using NUL dir on Windows messed up my Git

Question

I had this code running in a Node.js codebase for awhile:

if (os.platform() === 'win32') {
    stdout = fs.openSync('NUL', 'a');

it put a ghost file ('NUL') on my machine at the root of the project, and I can't git rm it or delete it permanently. Having this file around seems to make git very unreliable; commits don't seem to work completely.

Does anybody know what do with this?

Answer 1

The good news is: it won't mess up your Git no more.
Because, with Git 2.25 (Q1 2020), Git will forbid pathnames that the platform's filesystem cannot represent on MinGW.

See commit 4dc42c6, commit 98d9b23 (21 Dec 2019) by Johannes Schindelin (dscho).
^{(Merged by Junio C Hamano -- gitster -- in commit 13432fc, 02 Jan 2020)}

mingw: refuse paths containing reserved names

^{Signed-off-by: Johannes Schindelin}

There are a couple of reserved names that cannot be file names on Windows, such as AUX, NUL, etc.
For an almost complete list, see "Naming Files, Paths, and Namespaces: 'Naming Conventions'".

If one would try to create a directory named NUL, it would actually "succeed", i.e. the call would return success, but nothing would be created.

Worse, even adding a file extension to the reserved name does not make it a valid file name.
To understand the rationale behind that behavior, see "What's the deal with those reserved filenames like NUL and CON?" from Raymond Chen.

Let's just disallow them all.

---

Git 2.27 (Q2 2020) fixes another issue with COM0.

See commit 3efc128 (09 Apr 2020), and commit b6852e1 (08 Apr 2020) by Johannes Schindelin (dscho).
See commit a748f3f (08 Apr 2020) by Matthias Aßhauer (rimrul).
^{(Merged by Junio C Hamano -- gitster -- in commit b3eb70e, 22 Apr 2020)}

mingw: do not treat COM0 as a reserved file name

^{Signed-off-by: Johannes Schindelin}

In 4dc42c6c186 ("mingw: refuse paths containing reserved names", 2019-12-21, Git v2.25.0-rc1 -- merge), we started disallowing file names that are reserved, e.g. NUL, CONOUT$, etc.

This included COM<n> where <n> is a digit.

Unfortunately, this includes COM0 but only COM1, ..., COM9 are reserved, according to the official documentation, COM0 is mentioned in the "NT Namespaces" section but it is explicitly omitted from the list of reserved names: https://learn.microsoft.com/en-us/windows/win32/fileio/naming-a-file#naming-conventions

Tests corroborate this: it is totally possible to write a file called com0.c on Windows 10, but not com1.c.

So let's tighten the code to disallow only the reserved COM<n> file names, but to allow COM0 again.

This fixes git-for-windows/git issue 2470 "Cannot add file COM0.c or checkout branch containing COM0.c file".