Re: [PATCH] t1500: ensure current --since= behavior remains

Jeff King [off-list ref] wrote:

On Wed, Feb 10, 2021 at 09:55:43PM +0000, Eric Wong wrote:

quoted

This behavior of git-rev-parse is observed since git 1.8.3.1
at least(*), and likely earlier versions.

At least one git-reliant project in-the-wild relies on this
current behavior of git-rev-parse being able to handle multiple
--since= arguments without squeezing identical results together.
So add a test to prevent the potential for regression in
downstream projects.

I had to read this a few times to understand what "this behavior" meant.
It is just: when given multiple --since options, output a --max-age for
each of them, even though internally, Git's revision traversal will only
use one (in the usual last-one-wins fashion).

I'm not sure if I was just being dense, or if this could be spelled out
more clearly. :)

*shrug* :>  My brain struggles with coherent thought so I'm
surprised anybody is able to understand me at all :x

Out of curiosity, why does the other project want that? From your
mention of libgit2's git__date_parse(), I assume it's something that
wants to parse approxidates into timestamps in a script. Maybe we ought
to provide a more direct and robust way of doing that. We have a similar
need in t0006, but we use a test-helper program for it.

It takes about 5ms for my system to run git-rev-parse once.  I
may be getting multiple approxidates at once, 1-2 in a typical
input (start..end); but a strange or malicious input could have
hundreds/thousands of approxidates to parse.

Thus, I'm batching up all the approxidates into one rev-parse
invocation (up to system argv limits right now).  With the
output lines split into an array, walking the output/input
arrays in parallel will match them up.  Hypothetically, if
rev-parse were to get clever and deduplicate or reject identical
inputs; then the parallel walk would be broken.

(I have no problem in the meantime with this patch, though; any new
method for accomplishing this would want to give other projects time to
adapt to its use).

Yes.  I think I've mentioned some years/decade ago having
general functionality along the lines of "git cat-file --batch"
or fast-import would be nice (even for some existing scripts and
tests shipped with git).  (v)fork+execve is painful even on
Linux.