Re: [PATCH] qga: return a more explicit error on why a command is disabled

[Peter Krempa]

On Tue, Feb 16, 2021 at 17:38:37 +0400, marcandre.lureau@redhat.com wrote:
> From: Marc-André Lureau <marcandre.lureau@redhat.com>
> 
> qmp_disable_command() now takes an optional error string to return a
> more explicit error message.
> 
> Fixes:
> https://bugzilla.redhat.com/show_bug.cgi?id=1928806
> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  include/qapi/qmp/dispatch.h | 3 ++-
>  qapi/qmp-dispatch.c         | 2 +-
>  qapi/qmp-registry.c         | 9 +++++----
>  qga/main.c                  | 4 ++--
>  4 files changed, 10 insertions(+), 8 deletions(-)

[...]

> diff --git a/qapi/qmp-dispatch.c b/qapi/qmp-dispatch.c
> index 0a2b20a4e4..ce4bf56b2c 100644
> --- a/qapi/qmp-dispatch.c
> +++ b/qapi/qmp-dispatch.c
> @@ -157,7 +157,7 @@ QDict *qmp_dispatch(const QmpCommandList *cmds, QObject 
> *request,
>      }
>      if (!cmd->enabled) {
>          error_set(&err, ERROR_CLASS_COMMAND_NOT_FOUND,
> -                  "The command %s has been disabled for this instance",
> +                  cmd->err_msg ?: "The command %s has been disabled for this 
> instance",

Passing in the formatting string from a variable looks shady and it's
hard to ensure that callers pass in the appropriate format modifier ...

>                    command);
>          goto out;
>      }

[...]

> diff --git a/qga/main.c b/qga/main.c
> index e7f8f3b161..c59b610691 100644
> --- a/qga/main.c
> +++ b/qga/main.c
> @@ -375,7 +375,7 @@ static void ga_disable_non_whitelisted(const QmpCommand 
> *cmd, void *opaque)
>      }
>      if (!whitelisted) {
>          g_debug("disabling command: %s", name);
> -        qmp_disable_command(&ga_commands, name);
> +        qmp_disable_command(&ga_commands, name, "The command was disabled 
> after fsfreeze.");

... such as here where '%s' is missing. Luckily that is not a problem
compared to when there would be more than one format modifier.

But the error message looks definitely better.

>      }
>  }

My suggestion would be to store a boolean flag that the command was
disabled due to an ongoing fsfreeze and then choose the appropriate
error message directly at the point where it's reported.