hmp: Allow for error message hints on HMP

Commits 7216ae3d and d2828429 disabled some error message hints,
all because a change to use modern error reporting meant that the
hint would be output prior to the actual error.  Fix this by making
hints a first-class member of Error.

For example, we are now back to the pleasant:

 $ qemu-system-x86_64 --nodefaults -S --vnc :0 --chardev null,id=,
 qemu-system-x86_64: --chardev null,id=,: Parameter 'id' expects an identifier
 Identifiers consist of letters, digits, '-', '.', '_', starting with a letter.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1441901956-21991-1-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

include/qapi/error.h

@@ -155,6 +155,13 @@ void error_setg_win32_internal(Error **errp,
  */
 void error_propagate(Error **dst_errp, Error *local_err);
 
+/**
+ * Append a printf-style human-readable explanation to an existing error.
+ * May be called multiple times, and safe if @errp is NULL.
+ */
+void error_append_hint(Error **errp, const char *fmt, ...)
+    GCC_FMT_ATTR(2, 3);
+
 /*
  * Convenience function to report open() failure.
  */

qdev-monitor.c

@@ -289,37 +289,35 @@ static Object *qdev_get_peripheral_anon(void)
     return dev;
 }
 
-#if 0 /* conversion from qerror_report() to error_set() broke their use */
-static void qbus_list_bus(DeviceState *dev)
+static void qbus_list_bus(DeviceState *dev, Error **errp)
 {
     BusState *child;
     const char *sep = " ";
 
-    error_printf("child buses at \"%s\":",
-                 dev->id ? dev->id : object_get_typename(OBJECT(dev)));
+    error_append_hint(errp, "child buses at \"%s\":",
+                      dev->id ? dev->id : object_get_typename(OBJECT(dev)));
     QLIST_FOREACH(child, &dev->child_bus, sibling) {
-        error_printf("%s\"%s\"", sep, child->name);
+        error_append_hint(errp, "%s\"%s\"", sep, child->name);
         sep = ", ";
     }
-    error_printf("\n");
 }
 
-static void qbus_list_dev(BusState *bus)
+static void qbus_list_dev(BusState *bus, Error **errp)
 {
     BusChild *kid;
     const char *sep = " ";
 
-    error_printf("devices at \"%s\":", bus->name);
+    error_append_hint(errp, "devices at \"%s\":", bus->name);
     QTAILQ_FOREACH(kid, &bus->children, sibling) {
         DeviceState *dev = kid->child;
-        error_printf("%s\"%s\"", sep, object_get_typename(OBJECT(dev)));
-        if (dev->id)
-            error_printf("/\"%s\"", dev->id);
+        error_append_hint(errp, "%s\"%s\"", sep,
+                          object_get_typename(OBJECT(dev)));
+        if (dev->id) {
+            error_append_hint(errp, "/\"%s\"", dev->id);
+        }
         sep = ", ";
     }
-    error_printf("\n");
 }
-#endif
 
 static BusState *qbus_find_bus(DeviceState *dev, char *elem)
 {
@@ -461,11 +459,7 @@ static BusState *qbus_find(const char *path, Error **errp)
         if (!dev) {
             error_set(errp, ERROR_CLASS_DEVICE_NOT_FOUND,
                       "Device '%s' not found", elem);
-#if 0 /* conversion from qerror_report() to error_set() broke this: */
-            if (!monitor_cur_is_qmp()) {
-                qbus_list_dev(bus);
-            }
-#endif
+            qbus_list_dev(bus, errp);
             return NULL;
         }
 
