Re: [PATCH v2 0/8] CDN offloading of fetch response

From: Jonathan Tan <jonathantanmy@google.com>
Date: Apr 23, 2019, 19:23 UTC
Message-ID: <20190423192321.145285-1-jonathantanmy@google.com>
In-Reply-To: <20190423052159.GA12530@sigill.intra.peff.net>

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> On Fri, Mar 08, 2019 at 01:55:12PM -0800, Jonathan Tan wrote:
> 
> > Here's my current progress - the only thing that is lacking is more
> > tests, maybe, so I think it's ready for review.
> 
> A bit belated, but here are some overall thoughts. A lot of my thinking
> comes from comparing this to previous work I had done on teaching the
> client to fetch first from a bundle and then do a follow-up fetch from
> the server.

Thanks for your thoughts.

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> > This code now starts CDN downloads after closing the first HTTP request,
> > and it holds on to the .keep files until after the refs are set. I'll
> > leave parallelization of the CDN downloads for later work.
> 
> Good. One of the problems with the bundle+followup approach was either
> having to hold open or re-establish the connection to the original
> server, since that fetch had to be put on hold.
> 
> I agree that parallelizing can come later. I do wonder if it's worth
> making a new tool rather than trying to reuse git-http-fetch. Yes, it
> basically does what we want already, but there's quite a lot of cruft in
> that dumb-http code base. Using http_get_file(), or even curl more
> directly might be a bit easier.
> 
> One problem in particular I'd worry about is that the http code
> generally expects authentication to happen through the initial
> ref-discovery, and then be set for all subsequent requests. So I doubt
> an http_pack_request actually handles auth at all, and retro-fitting it
> may be tricky.

Thanks - I'll keep that in mind. If auth isn't handled, then we'll definitely need something new.

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> > One relatively significant change: someone pointed out that the issue fixed by 
> > 50d3413740 ("http: make redirects more obvious", 2016-12-06) could also
> > occur here, so I have changed it so that the server is required to send
> > the packfile's hash along with the URI.
> 
> I have mixed feelings on that. I like how it clearly makes the server
> the source of authority. You don't have to care about CDN tampering,
> because you know you are getting the bytes the server wanted you to.

I was thinking not of CDN tampering but of a malicious server. Suppose a server knew of (1) the hash of a target commit, and (2) the URL of a packfile containing that target commit. The server can craft a commit whose parent is that target commit and advertise it, and serves both that commit as a packfile and the URL as packfile-uri. When the user subsequently pushes, they will probably inadvertently push everything including the target commit.

This is similar to the 50d3413740 case except that now the server needs an additional datum (2). The packfile's hash is yet another additional datum. I admit that it may not be that useful, though.

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> I think even without that this is still _mostly_ true, though. You're
> going to compute the hash of every object the CDN sends you, and those
> objects must fit into the missing gaps in what the server sends you. So
> the worst case is that a malicious CDN could send you some extra
> objects. That's probably not the end of the world, but I do like the
> extra guarantee of knowing that you got byte-for-byte what the server
> wanted you to.

Yes, the guarantee is nice.

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> > This does mean that Ævar's workflow described in [1] would not work.
> > Quoting Ævar:
> > 
> > > More concretely, I'd like to have a setup where a server can just dumbly
> > > point to some URL that probably has most of the data, without having any
> > > idea what OIDs are in it. So that e.g. some machine entirely
> > > disconnected from the server (and with just a regular clone) can
> > > continually generating an up-to-date-enough packfile.
> > 
> > With 50d3413740, it seems to me that it's important for the server to
> > know details about the URIs that it points to, so such a disconnection
> > would not work.
> 
> I think even without 50d3413740, this is important for your scheme. One
> of the weaknesses (and strengths, I suppose) of the bundle+followup
> scheme was that the initial bundle generally had to meet the git
> repository guarantee. After you fetched the bundle, you'd tell the
> server "OK, now I have commit X" just like you were a regular client.
> 
> But I really like that your proposal does away with that, and asks for
> tighter cooperation between the server and the CDN. It means the CDN can
> serve some random subset of the objects. But once we do that, now the
> server _has_ to know what was in those URLs it pointed to, because our
> protocol has no good way of communicating a random subset of objects (if
> they're just blobs, we could enumerate all of them to the server as
> haves; yuck.  But as soon as you get a tree or commit from the CDN, you
> have to have all of the reachable objects to be able to claim it).

Ah...you're right that the server still needs some inkling of what the CDN's packfile has.

Without the packfile hash, the knowledge needed is slightly less: the server just has to know a subset of the objects instead of the exact list of objects, but I can't think of how this can benefit us other than that we don't require so much consistency (it's OK if the server's knowledge of the CDN is updated significantly later than the CDN is updated, if the CDN reuses the same URL for each version of the packfile).

> So I think this is pretty inherent to your proposal, and but it's the
> right tradeoff to be making here (I think there could still be room for
> the other thing, too, but it's just a different feature).

Thanks.