Re: [PATCH 3/4] char: Flat alternative to overly nested chardev-add arguments

From: Eric Blake <eblake@redhat.com>
To: Markus Armbruster <armbru@redhat.com>, qemu-devel@nongnu.org
Cc: kwolf@redhat.com, marcandre.lureau@redhat.com,
	qemu-block@nongnu.org, pbonzini@redhat.com
Subject: Re: [PATCH 3/4] char: Flat alternative to overly nested chardev-add arguments
Date: Tue, 27 Oct 2020 13:23:13 -0500	[thread overview]
Message-ID: <df6a32e6-3b4c-d9d7-b473-9bdd7c674843@redhat.com> (raw)
In-Reply-To: <20201026101005.2940615-4-armbru@redhat.com>

On 10/26/20 5:10 AM, Markus Armbruster wrote:
> chardev-add's arguments use an annoying amount of nesting.  Example:
> 
>     {"execute": "chardev-add",
>      "arguments": {
>          "id":"sock0",
> 	 "backend": {
> 	     "type": "socket",
> 	     "data": {
> 	         "addr": {
> 		     "type": "inet",
> 		     "data": {
> 		         "host": "0.0.0.0",
> 			 "port": "2445"}}}}}}
> 
> This is because chardev-add predates QAPI features that enable flatter
> data structures, both on the wire and in C: base types, flat unions,
> commands taking a union or alternate as 'data'.
> 
> The nesting would be even more annoying in dotted key syntax:
> 
>     id=sock0,\
>     backend.type=socket,\
>     backend.data.addr.type=inet,\
>     backend.data.addr.data.host=0.0.0.0,\
>     backend.data.addr.data.port=2445
> 
> Relevant, because the next commit will QAPIfy qemu-storage-daemon
> --chardev.  We really want this instead:
> 
>     --chardev socket,id=sock0,\
>     addr.type=inet,\
>     addr.host=0.0.0.0,\
>     addr.port=2445
> 
> To get it, define a new QAPI type ChardevOptions that is the flat
> equivalent to chardev-add's arguments.
> 
> What we should do now is convert the internal interfaces to take this
> new type, and limit the nested old type to the external interface,
> similar to what commit bd269ebc82 "sockets: Limit SocketAddressLegacy
> to external interfaces" did.  But we're too close to the freeze to
> pull that off safely.
> 
> What I can do now is convert the new type to the old nested type, and
> promise to replace this by what should be done in the next development
> cycle.

Nice evaluation of the trade-off.

> 
> In more detail:
> 
> * Flat union ChardevOptions corresponds to chardev-add's implicit
>   arguments type.  It flattens a struct containing a simple union into
>   a flat union.
> 
> * The flat union's discriminator is named @backend, not @type.  This
>   avoids clashing with member @type of ChardevSpiceChannel.  For what
>   it's worth, -chardev also uses this name.
> 
> * Its branches @socket, @udp use ChardevSocketFlat, ChardevUdpFlat
>   instead of ChardevSocket, ChardevUdp.  This flattens simple union
>   SocketAddressLegacy members to flat union SocketAddress members.
> 
> * New chardev_options_crumple() converts ChardevOptions to
>   chardev-add's implict arguments type.

implicit

> 
> Only one existing QAPI definition is affected: some of ChardevSocket's
> members get moved to a new base type ChardevSocketBase, to reduce
> duplication.  No change to the generated C type and the wire format.
> 
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---
>  qapi/char.json         | 106 ++++++++++++++++++++++++++++---
>  include/chardev/char.h |   5 ++
>  include/qemu/sockets.h |   3 +
>  chardev/char-legacy.c  | 140 +++++++++++++++++++++++++++++++++++++++++
>  chardev/char-socket.c  |   3 +-
>  util/qemu-sockets.c    |  38 +++++++++++
>  chardev/meson.build    |   1 +
>  7 files changed, 287 insertions(+), 9 deletions(-)
>  create mode 100644 chardev/char-legacy.c

Big but worth it.  I'm liking the simplicity of this alternative over
Kevin's proposal, especially if we're aiming to get this in 5.2 soft freeze.

