`SignalFd::set_mask` closes the signal file descriptor

[DJMcNab]

nix/src/sys/signalfd.rs

Lines 103 to 105 in b2e45a9

	pub fn set_mask(&mut self, mask: &SigSet) -> Result<()> {
	_signalfd(Some(self.0.as_fd()), mask, SfdFlags::empty()).map(drop)
	}

This is because _signalfd returns an OwnedFd as of #1874 (cc @JonathanWoollett-Light ):

nix/src/sys/signalfd.rs

Line 52 in b2e45a9

fn _signalfd<F: AsFd>(fd: Option<F>, mask: &SigSet, flags: SfdFlags) -> Result<OwnedFd> {

This has caused issues in Smithay/calloop, and required backing out of the nix 0.27 update (+ #2112, of course).

This can be worked around in user code by using the newly-deprecated signalfd function, so it's not too bad:

-        file.set_mask(&mask)?;
+        // The nix crate is currently broken. We would like to use:
+        // file.set_mask(&mask)?;
+        // but this closes the file descriptor
+        #[allow(deprecated)]
+        signalfd::signalfd(Some(file.as_fd()), &mask, SfdFlags::empty())
+            .map(std::mem::forget)?;