Re: [Qemu-devel] [PATCH 1/3] qmp/hmp: Add QMP getfd command that returns fd

[Luiz Capitulino <lcapitulino@redhat.com>]

On Mon,  4 Jun 2012 09:10:08 -0400
Corey Bryant <coreyb@linux.vnet.ibm.com> wrote:

> This patch adds QMP support for the getfd command using the QAPI framework.
> Like the HMP getfd command, it is used to pass a file descriptor via
> SCM_RIGHTS.  However, the QMP getfd command also returns the received file
> descriptor, which is a difference in behavior from the HMP getfd command,
> which returns nothing.

I have a few comments regarding the qapi conversion below, but something
important to discuss is that returning an int the way you're doing it is
certainly incompatible.

Today, we return a dict on success:

 { "return": {} }

But this patch changes it to:

 { "return": 42 }

There are two ways to do this without breaking compatibility:

 1. Add a new command (say get-file-descriptor)

 2. Return a type instead, like:

     { "return": { "file-descriptor": 42 } }

I think I prefer item 1, as we could also take the opportunity to fix the
argument type and improve its name. Besides, we don't have a schema to do 2.

Also, please split the getfd conversion to the qapi from changing its
return type in two patches (even if we go for a new command, please do
convert getfd too, for completeness); and also convert closefd, please.

More comments below.

> Signed-off-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
> ---
>  hmp-commands.hx  |    2 +-
>  monitor.c        |   37 ++++++++++++++++++++++++++++++++++++-
>  qapi-schema.json |   13 +++++++++++++
>  qmp-commands.hx  |    6 ++++--
>  4 files changed, 54 insertions(+), 4 deletions(-)
> 
> diff --git a/hmp-commands.hx b/hmp-commands.hx
> index 18cb415..dfab369 100644
> --- a/hmp-commands.hx
> +++ b/hmp-commands.hx
> @@ -1211,7 +1211,7 @@ ETEXI
>          .params     = "getfd name",
>          .help       = "receive a file descriptor via SCM rights and assign it a name",
>          .user_print = monitor_user_noop,

The .user_print line should be dropped.

> -        .mhandler.cmd_new = do_getfd,
> +        .mhandler.cmd_new = hmp_getfd,

You should use 'cmd' (instead of 'cmd_new'). Please, look for other qapi
conversions for examples (like a15fef21c).

>      },
>  
>  STEXI
> diff --git a/monitor.c b/monitor.c
> index 12a6fe2..6acf5a3 100644
> --- a/monitor.c
> +++ b/monitor.c
> @@ -2199,7 +2199,7 @@ static void do_inject_mce(Monitor *mon, const QDict *qdict)
>  }
>  #endif
>  
> -static int do_getfd(Monitor *mon, const QDict *qdict, QObject **ret_data)
> +static int hmp_getfd(Monitor *mon, const QDict *qdict, QObject **ret_data)

This should be moved to hmp.c and the function signature should be different,
looking at other qapi conversions will help you.

>  {
>      const char *fdname = qdict_get_str(qdict, "fdname");
>      mon_fd_t *monfd;
> @@ -2235,6 +2235,41 @@ static int do_getfd(Monitor *mon, const QDict *qdict, QObject **ret_data)
>      return 0;
>  }
>  
> +int64_t qmp_getfd(const char *fdname, Error **errp)
> +{
> +    mon_fd_t *monfd;
> +    int fd;
> +
> +    fd = qemu_chr_fe_get_msgfd(cur_mon->chr);
> +    if (fd == -1) {
> +        error_set(errp, QERR_FD_NOT_SUPPLIED);
> +        return -1;
> +    }
> +
> +    if (qemu_isdigit(fdname[0])) {
> +        error_set(errp, QERR_INVALID_PARAMETER_VALUE, "fdname",
> +                  "a name not starting with a digit");
> +        return -1;
> +    }
> +
> +    QLIST_FOREACH(monfd, &cur_mon->fds, next) {
> +        if (strcmp(monfd->name, fdname) != 0) {
> +            continue;
> +        }
> +
> +        close(monfd->fd);
> +        monfd->fd = fd;
> +        return fd;
> +    }
> +
> +    monfd = g_malloc0(sizeof(mon_fd_t));
> +    monfd->name = g_strdup(fdname);
> +    monfd->fd = fd;
> +
> +    QLIST_INSERT_HEAD(&cur_mon->fds, monfd, next);
> +    return fd;
> +}
> +
>  static int do_closefd(Monitor *mon, const QDict *qdict, QObject **ret_data)
>  {
>      const char *fdname = qdict_get_str(qdict, "fdname");
> diff --git a/qapi-schema.json b/qapi-schema.json
> index 2ca7195..5f26ba2 100644
> --- a/qapi-schema.json
> +++ b/qapi-schema.json
> @@ -1755,3 +1755,16 @@
>  # Since: 0.14.0
>  ##
>  { 'command': 'device_del', 'data': {'id': 'str'} }
> +
> +##
> +# @getfd:
> +#
> +# Receive a file descriptor via SCM rights and assign it a name
> +#
> +# @fdname: file descriptor name
> +#
> +# Returns: The QEMU @fd that was received
> +#
> +# Since: 1.2

This command exists since 0.14 if I'm not mistaken, only the return type
is new.

> +##
> +{ 'command': 'getfd', 'data': {'fdname': 'str'}, 'returns': 'int' }
> diff --git a/qmp-commands.hx b/qmp-commands.hx
> index db980fa..e13d583 100644
> --- a/qmp-commands.hx
> +++ b/qmp-commands.hx
> @@ -844,7 +844,7 @@ EQMP
>          .params     = "getfd name",
>          .help       = "receive a file descriptor via SCM rights and assign it a name",
>          .user_print = monitor_user_noop,
> -        .mhandler.cmd_new = do_getfd,
> +        .mhandler.cmd_new = qmp_marshal_input_getfd,
>      },
>  
>  SQMP
> @@ -857,10 +857,12 @@ Arguments:
>  
>  - "fdname": file descriptor name (json-string)
>  
> +Return a json-int with the QEMU file descriptor that was received.
> +
>  Example:
>  
>  -> { "execute": "getfd", "arguments": { "fdname": "fd1" } }
> -<- { "return": {} }
> +<- { "return": 42 }
>  
>  EQMP
>