> 
> diff --git a/qapi/char.json b/qapi/char.json
> index 43486d1daa..31b693bbb2 100644
> --- a/qapi/char.json
> +++ b/qapi/char.json
> @@ -244,12 +244,8 @@
>    'base': 'ChardevCommon' }
>  
>  ##
> -# @ChardevSocket:
> +# @ChardevSocketBase:
>  #
> -# Configuration info for (stream) socket chardevs.
> -#
> -# @addr: socket address to listen on (server=true)
> -#        or connect to (server=false)
>  # @tls-creds: the ID of the TLS credentials object (since 2.6)
>  # @tls-authz: the ID of the QAuthZ authorization object against which
>  #             the client's x509 distinguished name will be validated. This
> @@ -274,9 +270,8 @@
>  #
>  # Since: 1.4
>  ##
> -{ 'struct': 'ChardevSocket',
> -  'data': { 'addr': 'SocketAddressLegacy',
> -            '*tls-creds': 'str',
> +{ 'struct': 'ChardevSocketBase',
> +  'data': { '*tls-creds': 'str',
>              '*tls-authz'  : 'str',
>              '*server': 'bool',
>              '*wait': 'bool',
> @@ -287,6 +282,35 @@
>              '*reconnect': 'int' },
>    'base': 'ChardevCommon' }

Here we are subdividing ChardevSocket into everything that is already
flat, and excluding the awkward 'addr'...

>  
> +##
> +# @ChardevSocket:
> +#
> +# Configuration info for (stream) socket chardevs.
> +#
> +# @addr: socket address to listen on (server=true)
> +#        or connect to (server=false)
> +#
> +# Since: 1.4
> +##
> +{ 'struct': 'ChardevSocket',
> +  # Do not add to 'data', it breaks chardev_options_crumple()!  Add to
> +  # ChardevSocketBase's 'data' instead.
> +  'data': { 'addr': 'SocketAddressLegacy' },
> +  'base': 'ChardevSocketBase' }

...legacy use pulls in the legacy 'addr'...

> +
> +##
> +# @ChardevSocketFlat:
> +#
> +# Note: This type should eventually replace ChardevSocket.  The
> +#       difference between the two: ChardevSocketFlat uses
> +#       SocketAddressLegacy, ChardevSocket uses SocketAddress.
> +##

Missing a 'Since: 5.2' tag, if you want one.

> +{ 'struct': 'ChardevSocketFlat',
> +  # Do not add to 'data', it breaks chardev_options_crumple()!  Add to
> +  # ChardevSocketBase's 'data' instead.
> +  'data': { 'addr': 'SocketAddress' },
> +  'base': 'ChardevSocketBase' }
> +

...and this is the new type with a saner 'addr'.  Works for me so far.

>  ##
>  # @ChardevUdp:
>  #
> @@ -298,10 +322,26 @@
>  # Since: 1.5
>  ##
>  { 'struct': 'ChardevUdp',
> +  # Do not add to 'data', it breaks chardev_options_crumple()!  Create
> +  # ChardevUdpBase instead, similar to ChardevSocketBase.
>    'data': { 'remote': 'SocketAddressLegacy',
>              '*local': 'SocketAddressLegacy' },
>    'base': 'ChardevCommon' }
>  
> +##
> +# @ChardevUdpFlat:
> +#
> +# Note: This type should eventually replace ChardevUdp.  The
> +#       difference between the two: ChardevUdpFlat uses
> +#       SocketAddressLegacy, ChardevUdp uses SocketAddress.
> +##

Another missing 'Since: 5.2'

