[v1] chardev: introduce 'reconnect-ms' and deprecate 'reconnect'

The 'reconnect' option only allows to specify the time in seconds,
which is way too long for certain workflows.

We have a lightweight disk backend server, which takes about 20ms to
live update, but due to this limitation in QEMU, previously the guest
disk controller would hang for one second because it would take this
long for QEMU to reinitialize the socket connection.

Introduce a new option called 'reconnect-ms', which is the same as
'reconnect', except the value is treated as milliseconds. These are
mutually exclusive and specifying both results in an error.

'reconnect' is also deprecated by this commit to make it possible to
remove it in the future as to not keep two options that control the
same thing.

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Acked-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
---

Changes since v0:
- Mention the deprecation in docs (Paolo)

---
 chardev/char-socket.c         | 33 ++++++++++++++++++++++++---------
 chardev/char.c                |  3 +++
 docs/about/deprecated.rst     |  6 ++++++
 include/chardev/char-socket.h |  2 +-
 qapi/char.json                | 17 +++++++++++++++--
 5 files changed, 49 insertions(+), 12 deletions(-)

Ping :) I think this one should be good to go now!

On 9/5/24 11:57 AM, Daniil Tatianin wrote:
> The 'reconnect' option only allows to specify the time in seconds,
> which is way too long for certain workflows.
>
> We have a lightweight disk backend server, which takes about 20ms to
> live update, but due to this limitation in QEMU, previously the guest
> disk controller would hang for one second because it would take this
> long for QEMU to reinitialize the socket connection.
>
> Introduce a new option called 'reconnect-ms', which is the same as
> 'reconnect', except the value is treated as milliseconds. These are
> mutually exclusive and specifying both results in an error.
>
> 'reconnect' is also deprecated by this commit to make it possible to
> remove it in the future as to not keep two options that control the
> same thing.
>
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> Acked-by: Peter Krempa <pkrempa@redhat.com>
> Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
> ---
>
> Changes since v0:
> - Mention the deprecation in docs (Paolo)
>
> ---
>   chardev/char-socket.c         | 33 ++++++++++++++++++++++++---------
>   chardev/char.c                |  3 +++
>   docs/about/deprecated.rst     |  6 ++++++
>   include/chardev/char-socket.h |  2 +-
>   qapi/char.json                | 17 +++++++++++++++--
>   5 files changed, 49 insertions(+), 12 deletions(-)
>
> diff --git a/chardev/char-socket.c b/chardev/char-socket.c
> index 1ca9441b1b..c24331ac23 100644
> --- a/chardev/char-socket.c
> +++ b/chardev/char-socket.c
> @@ -74,7 +74,7 @@ static void qemu_chr_socket_restart_timer(Chardev *chr)
>       assert(!s->reconnect_timer);
>       name = g_strdup_printf("chardev-socket-reconnect-%s", chr->label);
>       s->reconnect_timer = qemu_chr_timeout_add_ms(chr,
> -                                                 s->reconnect_time * 1000,
> +                                                 s->reconnect_time_ms,
>                                                    socket_reconnect_timeout,
>                                                    chr);
>       g_source_set_name(s->reconnect_timer, name);
> @@ -481,7 +481,7 @@ static void tcp_chr_disconnect_locked(Chardev *chr)
>       if (emit_close) {
>           qemu_chr_be_event(chr, CHR_EVENT_CLOSED);
>       }
> -    if (s->reconnect_time && !s->reconnect_timer) {
> +    if (s->reconnect_time_ms && !s->reconnect_timer) {
>           qemu_chr_socket_restart_timer(chr);
>       }
>   }
> @@ -1080,9 +1080,9 @@ static int tcp_chr_wait_connected(Chardev *chr, Error **errp)
>           } else {
>               Error *err = NULL;
>               if (tcp_chr_connect_client_sync(chr, &err) < 0) {
> -                if (s->reconnect_time) {
> +                if (s->reconnect_time_ms) {
>                       error_free(err);
> -                    g_usleep(s->reconnect_time * 1000ULL * 1000ULL);
> +                    g_usleep(s->reconnect_time_ms * 1000ULL);
>                   } else {
>                       error_propagate(errp, err);
>                       return -1;
> @@ -1267,13 +1267,13 @@ skip_listen:
>   
>   
>   static int qmp_chardev_open_socket_client(Chardev *chr,
> -                                          int64_t reconnect,
> +                                          int64_t reconnect_ms,
>                                             Error **errp)
>   {
>       SocketChardev *s = SOCKET_CHARDEV(chr);
>   
> -    if (reconnect > 0) {
> -        s->reconnect_time = reconnect;
