json: Pass lexical errors and limit violations to callback

The callback to consume JSON values takes QObject *json, Error *err.
If both are null, the callback is supposed to make up an error by
itself.  This sucks.

qjson.c's consume_json() neglects to do so, which makes
qobject_from_json() null instead of failing.  I consider that a bug.

The culprit is json_message_process_token(): it passes two null
pointers when it runs into a lexical error or a limit violation.  Fix
it to pass a proper Error object then.  Update the callbacks:

* monitor.c's handle_qmp_command(): the code to make up an error is
  now dead, drop it.

* qga/main.c's process_event(): lumps the "both null" case together
  with the "not a JSON object" case.  The former is now gone.  The
  error message "Invalid JSON syntax" is misleading for the latter.
  Improve it to "Input must be a JSON object".

* qobject/qjson.c's consume_json(): no update; check-qjson
  demonstrates qobject_from_json() now sets an error on lexical
  errors, but still doesn't on some other errors.

* tests/libqtest.c's qmp_response(): the Error object is now reliable,
  so use it to improve the error message.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180823164025.12553-40-armbru@redhat.com>

2 files changed, 13 insertions, 9 deletions

diff --git a/tests/check-qjson.c b/tests/check-qjson.c
index 604886a1a2..d6fda0786f 100644
--- a/tests/check-qjson.c
+++ b/tests/check-qjson.c
@@ -1021,6 +1021,7 @@ static void interpolation_unknown(void)
     }
     g_test_trap_subprocess(NULL, 0, 0);
     g_test_trap_assert_failed();
+    g_test_trap_assert_stderr("*Unexpected error*stray '%x'*");
 }
 
 static void interpolation_string(void)
@@ -1296,11 +1297,11 @@ static void junk_input(void)
     QObject *obj;
 
     obj = qobject_from_json("@", &err);
-    g_assert(!err);             /* BUG */
+    error_free_or_abort(&err);
     g_assert(obj == NULL);
 
     obj = qobject_from_json("{\x01", &err);
-    g_assert(!err);             /* BUG */
+    error_free_or_abort(&err);
     g_assert(obj == NULL);
 
     obj = qobject_from_json("[0\xFF]", &err);
@@ -1308,11 +1309,11 @@ static void junk_input(void)
     g_assert(obj == NULL);
 
     obj = qobject_from_json("00", &err);
-    g_assert(!err);             /* BUG */
+    error_free_or_abort(&err);
     g_assert(obj == NULL);
 
     obj = qobject_from_json("[1e", &err);
-    g_assert(!err);             /* BUG */
+    error_free_or_abort(&err);
     g_assert(obj == NULL);
 
     obj = qobject_from_json("truer", &err);
@@ -1324,7 +1325,7 @@ static void unterminated_string(void)
 {
     Error *err = NULL;
     QObject *obj = qobject_from_json("\"abc", &err);
-    g_assert(!err);             /* BUG */
+    error_free_or_abort(&err);
     g_assert(obj == NULL);
 }
 
@@ -1332,7 +1333,7 @@ static void unterminated_sq_string(void)
 {
     Error *err = NULL;
     QObject *obj = qobject_from_json("'abc", &err);
-    g_assert(!err);             /* BUG */
+    error_free_or_abort(&err);
     g_assert(obj == NULL);
 }
 
@@ -1340,7 +1341,7 @@ static void unterminated_escape(void)
 {
     Error *err = NULL;
     QObject *obj = qobject_from_json("\"abc\\\"", &err);
-    g_assert(!err);             /* BUG */
+    error_free_or_abort(&err);
     g_assert(obj == NULL);
 }
 
diff --git a/tests/libqtest.c b/tests/libqtest.c
index 1f3b0cb1b1..5973a67652 100644
--- a/tests/libqtest.c
+++ b/tests/libqtest.c
@@ -450,8 +450,11 @@ static void qmp_response(void *opaque, QObject *obj, Error *err)
 {
     QMPResponseParser *qmp = opaque;
 
-    if (!obj) {
-        fprintf(stderr, "QMP JSON response parsing failed\n");
+    assert(!obj != !err);
+
+    if (err) {
+        error_prepend(&err, "QMP JSON response parsing failed: ");
+        error_report_err(err);
         abort();
     }