Re: [Qemu-devel] [PATCH v3] hmp: Allow for error message hints on HMP

[Paolo Bonzini <pbonzini@redhat.com>]

On 19/08/2015 18:34, Eric Blake wrote:
> 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>
> ---
> 
> v3: don't crash when freeing error without hint [Eric]
> v2: use GString instead of rolling our own O(n^2) string concast [Paolo]
> 
>  include/qapi/error.h |  7 +++++++
>  qdev-monitor.c       | 42 ++++++++++++++----------------------------
>  util/error.c         | 32 ++++++++++++++++++++++++++++++++
>  util/qemu-option.c   | 11 ++++-------
>  4 files changed, 57 insertions(+), 35 deletions(-)
> 
> diff --git a/include/qapi/error.h b/include/qapi/error.h
> index f44c451..00daea3 100644
> --- a/include/qapi/error.h
> +++ b/include/qapi/error.h
> @@ -67,6 +67,13 @@ void error_set_win32(Error **errp, int win32_err, ErrorClass err_class,
>   */
>  void error_setg_file_open(Error **errp, int os_errno, const char *filename);
> 
> +/**
> + * 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);
> +
>  /*
>   * Get the error class of an error object.
>   */
> diff --git a/qdev-monitor.c b/qdev-monitor.c
> index f9e2d62..0bf7f83 100644
> --- a/qdev-monitor.c
> +++ b/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;
>          }
>      }
> diff --git a/util/error.c b/util/error.c
> index 14f4351..df5d640 100644
> --- a/util/error.c
> +++ b/util/error.c
> @@ -18,6 +18,7 @@ struct Error
>  {
>      char *msg;
>      ErrorClass err_class;
> +    GString *hint;
>  };
> 
>  Error *error_abort;
> @@ -91,6 +92,28 @@ void error_setg_file_open(Error **errp, int os_errno, const char *filename)
>      error_setg_errno(errp, os_errno, "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_set_win32(Error **errp, int win32_err, ErrorClass err_class,
> @@ -138,6 +161,9 @@ 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;
>  }
> @@ -155,6 +181,9 @@ 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);
>  }
> 
> @@ -162,6 +191,9 @@ void error_free(Error *err)
>  {
>      if (err) {
>          g_free(err->msg);
> +        if (err->hint) {
> +            g_string_free(err->hint, true);
> +        }
>          g_free(err);
>      }
>  }
> diff --git a/util/qemu-option.c b/util/qemu-option.c
> index efe9d27..03201ad 100644
> --- a/util/qemu-option.c
> +++ b/util/qemu-option.c
> @@ -200,10 +200,8 @@ 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 {
> @@ -643,9 +641,8 @@ 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);
> 

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>