Re: [PATCH 0/2] describe and diff: implement --no-optional-locks

[Junio C Hamano <gitster@pobox.com>]

"Benjamin Woodruff via GitGitGadget" <gitgitgadget@gmail.com>
writes:

> git describe and git diff may update the index in the background for similar
> performance reasons to git-status.

That is a wrong reasoning that is completely opposite, though.

The commands at the Porcelain level, like "status" and "diff",
refresh the index for the CORRECTNESS purposes.

The commands at the plumbing level, which are designed to be used in
your own scripts, like "diff-files" and "diff-index", do not refresh
the index for the performance purposes.  If your own script wants to
produce correct result, your script IS responsible for refreshing
the index after its last modification to working tree files before
it starts to use the plumbing commands to inspect which ones are
modified and which ones are not.  This is so that your script has
more control over when the index is refreshed.  It does not have to
pay cost to run refresh for each Git command it invokes, if it knows
that it does not make any modification between the two invocations;
it can instead refresh just once and then run these two plumbing
commands.

So, it would be absolute no-no to make the Porcelain commands like
"describe" and "diff" not to refresh the index before they work by
default.  As an optional behaviour, it might be acceptable if there
is no other reasonable solutions (like, using plumbing commands if
the callers are not you typing but your scripts calling them).