=head1 NAME

nbd_set_handshake_flags - control use of handshake flags

=head1 SYNOPSIS

 #include <libnbd.h>

 int nbd_set_handshake_flags (
       struct nbd_handle *h, uint32_t flags
     );

=head1 DESCRIPTION

By default, libnbd tries to negotiate
all possible handshake flags
that are also supported by the server, since omitting a handshake
flag can prevent the use of other functionality such as TLS encryption
or structured replies.  However, for integration testing, it can be
useful to reduce the set of flags supported by the client to test that
a particular server can handle various clients that were compliant to
older versions of the NBD specification.

The C<flags> argument is a bitmask, including zero or more of the
following handshake flags:

=over 4

=item C<LIBNBD_HANDSHAKE_FLAG_FIXED_NEWSTYLE> = 1

The server gracefully handles unknown option requests from the
client, rather than disconnecting.  Without this flag, a client
cannot safely request to use extensions such as TLS encryption or
structured replies, as the request may cause an older server to
drop the connection.

=item C<LIBNBD_HANDSHAKE_FLAG_NO_ZEROES> = 2

If the client is forced to use C<NBD_OPT_EXPORT_NAME> instead of
the preferred C<NBD_OPT_GO>, this flag allows the server to send
fewer all-zero padding bytes over the connection.

=back

For convenience, the constant C<LIBNBD_HANDSHAKE_FLAG_MASK> is
available to describe all flags supported by this build of libnbd.
Future NBD extensions may add further flags, which in turn may
be enabled by default in newer libnbd.  As such, when attempting
to disable only one specific bit, it is wiser to first call
L<nbd_get_handshake_flags(3)> and modify that value, rather than
blindly setting a constant value.

=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_set_handshake_flags
can be called when the handle is in the following state:

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

=head1 VERSION

This function first appeared in libnbd 1.2.

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

 #define LIBNBD_HAVE_NBD_SET_HANDSHAKE_FLAGS 1

=head1 SEE ALSO

L<nbd_create(3)>,
L<nbd_get_handshake_flags(3)>,
L<nbd_set_request_structured_replies(3)>,
L<libnbd(3)>.

=head1 AUTHORS

Eric Blake

Richard W.M. Jones

=head1 COPYRIGHT

Copyright Red Hat
