Re: RFC: Resumable clone based on hybrid "smart" and "dumb" HTTP

From: Stefan Beller <sbeller@google.com>
Date: Feb 10, 2016, 20:23 UTC
Message-ID: <CAGZ79kZMvxa5Np4GbShv_A6NZwVAqff94+d8MFTZwrZS+2CqeQ@mail.gmail.com>
In-Reply-To: <CAJo=hJuRxoe6tXe65ci-A35c_PWJEP7KEPFu5Ocn147HwVuo3A@mail.gmail.com>

On Wed, Feb 10, 2016 at 12:11 PM, Shawn Pearce <spearce@spearce.org> wrote:

Show 12 quoted lines

> On Wed, Feb 10, 2016 at 10:59 AM, Shawn Pearce <spearce@spearce.org> wrote:
>>
>> ... Thoughts?
>
> Several of us at $DAY_JOB talked about this more today and thought a
> variation makes more sense:
>
> 1. Clients attempting clone ask for /info/refs?service=git-upload-pack
> like they do today.
>
> 2. Servers that support resumable clone include a "resumable"
> capability in the advertisement.

like "resumable-token=hash" similar to a push cert advertisement?

>
> 3. Updated clients on clone request GET /info/refs?service=git-resumable-clone.

Or just in the non-http case, they would terminate after the ls-remote (including capability advertisement) was done and connect again to a different service such as git-upload-stale-pack with the resumable token to identify the pack.

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>
> 4. The server may return a 302 Redirect to its current "mostly whole"
> pack file. This can be more flexible than "refs/heads/*", it just
> needs to be a mostly complete pack file that contains a complete graph
> from any arbitrary roots.
>
> 5. Clients fetch the file using standard HTTP GET, possibly with
> byte-ranges to resume.

In the non-http case the git-upload-stale-pack would be rsync with the resume token to determine the file name of the pack, such that we have resumeability.

>
> 6. Once stored and indexed with .idx, clients run `git fsck
> --lost-found` to discover the roots of the pack it downloaded. These
> are saved as temporary references.

jrn:

> I suspect we can do even faster by making index-pack do the work

    index-pack --check-self-contained-and-connected

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>
> 7. Client runs incremental fetch, and then deletes the temporary
> references from 6.
>
>
> An advantage to this process is its much more flexible for the server.
> There is no additional pack-*.info file required. GC can organize
> packs anyway it wants, etc.
>
> To make step 4 really resume well, clients may need to save the first
> Location header it gets back from
> /info/refs?service=git-resumable-clone and use that on resume. Servers
> are likely to embed the pack SHA-1 in the Location header, and the
> client wants to use this on subsequent GET attempts to abort early if
> the server has deleted the pack the client is trying to obtain.
> --
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