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

Hi Ævar,

On Mon, 22 Aug 2022, Johannes Schindelin wrote:

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