Re: [PATCH 1/6] add open_nofollow() helper

[Jeff King <peff@peff.net>]

On Tue, Feb 16, 2021 at 10:44:00AM -0500, Taylor Blau wrote:

> On Tue, Feb 16, 2021 at 09:54:24AM -0500, Jeff King wrote:
> > This is all on Linux, of course. Perhaps other systems with slower
> > syscalls may be more impacted.
> 
> I timed it on macOS, which (as you know) I don't use for daily
> development, but it's a useful testbed from time to time.
> 
> On your branch, 'git check-attr -a' took 193.4ms with O_NOFOLLOW, and
> 245.3ms without. After touching every .gitattributes file, those numbers
> shot up to 340.9ms and 346.6ms, respectively.
> 
> (All numbers on linux.git, of course).

That seems...really weird. Your "after touching every file" case is
slightly slower, which is totally expected. But why would the _before_
case be so different? It should have made exactly one extra lstat()
call, and replaced a bunch of open(NOFOLLOW) calls with lstat() (and if
those were so expensive, we'd have presumably seen it in the "touching
every file" case which is running _both_ syscalls).

Can you double-check your initial timings?

> There isn't an apples-to-apples comparison between my numbers and yours
> (since my laptop is much slower than yours), but the relative numbers
> are quite clear that only doing a single syscall is worth it in the
> non-pathological case.

I guess in a sense it doesn't matter that much how macos performs. If it
has O_NOFOLLOW, then it's a no-brainer to use it. The bigger question
is: how many platforms don't have it, and what are _their_ syscalls like
(and are they niche enough that we can accept a small slowdown)?

The obvious one we'd care the most about is Windows, but I still hold
out hope that there's some equivalent mechanism there we can use.

-Peff