I had to double-check this line, and the equivalent below. Maybe we need a note that root includes a trailing separator, and path does not include a leading separator? If we ever move to some globbing code that doesn't depend on fs.FS, I could see missing that as a mistake we might make.

Naïvely I would've expected this to be filepath.Join. Maybe it should be anyways?

So, in an earlier iteration, I had the same hypothesis thatJoin with an empty string would do the trick. But no, Go strikes again!

Really though, since Join hits the entirety of Clean underneath the hood, we should keep it as string concatenation. I'll do a better job explaining this.

Can you add a comment as to why we're stripping the leading character? I understand it's due to fs.FS not being rooted, but I think we should call it out.

Comment in place and split to separate steps to make it more clear.

This looks like it's changing what we're testing. Previously we were passing in relative paths, and now we're passing in absolute paths. Have we changed what the production code is doing?

Previously afero's OsFs implicitly relied on the current working directory. Eventually it delegates down to Os.Open() and it just kind of happens to work even though it's not supposed to be a valid path inside of an fs.FS filesystem (and happily broke everything when I swapped in a "valid" fs.FS implementation). In fact, the leading path traversal is also not allowed for fs.FS filesystems, which afero.OsFs claims to be.

We're lucky, however, the place that consumes this code is already set up to do the right thing:

workspaces coming back fully-qualified immediately get relative-ized to make sure that they're children, and from that point forward the workspaces are no longer used.

So yes, in this case we are changing what we're testing, but to correct behavior, and I traced through the code to ensure that the consuming location didn't care. This change serves to lock in the current state of the world.

That this combination of edge cases and what amount to bugs in afero worked to begin with astounds me. Like playing Minesweeper with 999 mines and randomly clicking your way to success.