Re: [GSoC/RFC] Ideas on git fetch --deepen

[Duy Nguyen <pclouds@gmail.com>]

On Sun, Mar 8, 2015 at 3:57 PM, Dongcan Jiang <dongcan.jiang@gmail.com> wrote:
> Hi, all
>
> After digging into how "git fetch" works, I have found that my previous
> understanding was too rash. I'm sorry for that.
>
> I find that the current workflow of "git fetch --depth" is as follows:
>
>   1. 'fetch module' calls 'git-upload-pack service' via 'transport module'
>      to fetch ref with `depth`.
>   2. Such call is finished by pipe I/O.
>   3. git-upload-pack receives arguments such as 'depth' to fetch commits,
>      and send back to the caller.
>
> Therefore, if we want to implement "--deepen" for "git fetch", we have to
> modify the arguments protocol of git-upload-pack service by adding the
> 'shallow commit' hash.
> Then, we can fetch `depth` commits before the 'shallow commit' in
> git-upload-pack service.
> Apparently, we have to output error message when the 'shallow commit' hash
> is not in the repository.

Close. You can't figure this shallow commit hash from client side
(client repo is shortened). So you can't send it. What you send is
exactly what the user gives you (e.g. --deepen=5, then you send
"deepen 5" or similar). The server side (git-upload-pack) knows about
the shallow boundary of the client and can walk its (i.e. server-side)
commit graph to find out the new, deepened boundary. Then the server
sends necessary objects plus instructions to update shallow boundary
to the client.

Also, I think  this work would include support for smart-http
protocol. It goes a slightly different route, "git fetch" ends up at
transport.c, but then transport-helper.c and calls git-remote-http(s)
which is implemented by remote-curl.c. This one handles http stuff
then passes control to git-fetch-pack.c. It's fetch-pack that talks to
upload-pack.c

Have a look at Documentation/technical/pack-protocol.txt and
protocol-capabilities.txt (and http-protocol.txt for smart-http). The
function that "draws" shallow boundary based on --depth is
get_shallow_commits() in shallow.c. I suspect you need to improve it a
bit to use with --deepen. I guess you can look at these [1]. Those
changes touch shallow repo in a bit different way, but the main code
path is more or less the same (smart-http not touched).

[1] https://github.com/pclouds/git/commits/7edde8b83a20abb3cd404e2c5f07e3c29a2891f7
-- 
Duy