> +{ 'struct': 'ChardevUdpFlat',
> +  # Do not add to 'data', it breaks chardev_options_crumple()!  Create
> +  # ChardevUdpBase instead, similar to ChardevSocketBase.
> +  'data': { 'remote': 'SocketAddress',
> +            '*local': 'SocketAddress' },
> +  'base': 'ChardevCommon' }
> +
>  ##
>  # @ChardevMux:
>  #
> @@ -422,6 +462,56 @@
>              # next one is just for compatibility
>              'memory': 'ChardevRingbuf' } }
>  
> +##
> +# @ChardevBackendType:
> +#
> +# Since: 5.2
> +##
> +{ 'enum': 'ChardevBackendType',
> +
> +  'data': [ 'file', 'serial', 'parallel', 'pipe', 'socket', 'udp',
> +            'pty', 'null', 'mux', 'msmouse', 'wctablet', 'braille',
> +            'testdev', 'stdio', 'console', 'spicevmc', 'spiceport',
> +            'vc', 'ringbuf' ] }
> +
> +##
> +# @ChardevOptions:
> +#
> +# Note: This type should eventually replace the implicit arguments
> +#       type of chardev-add and chardev-chardev.  The differences
> +#       between the two: 1. ChardevSocketOptions is a flat union
> +#       rather than a struct with a simple union member, and 2. it
> +#       uses SocketAddress instead of SocketAddressLegacy.  This
> +#       avoids nesting on the wire, i.e. we need fewer {}.
> +#
> +# Since: 5.2
> +##
> +{ 'union': 'ChardevOptions',
> +  'base': { 'backend': 'ChardevBackendType',
> +            'id': 'str' },
> +  'discriminator': 'backend',
> +  'data': { 'file': 'ChardevFile',
> +            'serial': 'ChardevHostdev',
> +            'parallel': 'ChardevHostdev',
> +            'pipe': 'ChardevHostdev',
> +            'socket': 'ChardevSocketFlat',
> +            'udp': 'ChardevUdpFlat',
> +            'pty': 'ChardevCommon',
> +            'null': 'ChardevCommon',
> +            'mux': 'ChardevMux',
> +            'msmouse': 'ChardevCommon',
> +            'wctablet': 'ChardevCommon',
> +            'braille': 'ChardevCommon',
> +            'testdev': 'ChardevCommon',
> +            'stdio': 'ChardevStdio',
> +            'console': 'ChardevCommon',
> +            'spicevmc': { 'type': 'ChardevSpiceChannel',
> +                          'if': 'defined(CONFIG_SPICE)' },
> +            'spiceport': { 'type': 'ChardevSpicePort',
> +                           'if': 'defined(CONFIG_SPICE)' },
> +            'vc': 'ChardevVC',
> +            'ringbuf': 'ChardevRingbuf' } }

Looks good from the QAPI point of view.

> +/*
> + * TODO Convert internal interfaces to ChardevOptions, replace this
> + * function by one that flattens (const char *str, ChardevBackend
> + * *backend) -> ChardevOptions.
> + */
> +q_obj_chardev_add_arg *chardev_options_crumple(ChardevOptions *chr)
> +{
> +    q_obj_chardev_add_arg *arg;
> +    ChardevBackend *be;
> +
> +    if (!chr) {
> +        return NULL;
> +    }
> +
> +    arg = g_malloc(sizeof(*arg));
> +    arg->id = g_strdup(chr->id);
> +    arg->backend = be = g_malloc(sizeof(*be));
> +
> +    switch (chr->backend) {
> +    case CHARDEV_BACKEND_TYPE_FILE:
> +        be->type = CHARDEV_BACKEND_KIND_FILE;
> +        be->u.file.data = QAPI_CLONE(ChardevFile, &chr->u.file);
> +        break;

Most branches are straightforward,...

> +    case CHARDEV_BACKEND_TYPE_SOCKET:
> +        be->type = CHARDEV_BACKEND_KIND_SOCKET;
> +        /*
> +         * Clone with SocketAddress crumpled to SocketAddressLegacy.
> +         * All other members are in the base type.
> +         */
> +        be->u.socket.data = g_memdup(&chr->u.socket, sizeof(chr->u.socket));
> +        QAPI_CLONE_MEMBERS(ChardevSocketBase,
> +                           qapi_ChardevSocket_base(be->u.socket.data),
> +                           qapi_ChardevSocketFlat_base(&chr->u.socket));
> +        be->u.socket.data->addr = socket_address_crumple(chr->u.socket.addr);
> +        break;

...and this looks correct as well.

> +++ b/chardev/char-socket.c
> @@ -1404,7 +1404,8 @@ static void qemu_chr_parse_socket(QemuOpts *opts, ChardevBackend *backend,
>  
>      backend->type = CHARDEV_BACKEND_KIND_SOCKET;
>      sock = backend->u.socket.data = g_new0(ChardevSocket, 1);
> -    qemu_chr_parse_common(opts, qapi_ChardevSocket_base(sock));
> +    qemu_chr_parse_common(opts,
> +            qapi_ChardevSocketBase_base(qapi_ChardevSocket_base(sock)));

The double function call (for a double cast) looks unusual, but I don't
see any shorter expression, so it is fine.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org