treefmt --stdin hangs if no filename is specified

[jfly]

This command hangs indefinitely:

treefmt --stdin

SIGTERM doesn't stop it, I have to send a SIGKILL.

Expected behavior

treefmt should never hang indefinitely. In the specific case above, I'd expect to see this error message:

only one path should be specified when using the --stdin flag

System information

I'm using the latest version of treefmt (at time of writing):

$ treefmt --version
treefmt v2.0.5

Additional context

• I dug into this, and we appear to be getting stuck here waiting for some goroutines (am I using that word correctly? sorry, total go noob here) to terminate: https://github.com/numtide/treefmt/blob/v2.0.5/cli/format.go#L196
• Specifically, the applyFormatters goroutine is stuck forever in this for loop reading from f.filesCh
• The contract seems to be that the walkFilesystem goroutine is responsible for closing f.filesCh, but it exited here before it sets up a defer to close the channel at exit.
• IIUC, the fix here is as simple as moving the defer close(f.filesCh) to the very top of the walkFilesystem goroutine, so there's no chance for it to return without closing f.filesCh. This is a very simple change, so I'll send in a PR with the change.