QMP: add human-readable description to error response

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

QMP/qmp-spec.txt

@@ -102,13 +102,16 @@ completed because of an error condition.
 
 The format is:
 
-{ "error": { "class": json-string, "data": json-value }, "id": json-value }
+{ "error": { "class": json-string, "data": json-value, "desc": json-string },
+  "id": json-value }
 
  Where,
 
 - The "class" member contains the error class name (eg. "ServiceUnavailable")
 - The "data" member contains specific error data and is defined in a
   per-command basis, it will be an empty json-object if the error has no data
+- The "desc" member is a human-readable error message.  Clients should
+  not attempt to parse this message.
 - The "id" member contains the transaction identification associated with
   the command execution (if issued by the Client)
 

monitor.c

@@ -305,6 +305,7 @@ static void monitor_protocol_emitter(Monitor *mon, QObject *data)
         }
     } else {
         /* error response */
+        qdict_put(mon->error->error, "desc", qerror_human(mon->error));
         qdict_put(qmp, "error", mon->error->error);
         QINCREF(mon->error->error);
         QDECREF(mon->error);

qerror.c

@@ -283,13 +283,11 @@ static const char *append_field(QString *outstr, const QError *qerror,
 }
 
 /**
- * qerror_print(): Print QError data
+ * qerror_human(): Format QError data into human-readable string.
  *
- * This function will print the member 'desc' of the specified QError object,
- * it uses qemu_error() for this, so that the output is routed to the right
- * place (ie. stderr or Monitor's device).
+ * Formats according to member 'desc' of the specified QError object.
  */
-void qerror_print(const QError *qerror)
+QString *qerror_human(const QError *qerror)
 {
     const char *p;
     QString *qstring;
@@ -309,6 +307,19 @@ void qerror_print(const QError *qerror)
         }
     }
 
+    return qstring;
+}
+
+/**
+ * qerror_print(): Print QError data
+ *
+ * This function will print the member 'desc' of the specified QError object,
+ * it uses qemu_error() for this, so that the output is routed to the right
+ * place (ie. stderr or Monitor's device).
+ */
+void qerror_print(const QError *qerror)
+{
+    QString *qstring = qerror_human(qerror);
     qemu_error("%s\n", qstring_get_str(qstring));
     QDECREF(qstring);
 }

qerror.h

@@ -13,6 +13,7 @@
 #define QERROR_H
 
 #include "qdict.h"
+#include "qstring.h"
 #include <stdarg.h>
 
 typedef struct QErrorStringTable {
@@ -32,6 +33,7 @@ typedef struct QError {
 QError *qerror_new(void);
 QError *qerror_from_info(const char *file, int linenr, const char *func,
                          const char *fmt, va_list *va);
+QString *qerror_human(const QError *qerror);
 void qerror_print(const QError *qerror);
 QError *qobject_to_qerror(const QObject *obj);