> +    if (reconnect_ms > 0) {
> +        s->reconnect_time_ms = reconnect_ms;
>           tcp_chr_connect_client_async(chr);
>           return 0;
>       } else {
> @@ -1371,7 +1371,7 @@ static void qmp_chardev_open_socket(Chardev *chr,
>       bool is_tn3270      = sock->has_tn3270  ? sock->tn3270  : false;
>       bool is_waitconnect = sock->has_wait    ? sock->wait    : false;
>       bool is_websock     = sock->has_websocket ? sock->websocket : false;
> -    int64_t reconnect   = sock->has_reconnect ? sock->reconnect : 0;
> +    int64_t reconnect_ms = 0;
>       SocketAddress *addr;
>   
>       s->is_listen = is_listen;
> @@ -1443,7 +1443,13 @@ static void qmp_chardev_open_socket(Chardev *chr,
>               return;
>           }
>       } else {
> -        if (qmp_chardev_open_socket_client(chr, reconnect, errp) < 0) {
> +        if (sock->has_reconnect) {
> +            reconnect_ms = sock->reconnect * 1000ULL;
> +        } else if (sock->has_reconnect_ms) {
> +            reconnect_ms = sock->reconnect_ms;
> +        }
> +
> +        if (qmp_chardev_open_socket_client(chr, reconnect_ms, errp) < 0) {
>               return;
>           }
>       }
> @@ -1509,6 +1515,15 @@ static void qemu_chr_parse_socket(QemuOpts *opts, ChardevBackend *backend,
>       sock->wait = qemu_opt_get_bool(opts, "wait", true);
>       sock->has_reconnect = qemu_opt_find(opts, "reconnect");
>       sock->reconnect = qemu_opt_get_number(opts, "reconnect", 0);
> +    sock->has_reconnect_ms = qemu_opt_find(opts, "reconnect-ms");
> +    sock->reconnect_ms = qemu_opt_get_number(opts, "reconnect-ms", 0);
> +
> +    if (sock->has_reconnect_ms && sock->has_reconnect) {
> +        error_setg(errp,
> +            "'reconnect' and 'reconnect-ms' are mutually exclusive");
> +        return;
> +    }
> +
>       sock->tls_creds = g_strdup(qemu_opt_get(opts, "tls-creds"));
>       sock->tls_authz = g_strdup(qemu_opt_get(opts, "tls-authz"));
>   
> diff --git a/chardev/char.c b/chardev/char.c
> index ba847b6e9e..35623c78a3 100644
> --- a/chardev/char.c
> +++ b/chardev/char.c
> @@ -888,6 +888,9 @@ QemuOptsList qemu_chardev_opts = {
>           },{
>               .name = "reconnect",
>               .type = QEMU_OPT_NUMBER,
> +        },{
> +            .name = "reconnect-ms",
> +            .type = QEMU_OPT_NUMBER,
>           },{
>               .name = "telnet",
>               .type = QEMU_OPT_BOOL,
> diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst
> index 88f0f03786..e5db9bc6e9 100644
> --- a/docs/about/deprecated.rst
> +++ b/docs/about/deprecated.rst
> @@ -430,6 +430,12 @@ Backend ``memory`` (since 9.0)
>   
>   ``memory`` is a deprecated synonym for ``ringbuf``.
>   
> +``reconnect`` (since 9.2)
> +^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +The ``reconnect`` option only allows specifiying second granularity timeouts,
> +which is not enough for all types of use cases, use ``reconnect-ms`` instead.
> +
>   CPU device properties
>   '''''''''''''''''''''
>   
> diff --git a/include/chardev/char-socket.h b/include/chardev/char-socket.h
> index 0708ca6fa9..d6d13ad37f 100644
> --- a/include/chardev/char-socket.h
> +++ b/include/chardev/char-socket.h
> @@ -74,7 +74,7 @@ struct SocketChardev {
>       bool is_websock;
>   
>       GSource *reconnect_timer;
> -    int64_t reconnect_time;
> +    int64_t reconnect_time_ms;
>       bool connect_err_reported;
>   
>       QIOTask *connect_task;
> diff --git a/qapi/char.json b/qapi/char.json
> index ef58445cee..7f117438c6 100644
> --- a/qapi/char.json
> +++ b/qapi/char.json
> @@ -273,7 +273,19 @@
>   #
>   # @reconnect: For a client socket, if a socket is disconnected, then
>   #     attempt a reconnect after the given number of seconds.  Setting
> -#     this to zero disables this function.  (default: 0) (Since: 2.2)
> +#     this to zero disables this function.  The use of this member is
> +#     deprecated, use @reconnect-ms instead. (default: 0) (Since: 2.2)
> +#
> +# @reconnect-ms: For a client socket, if a socket is disconnected,
> +#     then attempt a reconnect after the given number of milliseconds.
> +#     Setting this to zero disables this function.  This member is
> +#     mutually exclusive with @reconnect.
> +#     (default: 0) (Since: 9.2)
> +#
> +# Features:
> +#
> +# @deprecated: Member @reconnect is deprecated.  Use @reconnect-ms
> +#     instead.
>   #
>   # Since: 1.4
>   ##
> @@ -287,7 +299,8 @@
>               '*telnet': 'bool',
>               '*tn3270': 'bool',
>               '*websocket': 'bool',
> -            '*reconnect': 'int' },
> +            '*reconnect': { 'type': 'int', 'features': [ 'deprecated' ] },
> +            '*reconnect-ms': 'int' },
>     'base': 'ChardevCommon' }
>   
>   ##

