json: Support %% in JSON strings when interpolating

The previous commit makes JSON strings containing '%' awkward to
express in templates: you'd have to mask the '%' with an Unicode
escape \u0025.  No template currently contains such JSON strings.
Support the printf conversion specification %% in JSON strings as a
convenience anyway, because it's trivially easy to do.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180823164025.12553-58-armbru@redhat.com>

qobject/json-parser.c

@@ -208,10 +208,11 @@ static QString *parse_string(JSONParserContext *ctxt, JSONToken *token)
             }
             break;
         case '%':
-            if (ctxt->ap) {
+            if (ctxt->ap && ptr[1] != '%') {
                 parse_error(ctxt, token, "can't interpolate into string");
                 goto out;
             }
+            ptr++;
             /* fall through */
         default:
             cp = mod_utf8_codepoint(ptr, 6, &end);

tests/check-qjson.c

@@ -1270,7 +1270,7 @@ static void simple_interpolation(void)
     QObject *obj;
     QLitObject decoded = QLIT_QLIST(((QLitObject[]){
             QLIT_QNUM(1),
-            QLIT_QNUM(2),
+            QLIT_QSTR("100%"),
             QLIT_QLIST(((QLitObject[]){
                         QLIT_QNUM(32),
                         QLIT_QNUM(42),
@@ -1280,7 +1280,7 @@ static void simple_interpolation(void)
     embedded_obj = qobject_from_json("[32, 42]", &error_abort);
     g_assert(embedded_obj != NULL);
 
-    obj = qobject_from_jsonf_nofail("[%d, 2, %p]", 1, embedded_obj);
+    obj = qobject_from_jsonf_nofail("[%d, '100%%', %p]", 1, embedded_obj);
     g_assert(qlit_equal_qobject(&decoded, obj));
 
     qobject_unref(obj);