Re: [PATCH 06/10] sparse-checkout: use oidset to prevent repeat blobs

From: Elijah Newren <newren@gmail.com>
Date: May 21, 2020, 03:49 UTC
Message-ID: <CABPp-BGcjpJOht7ip_cPcHEtd3kx5fCwm=i19narWCEfCUwWAQ@mail.gmail.com>
In-Reply-To: <CABPp-BEkf0TVTt4=adJ9x70j814frL932vxyQCpm74AQiHWwGQ@mail.gmail.com>

Replying to my own questions...

On Wed, May 20, 2020 at 9:40 AM Elijah Newren <newren@gmail.com> wrote:

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>
> On Thu, May 7, 2020 at 6:21 AM Derrick Stolee via GitGitGadget
> <gitgitgadget@gmail.com> wrote:
> >
> > From: Derrick Stolee <dstolee@microsoft.com>
> >
> > As we parse the in-tree config files that store the sparse.dir values
> > used to create an in-tree sparse-checkout definition, we can easily
> > avoid parsing the same file multiple times by using an oidset on those
> > blobs. We only parse if the oid is new to the oidset.
> >
> > This is unlikely to have a major performance benefit right now, but will
> > be extremely important when we introduce the sparse.inherit options to
> > link multiple files in a directed graph. This oidset will prevent
> > infinite loops when cycles exist in that digraph, or exponential blowups
> > even in the case of a directed acyclic graph.
>
> I'm still not sure if I like the idea of having a mirror dependency
> structure separate from (and duplicative of) the build code; I'm still
> mulling that over.

I mentioned this to a few other buildsystem folks at $DAYJOB. They were strongly opposed to having more than one source of truth, but generating the git in-tree sparse values from the official build system files, with commit hooks and build system checks to make sure they get updated seemed like it'd be fine or not concern them much.

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> It's good that you've protected against infinite loops.
>
> Is there any reason to prefer swallowing infinite loops rather than
> warning or flagging as an error?  (I'm not sure, just thinking my
> questions out loud.)

The buildsystem folks also reminded me that we have cylic dependencies already, and although it's absolutely ugly, it is somewhat forced on us by a combination of different external tools that we can't change. As such, warnings or errors would be really annoying and we'd be one of the ones to want to turn them off. So drop that idea from me. :-)