Daniil Tatianin <d-tatianin@yandex-team.ru> writes:

> The 'reconnect' option only allows to specify the time in seconds,
> which is way too long for certain workflows.
>
> We have a lightweight disk backend server, which takes about 20ms to
> live update, but due to this limitation in QEMU, previously the guest
> disk controller would hang for one second because it would take this
> long for QEMU to reinitialize the socket connection.
>
> Introduce a new option called 'reconnect-ms', which is the same as
> 'reconnect', except the value is treated as milliseconds. These are
> mutually exclusive and specifying both results in an error.

Good:

    $ upstream-qemu -nodefaults -chardev socket,id=chr0,path=test-hmp,server=off,reconnect=1,reconnect-ms=2
    upstream-qemu: -chardev socket,id=chr0,path=test-hmp,server=off,reconnect=1,reconnect-ms=2: 'reconnect' and 'reconnect-ms' are mutually exclusive

Bad:

    $ upstream-qemu -nodefaults -S -display none -qmp stdio
    {"QMP": {"version": {"qemu": {"micro": 50, "minor": 1, "major": 9}, "package": "v9.1.0-211-ga0866249bd"}, "capabilities": ["oob"]}}
    {"execute": "qmp_capabilities", "arguments": {"enable": ["oob"]}}
    {"return": {}}
    {"execute":"chardev-add", "arguments": {"id":"chr0", "backend": {"type": "socket", "data": {"server": false, "addr": {"type": "unix", "data": {"path": "xyz"}}, "reconnect": 1, "reconnect-ms": 2}}}}
    {"return": {}}
    upstream-qemu: Unable to connect character device chr0: Failed to connect to 'xyz': No such file or directory

We're not rejecting simultaneous use of @reconnect and @reconnect-ms
here.

