Fix another crash in json{b}_populate_recordset and json{b}_to_recordset.

populate_recordset_worker() failed to consider the possibility that the
supplied JSON data contains no rows, so that update_cached_tupdesc never
got called.  This led to a null-pointer dereference since commit 9a5e8ed28;
before that it led to a bogus "set-valued function called in context that
cannot accept a set" error.  Fix by forcing the update to happen.

Per bug #15514.  Back-patch to v11 as 9a5e8ed28 was.  (If we were excited
about the bogus error, we could perhaps go back further, but it'd take more
work to figure out how to fix it in older branches.  Given the lack of
field complaints about that aspect, I'm not excited.)

Discussion: https://postgr.es/m/15514-59d5b4c4065b178b@postgresql.org

src/backend/utils/adt/jsonfuncs.c

@@ -3658,6 +3658,12 @@ populate_recordset_worker(FunctionCallInfo fcinfo, const char *funcname,
 	if (PG_ARGISNULL(json_arg_num))
 		PG_RETURN_NULL();
 
+	/*
+	 * Forcibly update the cached tupdesc, to ensure we have the right tupdesc
+	 * to return even if the JSON contains no rows.
+	 */
+	update_cached_tupdesc(&cache->c.io.composite, cache->fn_mcxt);
+
 	state = palloc0(sizeof(PopulateRecordsetState));
 
 	/* make tuplestore in a sufficiently long-lived memory context */

src/test/regress/expected/json.out

@@ -1851,6 +1851,19 @@ FROM (VALUES (1),(2)) v(i);
  2 | (2,43)
 (4 rows)
 
+-- empty array is a corner case
+SELECT json_populate_recordset(null::record, '[]');
+ERROR:  record type has not been registered
+SELECT json_populate_recordset(row(1,2), '[]');
+ json_populate_recordset 
+-------------------------
+(0 rows)
+
+SELECT * FROM json_populate_recordset(NULL::jpop,'[]') q;
+ a | b | c 
+---+---+---
+(0 rows)
+
 -- composite domain
 SELECT json_populate_recordset(null::j_ordered_pair, '[{"x": 0, "y": 1}]');
  json_populate_recordset 

src/test/regress/expected/jsonb.out

@@ -2533,6 +2533,19 @@ FROM (VALUES (1),(2)) v(i);
  2 | (2,43)
 (4 rows)
 
+-- empty array is a corner case
+SELECT jsonb_populate_recordset(null::record, '[]');
+ERROR:  record type has not been registered
+SELECT jsonb_populate_recordset(row(1,2), '[]');
+ jsonb_populate_recordset 
+--------------------------
+(0 rows)
+
+SELECT * FROM jsonb_populate_recordset(NULL::jbpop,'[]') q;
+ a | b | c 
+---+---+---
+(0 rows)
+
 -- composite domain
 SELECT jsonb_populate_recordset(null::jb_ordered_pair, '[{"x": 0, "y": 1}]');
  jsonb_populate_recordset 

src/test/regress/sql/json.sql

@@ -550,6 +550,11 @@ SELECT json_populate_recordset(row(1,2), '[{"f1": 0, "f2": 1}]');
 SELECT i, json_populate_recordset(row(i,50), '[{"f1":"42"},{"f2":"43"}]')
 FROM (VALUES (1),(2)) v(i);
 
+-- empty array is a corner case
+SELECT json_populate_recordset(null::record, '[]');
+SELECT json_populate_recordset(row(1,2), '[]');
+SELECT * FROM json_populate_recordset(NULL::jpop,'[]') q;
+
 -- composite domain
 SELECT json_populate_recordset(null::j_ordered_pair, '[{"x": 0, "y": 1}]');
 SELECT json_populate_recordset(row(1,2)::j_ordered_pair, '[{"x": 0}, {"y": 3}]');

src/test/regress/sql/jsonb.sql

@@ -666,6 +666,11 @@ SELECT jsonb_populate_recordset(row(1,2), '[{"f1": 0, "f2": 1}]');
 SELECT i, jsonb_populate_recordset(row(i,50), '[{"f1":"42"},{"f2":"43"}]')
 FROM (VALUES (1),(2)) v(i);
 
+-- empty array is a corner case
+SELECT jsonb_populate_recordset(null::record, '[]');
+SELECT jsonb_populate_recordset(row(1,2), '[]');
+SELECT * FROM jsonb_populate_recordset(NULL::jbpop,'[]') q;
+
 -- composite domain
 SELECT jsonb_populate_recordset(null::jb_ordered_pair, '[{"x": 0, "y": 1}]');
 SELECT jsonb_populate_recordset(row(1,2)::jb_ordered_pair, '[{"x": 0}, {"y": 3}]');