Re: [PATCH 3/5] tests: explicitly skip `chmod` calls on Windows

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

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Hi Ævar,

On Mon, 22 Aug 2022, Johannes Schindelin wrote:

> Instead of wasting the run time (even on non-Windows platforms!) to
> determine whether the `MINGW` prereq is set in order to skip the `chmod`
> call or not, we can make `chmod` a no-op explicitly in that `case
> $uname_s` block.
>
> I will make it so.

tl;dr this patch needs to be dropped, without the suggested replacement.

Gaaah!

After struggling with this for much longer than I care to admit, and even
debugging inside the MSYS2 runtime (which is a level boss if there ever
was one), I found out that all about this patch was wrong.

The suggested patch (making `chmod` a no-op) was wrong: the test suite
started failing left and right. Why? Because `chmod` is not a _complete_
no-op. The test suite does not use the `-o` flag you suggested (which
would not make sense in Git's test suite, whether or not you remove file
permissions for "others" than the current user or group, Git behaves
identically), but it _does_ use the `-w`/`+w` flags, and those _are_
respected, even on Windows.

The way the write permission bits are translated to ACLs is admittedly
somewhat magical or all kinds of wrong, too, depending on your point of
view.

But my original patch was also wrong. Why? Because it claimed that `chmod
+x` does not work outside of MSYS2's pseudo Unix root directory tree. And
that's simply not true.

Whether you call `chmod +x C:/Users/avar/my-file.txt` or `chmod +x
/tmp/avar.sh`, it "succeeds" (by silently ignoring the flag that is
inapplicable on Windows, whether or not a file is executable is determined
by its file extension, and yes, that means that shell scripts are never
executable and we have to live with a nasty hack in Git to pretend that
they are, based on their contents starting with a hash-bang line).

So why did I claim that the `chmod +x` invocation does not work outside of
that pseudo Unix root directory tree? Because it actually did not work, at
least for me! But it worked on the build agents. Why? Because I have a
command in my `~/.profile` that mounts several of my Git for Windows SDKs
as "short-cuts" like `/sdk64`, `/sdk32` and the likes (think of these as
bind-mounts). And those `mount` commands did not specify the `noacl` flag.

So what's that `noacl` flag? It is magic, I can tell you. It basically
makes all this pretense work where MSYS2 pretends that we are in a Unix
environment with Unix permissions when we're not actually. The details are
too gnarly and involved to explain, I won't write them up here in order to
avoid putting even more readers to sleep than I must have done already.

So my analysis was based on observation instead of inspection, cutting
corners, and I dearly regret that now. By default, MSYS2 "mounts" the
system drives as `/c`, `/d`, etc with that `noacl` flag, the same as it
mounts the pseudo Unix root as `/`. And that is why the Git test suite
does not throw up every time `chmod +x` is called. And it was a simple
pilot error on my part that caused it to fail on my system, and I did not
realize that the problem was confined to my system, and the bug was in my
`~/.profile` and not in Git's code base.

Working on Git for Windows is never boring.

Ciao,
Johannes