qjson: Have qobject_from_json() & friends reject empty and blank

The last case where qobject_from_json() & friends return null without
setting an error is empty or blank input.  Callers:

* block.c's parse_json_protocol() reports "Could not parse the JSON
  options".  It's marked as a work-around, because it also covered
  actual bugs, but they got fixed in the previous few commits.

* qobject_input_visitor_new_str() reports "JSON parse error".  Also
  marked as work-around.  The recent fixes have made this unreachable,
  because it currently gets called only for input starting with '{'.

* check-qjson.c's empty_input() and blank_input() demonstrate the
  behavior.

* The other callers are not affected since they only pass input with
  exactly one JSON value or, in the case of negative tests, one error.

Fail with "Expecting a JSON value" instead of returning null, and
simplify callers.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180823164025.12553-48-armbru@redhat.com>

1 files changed, 10 insertions, 2 deletions

diff --git a/tests/check-qjson.c b/tests/check-qjson.c
index 0ca4b3c823..936258ddd4 100644
--- a/tests/check-qjson.c
+++ b/tests/check-qjson.c
@@ -1291,13 +1291,21 @@ static void simple_interpolation(void)
 
 static void empty_input(void)
 {
-    QObject *obj = qobject_from_json("", &error_abort);
+    Error *err = NULL;
+    QObject *obj;
+
+    obj = qobject_from_json("", &err);
+    error_free_or_abort(&err);
     g_assert(obj == NULL);
 }
 
 static void blank_input(void)
 {
-    QObject *obj = qobject_from_json("\n ", &error_abort);
+    Error *err = NULL;
+    QObject *obj;
+
+    obj = qobject_from_json("\n ", &err);
+    error_free_or_abort(&err);
     g_assert(obj == NULL);
 }