Fix test_decoding's handling of nonexistant columns in old tuple versions.

test_decoding used fastgetattr() to extract column values. That's wrong
when decoding updates and deletes if a table's replica identity is set
to FULL and new columns have been added since the old version of the
tuple was created. Due to the lack of a crosscheck with the datum's
natts values an invalid value will be output, leading to errors or
worse.

Bug: #13470
Reported-By: Krzysztof Kotlarski
Discussion: 20150626100333.3874.90852@wrigleys.postgresql.org

Backpatch to 9.4, where the feature, including the bug, was added.

contrib/test_decoding/sql/ddl.sql

@@ -277,6 +277,10 @@ ALTER TABLE table_without_key REPLICA IDENTITY FULL;
 UPDATE table_without_key SET data = 3 WHERE data = 2;
 UPDATE table_without_key SET id = -id;
 UPDATE table_without_key SET id = -id;
+-- ensure that FULL correctly deals with new columns
+ALTER TABLE table_without_key ADD COLUMN new_column text;
+UPDATE table_without_key SET id = -id;
+UPDATE table_without_key SET id = -id, new_column = 'someval';
 DELETE FROM table_without_key WHERE data = 3;
 
 CREATE TABLE table_with_pkey(id serial primary key, data int);

contrib/test_decoding/test_decoding.c

@@ -345,7 +345,7 @@ tuple_to_stringinfo(StringInfo s, TupleDesc tupdesc, HeapTuple tuple, bool skip_
 		typid = attr->atttypid;
 
 		/* get Datum from tuple */
-		origval = fastgetattr(tuple, natt + 1, tupdesc, &isnull);
+		origval = heap_getattr(tuple, natt + 1, tupdesc, &isnull);
 
 		if (isnull && skip_nulls)
 			continue;