[nfsv4] can a server replace a read deleg with a write deleg?

[Rick Macklem <[e-mail]>]

Somewhat tangencial to the recent thread, what about the following
simple case:
- client opens foo for reading and gets read_deleg_stateid_foo
- client opens foo for writing against the server (which it must do
   when it holds a read delegation)

Now, if the server sees that this client is the only one with a read
delegation for (and opens on) "foo", it could issue a write delegation
to the client for "foo".
- To do this does it first need to CBRecall the read delegation? (If so,
   it probably cannot issue the write delegation for this open, since it
   would take too long to reply to the Open unless it returns NFS4ERR_DELAY
   for the Open for a while. Not an ideal situation.)
OR
- Can it return a write delegation for "foo" in the write open reply?
   - If it is allowed to do this, does this delegation replace
     read_delegation_stateid_foo or is there now multiple delegations for
     "foo" issued to the same client?
     (I don't like the concept of having multiple delegations issued to
      the same client for the same file concurrently and I'm pretty sure
      my client isn't implemented to handle this case. Without looking at
      the code to be sure, I think it logs an error and throws away the
      new second delegation.)

I don't think this is clarified in RFC3530, but please correct me if I'm
incorrect w.r.t. this.

rick