Re: [PATCH 1/1] t/test-lib: make `test_dir_is_empty` more robust

[William Chargin <wchargin@gmail.com>]

Hi,

> This subject line will appear out of context in "git shortlog" output,
> so it's useful to pack in enough information to briefly summarize what
> the change does.

I'm happy to do so. I think that "simplify" is misleading, because this
is a bug fix, not a refactoring. I like your first suggestion, though it
exceeds the 50-character soft limit. What do you think of:

        test_dir_is_empty: find files with newline in name

?

> I don't think "xargs -0" is present on all supported platforms

Wow---I'm shocked that it's not POSIX, but you're right. (That makes
`xargs` so much less useful!)

t/t3600-rm.sh seems to just literally embed the newline into the
argument to `touch`. I can do that. (I intentionally avoided $'' for the
reason that you mention.)

> Not all filesystems support newlines in filenames.  I think we'd want
> to factor out the FUNNYNAMES prereq from there into a test_lazy_prereq
> so this test can be skipped on such filenames.

This makes sense. Would you like me to send in a separate patch to add
this `test_lazy_prereq` to `t/test-lib.sh`, fixing up existing uses (of
which there are several), and then rebase this patch on top of it?

> Another portability gotcha: wc output includes a space on Mac so this
> test would always return true there.

That's good to know. I can use `test -n "$(ls -A1 "$1")"` as you
suggest, although...

> "ls -A" was added in POSIX.1-2017. [...]
> That's very recent, but the widespread implementation it mentions is
> less so.  This would be our first use of "ls -A", so I'd be interested
> to hear from people on more obscure platforms.  It does seem to be
> widespread.

...if you prefer, a variant of `test "$(ls -a1 | wc -l)" -eq 2` should
satisfy all the crtieria that you mention above (POSIX since forever,
should work on Mac). The assumption is that a file with a newline
character may take up more than one line, but every file must take up
_at least_ one line, and we get two lines from `.` and `..`. If this
assumption is false, then I will have learned yet another new thing!

> Can you say a little more about where you ran into this?  Did you
> discover it by code inspection?

Sure. Yes. I have a build script that accepts a target output directory,
and rejects if the directory is nonempty. I used `ls -A | wc -l` to
implement this check. When testing the script with Sharness, I ran
across `test_dir_is_empty`. I was curious about the implementation,
having recently implemented the same test myself. The `egrep` in the
implementation stood out to me as suspicious, and so it was easy to come
up with an explicit counterexample.

> I do think the resulting implementation using -A is simpler.  Thanks
> for writing it.

You're welcome. Thank you for the detailed and helpful review.

Best,
WC