[Libguestfs] [libnbd] Simultaneous read and write

[Richard W.M. Jones]

OK I think I have a way to solve this.  However we need to post the
writes to another thread, not the reads.  The reason is that writes
can be spooled up almost indefinitely so never block, whereas we may
block on reads if the server doesn't respond fast enough.

The actual change is very simple - I'll write a patch up later.

The caller calls a new API such as:

  int nbd_set_concurrent_writer (void *data,
      				 void (*writer) (void *data,
				      		 void *buf, size_t len));

‘struct socket’ is modified here so that if a concurrent writer
callback is set in the handle it calls the writer function instead of
send(2):

https://github.com/libguestfs/libnbd/blob/9d05f54f4daa892a1e581e1d68bc2dd4dad35e50/lib/socket.c#L48

There also has to be an error notification function that the writer
can call if it encounters an error (which are essentially
unrecoverable - it just moves the state to DEAD).

This pushes the implementation complexity to the caller, but I think
that is necessary and justified:

 - libraries shouldn't be creating threads

 - the caller has a choice of how to implement this, eg. using
   one writer thread over all handles, or one thread per handle

 - the caller can do clever stuff like pinning the writer thread to a
   physical core or vectoring interrupts, which would be almost
   impossible if the thread was hidden inside the library

By including an example it should be possible to get over some of the
complexity.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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