RFC: Enable delayed responses to Git clean/smudge filter requests

[Lars Schneider <larsxschneider@gmail.com>]

Hi,

Git always performs a clean/smudge filter on files in sequential order.
Sometimes a filter operation can take a noticeable amount of time. 
This blocks the entire Git process.

I would like to give a filter process the possibility to answer Git with
"I got your request, I am processing it, ask me for the result later!".

I see the following way to realize this:

In unpack-trees.c:check_updates() [1] we loop through the cache 
entries and "ask me later" could be an acceptable return value of the 
checkout_entry() call. The loop could run until all entries returned
success or error.

The filter machinery is triggered in various other places in Git and
all places that want to support "ask me later" would need to be patched 
accordingly.

--

Do you think this could be a viable approach?
Do you see a better way?

Thanks,
Lars


[1] https://github.com/git/git/blob/3ab228137f980ff72dbdf5064a877d07bec76df9/unpack-trees.c#L267