=head1 NAME

nbd_opt_list_meta_context_queries - list available 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_list_meta_context_queries (
       struct nbd_handle *h, char **queries,
       nbd_context_callback context_callback
     );

=head1 DESCRIPTION

Request that the server list
available meta contexts associated with
the export previously specified by the most recent
L<nbd_set_export_name(3)> or L<nbd_connect_uri(3)>, and with an
explicit list of queries provided as a parameter (see
L<nbd_opt_list_meta_context(3)> if you want to reuse an
implicit query list instead).  This can only be used if
L<nbd_set_opt_mode(3)> enabled option mode.

The NBD protocol allows a client to decide how many queries to ask
the server.  For this function, the list is explicit in the C<queries>
parameter.  When the list is empty, a server will typically reply with all
contexts that it supports; when the list is non-empty, the server
will reply only with supported contexts that match the client's
request.  Note that a reply by the server might be encoded to
represent several feasible contexts within one string, rather than
multiple strings per actual context name that would actually succeed
during L<nbd_opt_go(3)>; so it is still necessary to use
L<nbd_can_meta_context(3)> after connecting to see which contexts
are actually supported.

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.  Remember that it is not safe to call
L<nbd_add_meta_context(3)> from within the context of the
callback function; rather, your code must copy any C<name> needed for
later use after this function completes.  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.  Likewise,
the NBD protocol does not specify an upper bound for the number of
replies that might be advertised, so client code should be aware that
a server may send a lengthy 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_list_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_LIST_META_CONTEXT_QUERIES 1

=head1 SEE ALSO

L<nbd_add_meta_context(3)>,
L<nbd_aio_opt_list_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_list_meta_context(3)>,
L<nbd_set_export_name(3)>,
L<nbd_set_opt_mode(3)>,
L<libnbd(3)>.

=head1 AUTHORS

Eric Blake

Richard W.M. Jones

=head1 COPYRIGHT

Copyright Red Hat
