On Mon, Feb 27, 2017 at 11:32:47AM +0100, Lars Schneider wrote:
> I completely agree - I need to change that. However, the goal of the v2
> iteration was to get the "convert" interface in an acceptable state.
> That's what I intended to say in the patch comment section:
>
> "Please ignore all changes behind async_convert_to_working_tree() and
> async_filter_finish() for now as I plan to change the implementation
> as soon as the interface is in an acceptable state."
Ah, sorry, I missed that. I would think the underlying approach would
influence the interface to some degree. But as long as the interface
is sufficiently abstract, I think it gives you enough flexibility.
> > From Git's side, the loop is something like:
> >
> > while (delayed_items > 0) {
> > /* issue a wait, and get back the status/index pair */
> > status = send_wait(&index);
> > delayed_items--;
> >
> > /*
> > * use "index" to find the right item in our list of files;
> > * the format can be opaque to the filter, so we could index
> > * it however we like. But probably numeric indices in an array
> > * are the simplest.
> > */
> > assert(index > 0 && index < nr_items);
> > item[index].status = status;
> > if (status == SUCCESS)
> > read_content(&item[index]);
> > }
> >
> > and the filter side just attaches the "index" string to whatever its
> > internal queue structure is, and feeds it back verbatim when processing
> > that item finishes.
>
> That could work! I had something like that in mind:
>
> I teach Git a new command "list_completed" or similar. The filter
> blocks this call until at least one item is ready for Git.
> Then the filter responds with a list of paths that identify the
> "ready items". Then Git asks for these ready items just with the
> path and not with any content. Could that work? Wouldn't the path
> be "unique" to identify a blob per filter run?
I think that could work, though I think there are few minor downsides
compared to what I wrote above:
- if you respond with "these items are ready", and then make Git ask
for each again, it's an extra round-trip for each set of ready
items. You could just say "an item is ready; here it is" in a single
response. For a local pipe the latency is probably negligible,
though.
- using paths as the index would probably work, but it means Git has
to use the path to find the "struct checkout_entry" again. Which
might mean a hashmap (though if you have them all in a sorted list,
I guess you could also do a binary search).
- Using an explicit index communicates to the filter not only what the
index is, but also that Git is prepared to accept a delayed response
for the item. For backwards compatibility, the filter would probably
advertise "I have the 'delayed' capability", and then Git could
choose to use it or not on a per-item basis. Realistically it would
not change from item to item, but rather operation to operation. So
that means we can easily convert the call-sites in Git to the async
approach incrementally. As each one is converted, it turns on the
flag that causes the filter code to send the "index" tag.
-Peff
