qmp: introduce QMPCapability

There were no QMP capabilities defined.  Define the first capability,
"oob", to allow out-of-band messages.

After this patch, we will allow QMP clients to enable QMP capabilities
when sending the first "qmp_capabilities" command.  Originally we are
starting QMP session with no arguments like:

  { "execute": "qmp_capabilities" }

Now we can enable some QMP capabilities using (take OOB as example,
which is the only capability that we support):

  { "execute": "qmp_capabilities",
    "arguments": { "enable": [ "oob" ] } }

When the "arguments" key is not provided, no capability is enabled.

For capability "oob", the monitor needs to be run on a dedicated IO
thread, otherwise the command will fail.  For example, trying to enable
OOB on a MUXed typed QMP monitor will fail.

One thing to mention is that QMP capabilities are per-monitor, and also
when the connection is closed due to some reason, the capabilities will
be reset.

Also, touch up qmp-test.c to test the new bits.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-11-peterx@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: touch up commit message]
Signed-off-by: Eric Blake <eblake@redhat.com>

1 files changed, 29 insertions, 3 deletions

diff --git a/qapi/misc.json b/qapi/misc.json
index 6150b9a..564216a 100644
--- a/qapi/misc.json
+++ b/qapi/misc.json
@@ -10,21 +10,47 @@
 #
 # Enable QMP capabilities.
 #
-# Arguments: None.
+# Arguments:
+#
+# @enable:   An optional list of QMPCapability values to enable.  The
+#            client must not enable any capability that is not
+#            mentioned in the QMP greeting message.  If the field is not
+#            provided, it means no QMP capabilities will be enabled.
+#            (since 2.12)
 #
 # Example:
 #
-# -> { "execute": "qmp_capabilities" }
+# -> { "execute": "qmp_capabilities",
+#      "arguments": { "enable": [ "oob" ] } }
 # <- { "return": {} }
 #
 # Notes: This command is valid exactly when first connecting: it must be
 # issued before any other command will be accepted, and will fail once the
 # monitor is accepting other commands. (see qemu docs/interop/qmp-spec.txt)
 #
+# The QMP client needs to explicitly enable QMP capabilities, otherwise
+# all the QMP capabilities will be turned off by default.
+#
 # Since: 0.13
 #
 ##
-{ 'command': 'qmp_capabilities' }
+{ 'command': 'qmp_capabilities',
+  'data': { '*enable': [ 'QMPCapability' ] } }
+
+##
+# @QMPCapability:
+#
+# Enumeration of capabilities to be advertised during initial client
+# connection, used for agreeing on particular QMP extension behaviors.
+#
+# @oob:   QMP ability to support Out-Of-Band requests.
+#         (Please refer to qmp-spec.txt for more information on OOB)
+#
+# Since: 2.12
+#
+##
+{ 'enum': 'QMPCapability',
+  'data': [ 'oob' ] }
 
 ##
 # @VersionTriple: