Re: [PATCH 0/2] cat-file: add a --stdin-cmd mode

[Taylor Blau <me@ttaylorr.com>]

On Wed, Jan 19, 2022 at 05:58:40PM +0000, John Cai via GitGitGadget wrote:
> I think this will be useful for other things, e.g. a not-trivial part of
> "cat-file --batch" time is spent on parsing its argument and seeing if it's
> a revision, ref etc.
>
> So we could e.g. add a command that only accepts a full-length 40 character
> SHA-1, or switch the --format output mid-request etc.

I would like to see a more concrete proposal or need for this sort of
thing before we go too far down adding a new mode to a command so
fundamental as cat-file is.

Between your two proposals for other commands that you could add, I am
not convinced that either of them needs to be in cat-file itself. IOW,
you could easily inject another process in between which verifies that
the provided objects are 40 character SHA-1s.

The latter, changing the output format in-process, seems dubious to me.
Is the start-up time of cat-file so slow (and you need to change formats
so often) that the two together are unbearable? I'd be surprised if they
were (and if so, we should focus our efforts on improving Git's start-up
time).

In the meantime, this is quite an invasive way to provide callers a way
to control the output stream. If there really is a need to just force
cat-file to flush more often, perhaps consider designating a special
signal that we could treat as a request to flush the output stream?

Thanks,
Taylor