Moreover, you somehow regressed the handling of the "unable to connect"
error.  Before the patch, behavior is correct:

    $ upstream-qemu -nodefaults -S -display none -qmp stdio
    {"QMP": {"version": {"qemu": {"micro": 50, "minor": 1, "major": 9}, "package": "v9.1.0-210-g4b7ea33074"}, "capabilities": ["oob"]}}
    {"execute": "qmp_capabilities", "arguments": {"enable": ["oob"]}}
    {"return": {}}
    {"execute":"chardev-add", "arguments": {"id":"chr0", "backend": {"type": "socket", "data": {"server": false, "addr": {"type": "unix", "data": {"path": "xyz"}}}}}}
    {"error": {"class": "GenericError", "desc": "Failed to add chardev 'chr0': Failed to connect to 'xyz': No such file or directory"}}

> 'reconnect' is also deprecated by this commit to make it possible to
> remove it in the future as to not keep two options that control the
> same thing.
>
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> Acked-by: Peter Krempa <pkrempa@redhat.com>
> Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>

On 9/13/24 11:57 AM, Markus Armbruster wrote:

> Daniil Tatianin <d-tatianin@yandex-team.ru> writes:
>
>> The 'reconnect' option only allows to specify the time in seconds,
>> which is way too long for certain workflows.
>>
>> We have a lightweight disk backend server, which takes about 20ms to
>> live update, but due to this limitation in QEMU, previously the guest
>> disk controller would hang for one second because it would take this
>> long for QEMU to reinitialize the socket connection.
>>
>> Introduce a new option called 'reconnect-ms', which is the same as
>> 'reconnect', except the value is treated as milliseconds. These are
>> mutually exclusive and specifying both results in an error.
> Good:
>
>      $ upstream-qemu -nodefaults -chardev socket,id=chr0,path=test-hmp,server=off,reconnect=1,reconnect-ms=2
>      upstream-qemu: -chardev socket,id=chr0,path=test-hmp,server=off,reconnect=1,reconnect-ms=2: 'reconnect' and 'reconnect-ms' are mutually exclusive
>
> Bad:
>
>      $ upstream-qemu -nodefaults -S -display none -qmp stdio
>      {"QMP": {"version": {"qemu": {"micro": 50, "minor": 1, "major": 9}, "package": "v9.1.0-211-ga0866249bd"}, "capabilities": ["oob"]}}
>      {"execute": "qmp_capabilities", "arguments": {"enable": ["oob"]}}
>      {"return": {}}
>      {"execute":"chardev-add", "arguments": {"id":"chr0", "backend": {"type": "socket", "data": {"server": false, "addr": {"type": "unix", "data": {"path": "xyz"}}, "reconnect": 1, "reconnect-ms": 2}}}}
>      {"return": {}}
>      upstream-qemu: Unable to connect character device chr0: Failed to connect to 'xyz': No such file or directory
>
> We're not rejecting simultaneous use of @reconnect and @reconnect-ms
> here.
>
> Moreover, you somehow regressed the handling of the "unable to connect"
> error.  Before the patch, behavior is correct:
>
>      $ upstream-qemu -nodefaults -S -display none -qmp stdio
>      {"QMP": {"version": {"qemu": {"micro": 50, "minor": 1, "major": 9}, "package": "v9.1.0-210-g4b7ea33074"}, "capabilities": ["oob"]}}
>      {"execute": "qmp_capabilities", "arguments": {"enable": ["oob"]}}
>      {"return": {}}
>      {"execute":"chardev-add", "arguments": {"id":"chr0", "backend": {"type": "socket", "data": {"server": false, "addr": {"type": "unix", "data": {"path": "xyz"}}}}}}
>      {"error": {"class": "GenericError", "desc": "Failed to add chardev 'chr0': Failed to connect to 'xyz': No such file or directory"}}

Oh wow, thanks for spotting! I'll be sure to take a look at these.

>> 'reconnect' is also deprecated by this commit to make it possible to
>> remove it in the future as to not keep two options that control the
>> same thing.
>>
>> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
>> Acked-by: Peter Krempa <pkrempa@redhat.com>
>> Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>