@@ -483,11 +477,7 @@ static BusState *qbus_find(const char *path, Error **errp)
             if (dev->num_child_bus) {
                 error_setg(errp, "Device '%s' has multiple child buses",
                            elem);
-#if 0 /* conversion from qerror_report() to error_set() broke this: */
-                if (!monitor_cur_is_qmp()) {
-                    qbus_list_bus(dev);
-                }
-#endif
+                qbus_list_bus(dev, errp);
             } else {
                 error_setg(errp, "Device '%s' has no child bus", elem);
             }
@@ -503,11 +493,7 @@ static BusState *qbus_find(const char *path, Error **errp)
         bus = qbus_find_bus(dev, elem);
         if (!bus) {
             error_setg(errp, "Bus '%s' not found", elem);
-#if 0 /* conversion from qerror_report() to error_set() broke this: */
-            if (!monitor_cur_is_qmp()) {
-                qbus_list_bus(dev);
-            }
-#endif
+            qbus_list_bus(dev, errp);
             return NULL;
         }
     }

util/error.c

@@ -20,6 +20,7 @@ struct Error
     ErrorClass err_class;
     const char *src, *func;
     int line;
+    GString *hint;
 };
 
 Error *error_abort;
@@ -115,6 +116,28 @@ void error_setg_file_open_internal(Error **errp,
                               "Could not open '%s'", filename);
 }
 
+void error_append_hint(Error **errp, const char *fmt, ...)
+{
+    va_list ap;
+    int saved_errno = errno;
+    Error *err;
+
+    if (!errp) {
+        return;
+    }
+    err = *errp;
+    assert(err && errp != &error_abort);
+
+    if (!err->hint) {
+        err->hint = g_string_new(NULL);
+    }
+    va_start(ap, fmt);
+    g_string_append_vprintf(err->hint, fmt, ap);
+    va_end(ap);
+
+    errno = saved_errno;
+}
+
 #ifdef _WIN32
 
 void error_setg_win32_internal(Error **errp,
@@ -151,6 +174,9 @@ Error *error_copy(const Error *err)
     err_new = g_malloc0(sizeof(*err));
     err_new->msg = g_strdup(err->msg);
     err_new->err_class = err->err_class;
+    if (err->hint) {
+        err_new->hint = g_string_new(err->hint->str);
+    }
 
     return err_new;
 }
@@ -168,6 +194,9 @@ const char *error_get_pretty(Error *err)
 void error_report_err(Error *err)
 {
     error_report("%s", error_get_pretty(err));
+    if (err->hint) {
+        error_printf_unless_qmp("%s\n", err->hint->str);
+    }
     error_free(err);
 }
 
@@ -175,6 +204,9 @@ void error_free(Error *err)
 {
     if (err) {
         g_free(err->msg);
+        if (err->hint) {
+            g_string_free(err->hint, true);
+        }
         g_free(err);
     }
 }

util/qemu-option.c

@@ -205,10 +205,8 @@ void parse_option_size(const char *name, const char *value,
             break;
         default:
             error_setg(errp, QERR_INVALID_PARAMETER_VALUE, name, "a size");
-#if 0 /* conversion from qerror_report() to error_set() broke this: */
-            error_printf_unless_qmp("You may use k, M, G or T suffixes for "
-                    "kilobytes, megabytes, gigabytes and terabytes.\n");
-#endif
+            error_append_hint(errp, "You may use k, M, G or T suffixes for "
+                    "kilobytes, megabytes, gigabytes and terabytes.");
             return;
         }
     } else {
@@ -648,9 +646,8 @@ QemuOpts *qemu_opts_create(QemuOptsList *list, const char *id,
         if (!id_wellformed(id)) {
             error_setg(errp, QERR_INVALID_PARAMETER_VALUE, "id",
                        "an identifier");
-#if 0 /* conversion from qerror_report() to error_set() broke this: */
-            error_printf_unless_qmp("Identifiers consist of letters, digits, '-', '.', '_', starting with a letter.\n");
-#endif
+            error_append_hint(errp, "Identifiers consist of letters, digits, "
+                              "'-', '.', '_', starting with a letter.");
             return NULL;
         }
         opts = qemu_opts_find(list, id);