=head1 NAME

nbd_block_status - send block status command, with 32-bit callback

=head1 SYNOPSIS

 #include <libnbd.h>

 typedef struct {
   int (*callback) (void *user_data,
                    const char *metacontext,
                    uint64_t offset, uint32_t *entries,
                    size_t nr_entries, int *error);
   void *user_data;
   void (*free) (void *user_data);
 } nbd_extent_callback;

 int nbd_block_status (
       struct nbd_handle *h, uint64_t count,
       uint64_t offset, nbd_extent_callback extent_callback,
       uint32_t flags
     );

=head1 DESCRIPTION

Issue the block status command to the NBD server.  If
supported by the server, this causes metadata context
information about blocks beginning from the specified
offset to be returned. The C<count> parameter is a hint: the
server may choose to return less status, or the final block
may extend beyond the requested range. If multiple contexts
are supported, the number of blocks and cumulative length
of those blocks need not be identical between contexts.

Note that not all servers can support a C<count> of 4GiB or larger;
L<nbd_get_extended_headers_negotiated(3)> indicates which servers
will parse a request larger than 32 bits.
The NBD protocol does not yet have a way for a client to learn if
the server will enforce an even smaller maximum block status size,
although a future extension may add a constraint visible in
L<nbd_get_block_size(3)>.  Furthermore, this function is inherently
limited to 32-bit values.  If the server replies with a larger
extent, the length of that extent will be truncated to just
below 32 bits and any further extents from the server will be
ignored.  If the server replies with a status value larger than
32 bits (only possible when extended headers are in use), the
callback function will be passed an C<EOVERFLOW> error.  To get
the full extent information from a server that supports 64-bit
extents, you must use L<nbd_block_status_64(3)>.

Depending on which metadata contexts were enabled before
connecting (see L<nbd_add_meta_context(3)>) and which are
supported by the server (see L<nbd_can_meta_context(3)>) this call
returns information about extents by calling back to the
C<extent> function.  The callback cannot call C<nbd_*> APIs on the
same handle since it holds the handle lock and will
cause a deadlock.  If the callback returns C<-1>, and no earlier
error has been detected, then the overall block status command
will fail with any non-zero value stored into the callback's
C<error> parameter (with a default of C<EPROTO>); but any further
contexts will still invoke the callback.

The C<extent> function is called once per type of metadata available,
with the C<user_data> passed to this function.  The C<metacontext>
parameter is a string such as C<"base:allocation">.  The C<entries>
array is an array of pairs of integers with the first entry in each
pair being the length (in bytes) of the block and the second entry
being a status/flags field which is specific to the metadata context.
The number of pairs passed to the function is C<nr_entries/2>.  The
NBD protocol document in the section about
C<NBD_REPLY_TYPE_BLOCK_STATUS> describes the meaning of this array;
for contexts known to libnbd, B<E<lt>libnbd.hE<gt>> contains constants
beginning with C<LIBNBD_STATE_> that may help decipher the values.
On entry to the callback, the C<error> parameter contains the errno
value of any previously detected error, but even if an earlier error
was detected, the current C<metacontext> and C<entries> are valid.

It is possible for the extent function to be called
more times than you expect (if the server is buggy),
so always check the C<metacontext> field to ensure you
are receiving the data you expect.  It is also possible
that the extent function is not called at all, even for
metadata contexts that you requested.  This indicates
either that the server doesn't support the context
or for some other reason cannot return the data.

The C<flags> parameter may be C<0> for no flags, or may contain
C<LIBNBD_CMD_FLAG_REQ_ONE> meaning that the server should
return only one extent per metadata context where that extent
does not exceed C<count> bytes; however, libnbd does not
validate that the server obeyed the flag.

By default, libnbd will reject attempts to use this function with
parameters that are likely to result in server failure, such as
requesting an unknown command flag.  The L<nbd_set_strict_mode(3)>
function can be used to alter which scenarios should await a server
reply rather than failing fast.

=head1 RETURN VALUE

If the call is successful the function returns 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_block_status
can be called when the handle is in the following state:

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

=head1 VERSION

This function first appeared in libnbd 1.0.

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

 #define LIBNBD_HAVE_NBD_BLOCK_STATUS 1

=head1 SEE ALSO

L<nbd_add_meta_context(3)>,
L<nbd_aio_block_status(3)>,
L<nbd_block_status_64(3)>,
L<nbd_can_meta_context(3)>,
L<nbd_create(3)>,
L<nbd_get_block_size(3)>,
L<nbd_get_extended_headers_negotiated(3)>,
L<nbd_set_strict_mode(3)>,
L<libnbd(3)>.

=head1 AUTHORS

Eric Blake

Richard W.M. Jones

=head1 COPYRIGHT

Copyright Red Hat
