on t_gin_test_tbl
+         Recheck Cond: (j @> '{}'::integer[])
+         ->  Bitmap Index Scan on t_gin_test_tbl_i_j_idx
+               Index Cond: (j @> '{}'::integer[])
+(5 rows)
+
+select count(*) from t_gin_test_tbl where j @> '{}'::int[];
+ count 
+-------
+ 20006
+(1 row)
+
 reset enable_seqscan;
 reset enable_bitmapscan;
 drop table t_gin_test_tbl;

src/test/regress/sql/gin.sql

@@ -138,4 +138,28 @@ from
 reset enable_seqscan;
 reset enable_bitmapscan;
 
+-- re-purpose t_gin_test_tbl to test scans involving posting trees
+insert into t_gin_test_tbl select array[1, g, g/10], array[2, g, g/10]
+  from generate_series(1, 20000) g;
+
+select gin_clean_pending_list('t_gin_test_tbl_i_j_idx') is not null;
+
+analyze t_gin_test_tbl;
+
+set enable_seqscan = off;
+set enable_bitmapscan = on;
+
+explain (costs off)
+select count(*) from t_gin_test_tbl where j @> array[50];
+select count(*) from t_gin_test_tbl where j @> array[50];
+explain (costs off)
+select count(*) from t_gin_test_tbl where j @> array[2];
+select count(*) from t_gin_test_tbl where j @> array[2];
+explain (costs off)
+select count(*) from t_gin_test_tbl where j @> '{}'::int[];
+select count(*) from t_gin_test_tbl where j @> '{}'::int[];
+
+reset enable_seqscan;
+reset enable_bitmapscan;
+
 drop table t_gin_test_tbl;