Re: [PATCH 1/5] t7300: add testcase showing unnecessary traversal into ignored directory

[Jeff King <peff@peff.net>]

On Thu, May 06, 2021 at 10:00:49PM -0700, Elijah Newren wrote:

> > > +               >directory-random-file.txt &&
> > > +               # Put this file under directory400/directory399/.../directory1/
> > > +               depth=400 &&
> > > +               for x in $(test_seq 1 $depth); do
> > > +                       mkdir "tmpdirectory$x" &&
> > > +                       mv directory* "tmpdirectory$x" &&
> > > +                       mv "tmpdirectory$x" "directory$x"
> > > +               done &&
> >
> > Is this expensive/slow loop needed because you'd otherwise run afoul
> > of command-line length limits on some platforms if you tried creating
> > the entire mess of directories with a single `mkdir -p`?
> 
> The whole point is creating a path long enough that it runs afoul of
> limits, yes.
> 
> If we had an alternative way to check whether dir.c actually recursed
> into a directory, then I could dispense with this and just have a
> single directory (and it could be named a single character long for
> that matter too), but I don't know of a good way to do that.  (Some
> possiibilities I considered along that route are mentioned at
> https://lore.kernel.org/git/CABPp-BF3e+MWQAGb6ER7d5jqjcV=kYqQ2stM_oDyaqvonPPPSw@mail.gmail.com/)

I don't have a better way of checking the dir.c behavior. But I think
the other half of Eric's question was: why can't we do this setup way
more efficiently with "mkdir -p"?

I'd be suspicious that it would work portably because of the long path.
But I think the perl I showed earlier would create it in much less time:

  $ touch directory-file
  $ time sh -c '
      for x in $(seq 1 400)
      do
        mkdir tmpdirectory$x &&
	mv directory* tmpdirectory$x &&
	mv tmpdirectory$x directory$x
      done
    '
    real	0m2.222s
    user	0m1.481s
    sys		0m0.816s

  $ time perl -e '
      for (reverse 1..400) {
        my $d = "directory$_";
	mkdir($d) and chdir($d) or die "mkdir($d): $!";
      }
      open(my $fh, ">", "some-file");
    '
    real	0m0.010s
    user	0m0.001s
    sys		0m0.009s

-Peff