Adding Permissions To MapFS Part 1

I decided to try and commit to golang, because I was feeling brave.

First step was identifying a github issue for a specific problem: The test.MapFS package does not enforce file permissions. I'd found this problem while working on Andrew, and John Arundel kindly identified a github issue tracking the problem.

"Hah", I thought, "This'll be easy."

There's no hubris quite like the hubrjis of estimating while never having working on a problem like the one before you.

Those wiser than me have noted that the moment of beginning is the moment of greatest ignorance.

First I compiled golang. Then I figured out how to source that same build repeatedly. It turned out that vs code was being weird about interacting with the custom build, so I had to add "go.goroot": "/Users/gwyn/Developer/go/bin" to settings.json

Unfortunately, settings.json is global to all instances of vs code, so I have to either remember to turn it off or have mostly broken package imports in other projectsfor a few weeks.

I also wrote a .env file to set up my command line environment

; cat ./src/gwyn.env
PATH=/Users/gwyn/Developer/go/bin:$PATH

I started with adding a test into go/src/testing/fstest/mapfs_test.go

func TestMapFSFileSystemPermissions(t *testing.T) {
	m := MapFS{
		"path/to/a.txt": &MapFile{Mode: 0o000},
	}
	_, err := m.Open("path/to/a.txt") // Use Open instead of Stat
	want := fs.ErrPermission

	if !errors.Is(err, fs.ErrPermission) {
		t.Errorf("MapFS should not open a file with permissions 0o000 want:\n%s\ngot:\n%v\n", want, err)
	}
}

So how do you check file system permissions anyway? Normally this'd be enforced by the kernel (the open system call returns an EACCES error if permission is denied, but a mapFS is just a hashmap that contains a bunch of a type called a MapFile

The kernel source for open() shows that it mostly dumps the work to a second function, after checking for whether it needs a special type to represent the size of a big file. The kernel shows that it populates a struct tracking file permissions, but I can't quite figure out from the kernel source precisely what else is happening.

So, I need to figure out how to verify if a string like "644" and validate it against a set of permissions. Fun!