=head1 NAME

nbd_opt_set_meta_context_queries - select specific meta contexts, using
explicit query list

=head1 SYNOPSIS

 #include <libnbd.h>

 typedef struct {
   int (*callback) (void *user_data, const char *name);
   void *user_data;
   void (*free) (void *user_data);
 } nbd_context_callback;

 int nbd_opt_set_meta_context_queries (
       struct nbd_handle *h, char **queries,
       nbd_context_callback context_callback
     );

=head1 DESCRIPTION

Request that the server supply all recognized meta contexts
passed in through C<queries>, in conjunction with the export
previously specified by the most recent L<nbd_set_export_name(3)>
or L<nbd_connect_uri(3)>.  This can only be used if
L<nbd_set_opt_mode(3)> enabled option mode.  Normally, this
function is redundant, as L<nbd_opt_go(3)> automatically does
the same task if structured replies or extended headers have
already been negotiated.  But manual control over meta context
requests can be useful for fine-grained testing of how a server
handles unusual negotiation sequences.  Often, use of this
function is coupled with L<nbd_set_request_meta_context(3)> to
bypass the automatic context request normally performed by
L<nbd_opt_go(3)>.

The NBD protocol allows a client to decide how many queries to ask
the server.  This function takes an explicit list of queries; to
instead reuse an implicit list, see L<nbd_opt_set_meta_context(3)>.
Since this function is primarily designed for testing servers,
libnbd does not prevent the use of this function on an empty
list or when L<nbd_set_request_structured_replies(3)> has
disabled structured replies, in order to see how a server behaves.

The C<context> function is called once per server reply, with any
C<user_data> passed to this function, and with C<name> supplied by
the server.  Additionally, each server name will remain visible through
L<nbd_can_meta_context(3)> until the next attempt at
L<nbd_set_export_name(3)> or L<nbd_opt_set_meta_context(3)>, as
well as L<nbd_opt_go(3)> or L<nbd_opt_info(3)> that trigger an
automatic meta context request.  Remember that it is not safe to
call any C<nbd_*> APIs from within the context of the callback
function.  At present, the return value of the callback is
ignored, although a return of -1 should be avoided.

For convenience, when this function succeeds, it returns the number
of replies returned by the server.

Not all servers understand this request, and even when it is understood,
the server might intentionally send an empty list because it does not
support the requested context, or may encounter a failure after
delivering partial results.  Thus, this function may succeed even when
no contexts are reported, or may fail but have a non-empty list.

=head1 RETURN VALUE

This call returns an integer E<ge> C<0>.

=head1 ERRORS

On error C<-1> is returned.

Refer to L<libnbd(3)/ERROR HANDLING>
for how to get further details of the error.

The following parameters must not be NULL: C<h>, C<queries>.
For more information see L<libnbd(3)/Non-NULL parameters>.

=head1 HANDLE STATE

nbd_opt_set_meta_context_queries
can be called when the handle is in the following state:

 ┌─────────────────────────────────────┬─────────────────────────┐
 │ Handle created, before connecting   │ ❌ error                │
 │ Connecting                          │ ❌ error                │
 │ Connecting & handshaking (opt_mode) │ ✅ allowed              │
 │ Connected to the server             │ ❌ error                │
 │ Connection shut down                │ ❌ error                │
 │ Handle dead                         │ ❌ error                │
 └─────────────────────────────────────┴─────────────────────────┘

=head1 VERSION

This function first appeared in libnbd 1.16.

If you need to test if this function is available at compile time
check if the following macro is defined:

 #define LIBNBD_HAVE_NBD_OPT_SET_META_CONTEXT_QUERIES 1

=head1 SEE ALSO

L<nbd_aio_opt_set_meta_context_queries(3)>,
L<nbd_can_meta_context(3)>,
L<nbd_connect_uri(3)>,
L<nbd_create(3)>,
L<nbd_opt_go(3)>,
L<nbd_opt_info(3)>,
L<nbd_opt_list_meta_context_queries(3)>,
L<nbd_opt_set_meta_context(3)>,
L<nbd_set_export_name(3)>,
L<nbd_set_opt_mode(3)>,
L<nbd_set_request_meta_context(3)>,
L<nbd_set_request_structured_replies(3)>,
L<libnbd(3)>.

=head1 AUTHORS

Eric Blake

Richard W.M. Jones

=head1 COPYRIGHT

Copyright Red Hat
