Re: [PATCH 00/27] [RFC] Sparse Index

From: Junio C Hamano <gitster@pobox.com>
Date: Jan 25, 2021, 20:10 UTC
Message-ID: <xmqqh7n4ydyw.fsf@gitster.c.googlers.com>
In-Reply-To: <pull.847.git.1611596533.gitgitgadget@gmail.com>

"Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com> writes:

> This RFC proposes an update to the index formats to allow "sparse directory
> entries". These entries correspond to directories that are completely
> excluded from the sparse checkout definition. We can detect that a directory
> is excluded when using "cone mode" patterns.

Yay.

> Since having directory entries is a radical departure from the existing
> index format, a new extension "extensions.sparseIndex" is added. Using a
> sparse index should cause incompatible tools to fail because they do not
> understand this extension.

Safety is good, but because the index is purely a local matter, we do not have to be so careful as updating the network protocols or pack/object formats.

I think the use of "extensions.*" mechanism to render the repository that uses the new feature unusable by older Git is safe enough, but it may be too draconian. For example, when things go wrong, don't you want to "fetch"/"clone" from it into another repository to first save the objects and refs? You do not need a version of the index file you understand in order to do that.

The index format has a mechanism to make older versions of Git bail when it encounters a file that uses newer feature that they do not understand. Perhaps using it is sufficient instead?