NAME

       nbd_opt_set_meta_context - select specific meta contexts, using implicit query list

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
            );

DESCRIPTION

       Request that the server supply all recognized meta contexts registered through prior calls
       to nbd_add_meta_context(3), in conjunction with the export previously specified by the
       most recent nbd_set_export_name(3) or nbd_connect_uri(3).  This can only be used if
       nbd_set_opt_mode(3) enabled option mode.  Normally, this function is redundant, as
       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 nbd_set_request_meta_context(3) to bypass the
       automatic context request normally performed by 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 nbd_add_meta_context(3); you can use
       nbd_clear_meta_contexts(3) to set up a different list of queries (see
       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 nbd_set_request_structured_replies(3) has
       disabled structured replies, in order to see how a server behaves.

       The "context" function is called once per server reply, with any "user_data" passed to
       this function, and with "name" supplied by the server.  Additionally, each server name
       will remain visible through nbd_can_meta_context(3) until the next attempt at
       nbd_set_export_name(3) or nbd_opt_set_meta_context(3), as well as nbd_opt_go(3) or
       nbd_opt_info(3) that trigger an automatic meta context request.  Remember that it is not
       safe to call any "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.

RETURN VALUE

       This call returns an integer ≥ 0.

ERRORS

       On error -1 is returned.

       Refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error.

       The following parameters must not be NULL: "h".  For more information see "Non-NULL
       parameters" in libnbd(3).

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                │
        └─────────────────────────────────────┴─────────────────────────┘

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

SEE ALSO

       nbd_add_meta_context(3), nbd_aio_opt_set_meta_context(3), nbd_can_meta_context(3),
       nbd_clear_meta_contexts(3), nbd_connect_uri(3), nbd_create(3), nbd_opt_go(3),
       nbd_opt_info(3), nbd_opt_list_meta_context(3), nbd_opt_set_meta_context(3),
       nbd_opt_set_meta_context_queries(3), nbd_set_export_name(3), nbd_set_opt_mode(3),
       nbd_set_request_meta_context(3), nbd_set_request_structured_replies(3), libnbd(3).

AUTHORS

       Eric Blake

       Richard W.M. Jones

COPYRIGHT

       Copyright Red Hat

LICENSE

       This library is free software; you can redistribute it and/or modify it under the terms of
       the GNU Lesser General Public License as published by the Free Software Foundation; either
       version 2 of the License, or (at your option) any later version.

       This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
       without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
       See the GNU Lesser General Public License for more details.

       You should have received a copy of the GNU Lesser General Public License along with this
       library; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth
       Floor, Boston, MA 02110-1301 USA