On 2/11/20 2:41 AM, Richard W.M. Jones wrote:
On Mon, Feb 10, 2020 at 01:52:25PM -0600, Eric Blake wrote:
On 2/10/20 1:39 PM, Richard W.M. Jones wrote:
https://github.com/libguestfs/nbdkit/blob/ecef5b16359fb5af7e7abf4fd2fb3ad5438b16be/TODO#L76
Already existing filters (readahead, cache) could be improved if
filters could open a background work thread or threads which connect
independently to the plugin. A proposed new filter (scan) cannot
really be written at all without this capability.
First of all the reason this can't be done today is because filters
are called with a next_ops structure which is only valid transiently
during the filter callback. It cannot safely be saved or passed to
another thread.
(https://github.com/libguestfs/nbdkit/blob/master/docs/nbdkit-filter.pod#next-plugin)
I was independently thinking of fixing this: we want next_ops to be
have a life equal to the connection itself, rather than
stack-allocated. I'm hoping to post a patch for that shortly, as
part of my experimentation with implementing ext2 as a filter
instead of a plugin, where ext2 has the limitation that when writing
a custom io_manager, you only get ONE spot where you can pass in an
opaque pointer: our .prepare will have to pass a long-lived nxdata
to ext2fs_open().
Yes I can see this could be tricky, especially in the presence of
multiple threads serving a single connection. The only alternative
way I can think to do it would be to use a pointer in thread-local
storage which would would update with next_ops / nxdata each time.
Do you have a preliminary patch for this yet, or do you want me to
look at it today?
Just sent the stable nxdata patch. I'm still working on the ext2 filter
patches, though.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
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