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=head1 NAME
nbd_opt_set_meta_context - select specific meta contexts, using
implicit 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 (
struct nbd_handle *h,
nbd_context_callback context_callback
);
=head1 DESCRIPTION
Request that the server supply all recognized meta contexts
registered through prior calls to L<nbd_add_meta_context(3)>, 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. Rather than taking that list of queries as a parameter
to this function, libnbd reuses the current list of requested meta
contexts as set by L<nbd_add_meta_context(3)>; you can use
L<nbd_clear_meta_contexts(3)> to set up a different list of queries
(see L<nbd_opt_set_meta_context_queries(3)> to pass an explicit
list of contexts instead). 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>.
For more information see L<libnbd(3)/Non-NULL parameters>.
=head1 HANDLE STATE
nbd_opt_set_meta_context
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 1
=head1 SEE ALSO
L<nbd_add_meta_context(3)>,
L<nbd_aio_opt_set_meta_context(3)>,
L<nbd_can_meta_context(3)>,
L<nbd_clear_meta_contexts(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(3)>,
L<nbd_opt_set_meta_context(3)>,
L<nbd_opt_set_meta_context_queries(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
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