Re: RFC: Missing blob hook might be invoked infinitely recursively

From: Jonathan Nieder <jrnieder@gmail.com>
Date: Jun 29, 2017, 19:24 UTC
Message-ID: <20170629192441.GA68015@aiede.mtv.corp.google.com>
In-Reply-To: <20170629114830.39dc2161@twelve2.svl.corp.google.com>

Hi,

Jonathan Tan wrote:

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> Suppose you have missing blob AB12 and CD34 that you now need, so
> fetch-blob is invoked. It sends the literals AB12 and CD34 to a new
> server endpoint and obtains a packfile, which it then pipes through "git
> index-pack". The issue is that "git index-pack" wants to try to access
> AB12 and CD34 in the local repo in order to do a SHA-1 collision check,
> and therefore fetch-blob is invoked once again, creating infinite
> recursion.

[...]

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> 2. Add support for an environment variable to Git that suppresses access
> to the missing blob manifest, in effect, suppressing invocation of the
> hook. This allows anyone (the person configuring Git or the hook writer)
> to suppress this access, although they might need in-depth knowledge to
> know whether the hook is meant to be run with such access suppressed or
> required.

Small tweak: what if Git itself sets that environment variable when invoking the hook? A fetch-blob hook author can then explicitly unset the environment variable to request recursion if they need it.

I should credit Shawn Pearce for saying this a little more clearly offline a moment ago.

Thanks, Jonathan