Re: [Libguestfs] [PATCH libnbd discussion only 4/5] api: Implement concurrent writer.
On Mon, Jun 03, 2019 at 01:27:47PM -0500, Eric Blake wrote:
On 6/3/19 10:29 AM, Richard W.M. Jones wrote:
> +=item 4. Modify main loop
> +
> +Finally your main loop can unconditionally call
> +C<nbd_aio_notify_write> when C<nbd_aio_get_direction> returns
C<WRITE>
> +or C<BOTH> (since the concurrent thread can always enqueue more data
> +and so is always "ready to write").
Will we ever actually reach a state that is blocked on a write
completion for aio_get_direction to ever even request a WRITE or BOTH?
Or will the state machine always manage to churn through requests in
their entirety and back to state READY which is just a READ state?
Right, I believe the answer is no, we never get WRITE/BOTH, and that
is also what I observed when closely examining the events in the debug
log earlier today. However I guess better to be safe.
> +int
> +nbd_unlocked_set_concurrent_writer (struct nbd_handle *h,
> + void *data, writer_cb writer)
> +{
> + /* I suppose we could allow this, but it seems more likely that
> + * it's an error rather than intentional.
> + */
> + if (h->writer != NULL) {
> + set_error (EINVAL, "concurrent writer was already set for this
handle");
> + return -1;
> + }
> +
> + h->writer = writer;
> + h->writer_data = data;
> + return 0;
> +}
Is it worth a get_concurrent_writer()? Not strictly necessary, though.
We could add it later but it raises the question of why anyone would
need it.
> + else {
> + /* Pass the buffer to the concurrent writer thread. */
> + h->writer (h->writer_data, buf, len);
> + return len;
[1] So h->writer is NOT allowed to fail directly (if it fails, it must
call nbd_concurrent_writer_error instead). Stems from the fact that the
generator doesn't allow callbacks with a return type, but livable. But
may warrant extra wording in the documentation.
Right, it's a limitation of the generator which I should probably fix.
If we had that then we could differentiate "immediate failure" (eg
memory allocation) from later failure of send(2) in the writer thread.
Rich.
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