nfsd4: don't pretend to support write delegations

The delegation code mostly pretends to support either read or write
delegations.  However, correct support for write delegations would
require, for example, breaking of delegations (and/or implementation of
cb_getattr) on stat.  Currently all that stops us from handing out
delegations is a subtle reference-counting issue.

Avoid confusion by adding an earlier check that explicitly refuses write
delegations.

For now, though, I'm not going so far as to rip out existing
half-support for write delegations, in case we get around to using that
soon.
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -170,6 +170,13 @@ alloc_init_deleg(struct nfs4_client *clp, struct nfs4_stateid *stp, struct svc_f
 	struct nfs4_cb_conn *cb = &stp->st_stateowner->so_client->cl_cb_conn;
 
 	dprintk("NFSD alloc_init_deleg\n");
+	/*
+	 * Major work on the lease subsystem (for example, to support
+	 * calbacks on stat) will be required before we can support
+	 * write delegations properly.
+	 */
+	if (type != NFS4_OPEN_DELEGATE_READ)
+		return NULL;
 	if (fp->fi_had_conflict)
 		return NULL;
 	if (num_delegations > max_delegations)