Fix gin_leafpage_items().

On closer inspection, commit 84ad68d6 broke gin_leafpage_items(),
because the aligned copy of the page got palloc'd in a short-lived
context whereas it needs to be in the SRF's multi_call_memory_ctx.
This was not exposed by the regression test, because the regression
test doesn't actually exercise the function in a meaningful way.
Fix the code bug, and extend the test in what I hope is a portable
fashion.

contrib/pageinspect/expected/gin.out

 CREATE TABLE test1 (x int, y int[]);
 INSERT INTO test1 VALUES (1, ARRAY[11, 111]);
-CREATE INDEX test1_y_idx ON test1 USING gin (y);
+CREATE INDEX test1_y_idx ON test1 USING gin (y) WITH (fastupdate = off);
 \x
 SELECT * FROM gin_metapage_info(get_raw_page('test1_y_idx', 0));
 -[ RECORD 1 ]----+-----------
@@ -27,4 +27,11 @@ flags     | {leaf}
 SELECT * FROM gin_leafpage_items(get_raw_page('test1_y_idx', 1));
 ERROR:  input page is not a compressed GIN data leaf page
 DETAIL:  Flags 0002, expected 0083
-DROP TABLE test1;
+INSERT INTO test1 SELECT x, ARRAY[1,10] FROM generate_series(2,10000) x;
+SELECT COUNT(*) > 0
+FROM gin_leafpage_items(get_raw_page('test1_y_idx',
+                        (pg_relation_size('test1_y_idx') /
+                         current_setting('block_size')::bigint)::int - 1));
+-[ RECORD 1 ]
+?column? | t
+

contrib/pageinspect/ginfuncs.c

@@ -197,6 +197,9 @@ gin_leafpage_items(PG_FUNCTION_ARGS)
 		Page		page;
 		GinPageOpaque opaq;
 
+		fctx = SRF_FIRSTCALL_INIT();
+		mctx = MemoryContextSwitchTo(fctx->multi_call_memory_ctx);
+
 		page = get_page_from_raw(raw_page);
 
 		if (PageGetSpecialSize(page) != MAXALIGN(sizeof(GinPageOpaqueData)))
@@ -216,9 +219,6 @@ gin_leafpage_items(PG_FUNCTION_ARGS)
 							   opaq->flags,
 							   (GIN_DATA | GIN_LEAF | GIN_COMPRESSED))));
 
-		fctx = SRF_FIRSTCALL_INIT();
-		mctx = MemoryContextSwitchTo(fctx->multi_call_memory_ctx);
-
 		inter_call_data = palloc(sizeof(gin_leafpage_items_state));
 
 		/* Build a tuple descriptor for our result type */

contrib/pageinspect/sql/gin.sql

 CREATE TABLE test1 (x int, y int[]);
 INSERT INTO test1 VALUES (1, ARRAY[11, 111]);
-CREATE INDEX test1_y_idx ON test1 USING gin (y);
+CREATE INDEX test1_y_idx ON test1 USING gin (y) WITH (fastupdate = off);
 
 \x
 
@@ -11,4 +11,9 @@ SELECT * FROM gin_page_opaque_info(get_raw_page('test1_y_idx', 1));
 
 SELECT * FROM gin_leafpage_items(get_raw_page('test1_y_idx', 1));
 
-DROP TABLE test1;
+INSERT INTO test1 SELECT x, ARRAY[1,10] FROM generate_series(2,10000) x;
+
+SELECT COUNT(*) > 0
+FROM gin_leafpage_items(get_raw_page('test1_y_idx',
+                        (pg_relation_size('test1_y_idx') /
+                         current_setting('block_size')::bigint)::int - 1));