Monitor: Debug stray prints the right way

QObject Monitor handlers should not call any Monitor print
function: they should only build objects, printing is done
by common code.

Current QMP code will ignore such calls, as we can't send
garbage to clients, additionally it will also emit an
undefined error on the assumption that print calls usually
report errors.

However, the right way to deal with this is to rely on a
return code. This has been fixed by other commit already.

Now, this commit drops the error from monitor_vprintf() and
adds a better debugging mechanism for those 'stray' prints:
we count them if debug is enabled and let the developer know
if a QObject handler is trying to print anything.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

monitor.c

@@ -137,6 +137,9 @@ struct Monitor {
     CPUState *mon_cpu;
     BlockDriverCompletionFunc *password_completion_cb;
     void *password_opaque;
+#ifdef CONFIG_DEBUG_MONITOR
+    int print_calls_nr;
+#endif
     QError *error;
     QLIST_HEAD(,mon_fd_t) fds;
     QLIST_ENTRY(Monitor) entry;
@@ -146,8 +149,27 @@ struct Monitor {
 #define MON_DEBUG(fmt, ...) do {    \
     fprintf(stderr, "Monitor: ");       \
     fprintf(stderr, fmt, ## __VA_ARGS__); } while (0)
+
+static inline void mon_print_count_inc(Monitor *mon)
+{
+    mon->print_calls_nr++;
+}
+
+static inline void mon_print_count_init(Monitor *mon)
+{
+    mon->print_calls_nr = 0;
+}
+
+static inline int mon_print_count_get(const Monitor *mon)
+{
+    return mon->print_calls_nr;
+}
+
 #else /* !CONFIG_DEBUG_MONITOR */
 #define MON_DEBUG(fmt, ...) do { } while (0)
+static inline void mon_print_count_inc(Monitor *mon) { }
+static inline void mon_print_count_init(Monitor *mon) { }
+static inline int mon_print_count_get(const Monitor *mon) { return 0; }
 #endif /* CONFIG_DEBUG_MONITOR */
 
 static QLIST_HEAD(mon_list, Monitor) mon_list;
@@ -230,8 +252,9 @@ void monitor_vprintf(Monitor *mon, const char *fmt, va_list ap)
     if (!mon)
         return;
 
+    mon_print_count_inc(mon);
+
     if (monitor_ctrl_mode(mon)) {
-        qemu_error_new(QERR_UNDEFINED_ERROR);
         return;
     }
 
@@ -3873,6 +3896,21 @@ static void handler_audit(Monitor *mon, const mon_cmd_t *cmd, int ret)
         MON_DEBUG("command '%s' returned success but passed an error\n",
                   cmd->name);
     }
+
+    if (mon_print_count_get(mon) > 0 && strcmp(cmd->name, "info") != 0) {
+        /*
+         * Handlers should not call Monitor print functions.
+         *
+         * Action: Ignore them in QMP.
+         *
+         * (XXX: we don't check any 'info' or 'query' command here
+         * because the user print function _is_ called by do_info(), hence
+         * we will trigger this check. This problem will go away when we
+         * make 'query' commands real and kill do_info())
+         */
+        MON_DEBUG("command '%s' called print functions %d time(s)\n",
+                  cmd->name, mon_print_count_get(mon));
+    }
 #endif
 }
 
@@ -3882,6 +3920,8 @@ static void monitor_call_handler(Monitor *mon, const mon_cmd_t *cmd,
     int ret;
     QObject *data = NULL;
 
+    mon_print_count_init(mon);
+
     ret = cmd->mhandler.cmd_new(mon, params, &data);
     handler_audit(mon, cmd, ret);