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>

block.c

@@ -1478,11 +1478,6 @@ static QDict *parse_json_filename(const char *filename, Error **errp)
 
     options_obj = qobject_from_json(filename, errp);
     if (!options_obj) {
-        /* Work around qobject_from_json() lossage TODO fix that */
-        if (errp && !*errp) {
-            error_setg(errp, "Could not parse the JSON options");
-            return NULL;
-        }
         error_prepend(errp, "Could not parse the JSON options: ");
         return NULL;
     }

qapi/qobject-input-visitor.c

@@ -725,11 +725,6 @@ Visitor *qobject_input_visitor_new_str(const char *str,
     if (is_json) {
         obj = qobject_from_json(str, errp);
         if (!obj) {
-            /* Work around qobject_from_json() lossage TODO fix that */
-            if (errp && !*errp) {
-                error_setg(errp, "JSON parse error");
-                return NULL;
-            }
             return NULL;
         }
         args = qobject_to(QDict, obj);

qobject/qjson.c

@@ -70,6 +70,10 @@ static QObject *qobject_from_jsonv(const char *string, va_list *ap,
     json_message_parser_flush(&state.parser);
     json_message_parser_destroy(&state.parser);
 
+    if (!state.result && !state.err) {
+        error_setg(&state.err, "Expecting a JSON value");
+    }
+
     error_propagate(errp, state.err);
     return state.result;
 }

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);
 }