Fix missing varlen header in DimensionInfo

The SQL data type _timescaledb_internal.dimension_info is declared with
INTERNALLENGTH = VARIABLE. This requires a varlen header, which was
missing. Therefore, the needed space of the function result was not
handled properly. This commit introduces the missing header.

src/dimension.c

@@ -1748,7 +1748,10 @@ ts_dimension_info_out(PG_FUNCTION_ARGS)
 static DimensionInfo *
 make_dimension_info(Name colname, DimensionType dimtype)
 {
-	DimensionInfo *info = palloc0(sizeof(DimensionInfo));
+	size_t size = sizeof(DimensionInfo);
+	DimensionInfo *info = palloc0(size);
+	SET_VARSIZE(info, size);
+
 	info->type = dimtype;
 	namestrcpy(&info->colname, NameStr(*colname));
 	return info;

src/dimension.h

@@ -93,6 +93,11 @@ typedef struct Hypertable Hypertable;
  */
 typedef struct DimensionInfo
 {
+	/* We declare the SQL type dimension_info with INTERNALLENGTH = VARIABLE.
+	 * So, PostgreSQL expects a proper length info field (varlena header).
+	 */
+	int32 vl_len_;
+
 	Oid table_relid;
 	int32 dimension_id;
 	NameData colname;

tsl/test/expected/hypertable_generalization.out

@@ -55,6 +55,31 @@ SELECT by_hash('id', 3, partition_func => 'part_func');
  hash//id//3//part_func
 (1 row)
 
+-- Check if we handle the varlength of the datatype properly
+SELECT * FROM by_range('id') WHERE by_range IS NOT NULL;
+    by_range     
+-----------------
+ range//id//-//-
+(1 row)
+
+SELECT * FROM by_range('id', partition_func => 'part_func') WHERE by_range IS NOT NULL;
+        by_range         
+-------------------------
+ range//id//-//part_func
+(1 row)
+
+SELECT * FROM by_hash('id', 3) WHERE by_hash IS NOT NULL;
+    by_hash     
+----------------
+ hash//id//3//-
+(1 row)
+
+SELECT * FROM by_hash('id', 3, partition_func => 'part_func') WHERE by_hash IS NOT NULL;
+        by_hash         
+------------------------
+ hash//id//3//part_func
+(1 row)
+
 \set ON_ERROR_STOP 0
 SELECT 'hash//id//3//-'::_timescaledb_internal.dimension_info;
 ERROR:  cannot construct type "dimension_info" from string at character 8

tsl/test/sql/hypertable_generalization.sql

@@ -27,6 +27,12 @@ SELECT by_range('id', '1 week'::interval, 'part_func'::regproc);
 SELECT by_hash('id', 3);
 SELECT by_hash('id', 3, partition_func => 'part_func');
 
+-- Check if we handle the varlength of the datatype properly
+SELECT * FROM by_range('id') WHERE by_range IS NOT NULL;
+SELECT * FROM by_range('id', partition_func => 'part_func') WHERE by_range IS NOT NULL;
+SELECT * FROM by_hash('id', 3) WHERE by_hash IS NOT NULL;
+SELECT * FROM by_hash('id', 3, partition_func => 'part_func') WHERE by_hash IS NOT NULL;
+
 \set ON_ERROR_STOP 0
 SELECT 'hash//id//3//-'::_timescaledb_internal.dimension_info;
 SELECT by_range(NULL::name);