Re: [PATCH 2/2] config: allow specifying config entries via envvar pairs

From: Jeff King <peff@peff.net>
To: Junio C Hamano <gitster@pobox.com>
Cc: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"Patrick Steinhardt" <ps@pks.im>,
	git@vger.kernel.org
Subject: Re: [PATCH 2/2] config: allow specifying config entries via envvar pairs
Date: Mon, 16 Nov 2020 21:34:54 -0500
Message-ID: <20201117023454.GA34754@coredump.intra.peff.net> (raw)
In-Reply-To: <xmqqy2j1851k.fsf@gitster.c.googlers.com>

On Mon, Nov 16, 2020 at 11:39:35AM -0800, Junio C Hamano wrote:

> >> While not document, it is currently possible to specify config entries
> >> [in GIT_CONFIG_PARAMETERS]
> [...]
>
> "While not documented" yes, for sure, but we do not document it for
> a good reason---it is a pure implementation detail between Git
> process that runs another one as its internal implementation detail.

I have actually been quite tempted to document and promise that it will
continue to work. Because it really is useful in some instances. The
thing that has held me back is that the documentation would reveal how
unforgiving the parser is. ;)

It insists that key/value pairs are shell-quoted as a whole. I think if
we made it accept a some reasonable inputs:

  - do not require quoting for values that do not need it

  - allow any amount of shell-style single-quoting (whole parameters,
    just values, etc).

  - do not bother allowing other quoting, like double-quotes with
    backslashes. However, document backslash and double-quote as
    meta-characters that must not appear outside of single-quotes, to
    allow for future expansion.

then I'd feel comfortable making it a public-facing feature. And for
most cases it would be pretty pleasant to use (and for the unpleasant
ones, I'm not sure that a little quoting is any worse than the paired
environment variables found here).

> I especially do not think we want to read from unbounded number of
> GIT_CONFIG_KEY_<N> variables like this patch does.  How would a
> script cleanse its environment to protect itself from stray such
> environment variable pair?  Count up from 1 to forever?  Run "env"
> and grep for "GIT_CONFIG_KEY_[0-9]*=" (the answer is No.  What if
> some environment variables have newline in its values?)

Yeah, scripts can currently assume that:

  unset $(git rev-parse --local-env-vars)

will drop any config from the environment. In some cases, having
rev-parse enumerate the GIT_CONFIG_KEY_* variables that are set would be
sufficient. But that implies that rev-parse is seeing the same
environment we're planning to clear. As it is now, a script is free to
use rev-parse to generate that list, hold on to it, and then use it
later.

-Peff