qapi: Introduce add-fd, remove-fd, query-fdsets

This patch adds support that enables passing of file descriptors
to the QEMU monitor where they will be stored in specified file
descriptor sets.

A file descriptor set can be used by a client like libvirt to
store file descriptors for the same file.  This allows the
client to open a file with different access modes (O_RDWR,
O_WRONLY, O_RDONLY) and add/remove the passed fds to/from an fd
set as needed.  This will allow QEMU to (in a later patch in this
series) "open" and "reopen" the same file by dup()ing the fd in
the fd set that corresponds to the file, where the fd has the
matching access mode flag that QEMU requests.

The new QMP commands are:
  add-fd: Add a file descriptor to an fd set
  remove-fd: Remove a file descriptor from an fd set
  query-fdsets: Return information describing all fd sets

Note: These commands are not compatible with the existing getfd
and closefd QMP commands.

Signed-off-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 122 insertions, 0 deletions

diff --git a/qmp-commands.hx b/qmp-commands.hx
index 527b9f7c24..2ce4ce6556 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -970,6 +970,128 @@ Example:
 
 EQMP
 
+     {
+        .name       = "add-fd",
+        .args_type  = "fdset-id:i?,opaque:s?",
+        .params     = "add-fd fdset-id opaque",
+        .help       = "Add a file descriptor, that was passed via SCM rights, to an fd set",
+        .mhandler.cmd_new = qmp_marshal_input_add_fd,
+    },
+
+SQMP
+add-fd
+-------
+
+Add a file descriptor, that was passed via SCM rights, to an fd set.
+
+Arguments:
+
+- "fdset-id": The ID of the fd set to add the file descriptor to.
+              (json-int, optional)
+- "opaque": A free-form string that can be used to describe the fd.
+            (json-string, optional)
+
+Return a json-object with the following information:
+
+- "fdset-id": The ID of the fd set that the fd was added to. (json-int)
+- "fd": The file descriptor that was received via SCM rights and added to the
+        fd set. (json-int)
+
+Example:
+
+-> { "execute": "add-fd", "arguments": { "fdset-id": 1 } }
+<- { "return": { "fdset-id": 1, "fd": 3 } }
+
+Notes:
+
+(1) The list of fd sets is shared by all monitor connections.
+(2) If "fdset-id" is not specified, a new fd set will be created.
+
+EQMP
+
+     {
+        .name       = "remove-fd",
+        .args_type  = "fdset-id:i,fd:i?",
+        .params     = "remove-fd fdset-id fd",
+        .help       = "Remove a file descriptor from an fd set",
+        .mhandler.cmd_new = qmp_marshal_input_remove_fd,
+    },
+
+SQMP
+remove-fd
+---------
+
+Remove a file descriptor from an fd set.
+
+Arguments:
+
+- "fdset-id": The ID of the fd set that the file descriptor belongs to.
+              (json-int)
+- "fd": The file descriptor that is to be removed. (json-int, optional)
+
+Example:
+
+-> { "execute": "remove-fd", "arguments": { "fdset-id": 1, "fd": 3 } }
+<- { "return": {} }
+
+Notes:
+
+(1) The list of fd sets is shared by all monitor connections.
+(2) If "fd" is not specified, all file descriptors in "fdset-id" will be
+    removed.
+
+EQMP
+
+    {
+        .name       = "query-fdsets",
+        .args_type  = "",
+        .help       = "Return information describing all fd sets",
+        .mhandler.cmd_new = qmp_marshal_input_query_fdsets,
+    },
+
+SQMP
+query-fdsets
+-------------
+
+Return information describing all fd sets.
+
+Arguments: None
+
+Example:
+
+-> { "execute": "query-fdsets" }
+<- { "return": [
+       {
+         "fds": [
+           {
+             "fd": 30,
+             "opaque": "rdonly:/path/to/file"
+           },
+           {
+             "fd": 24,
+             "opaque": "rdwr:/path/to/file"
+           }
+         ],
+         "fdset-id": 1
+       },
+       {
+         "fds": [
+           {
+             "fd": 28
+           },
+           {
+             "fd": 29
+           }
+         ],
+         "fdset-id": 0
+       }
+     ]
+   }
+
+Note: The list of fd sets is shared by all monitor connections.
+
+EQMP
+
     {
         .name       = "block_passwd",
         .args_type  = "device:B,password:s",