syscallcompat: OSX: add Mknodat wrapper

Protip: find naked *at syscalls using:

   git grep "syscall." | grep "at(" | grep -v syscallcompat

internal/fusefrontend/fs.go

@@ -205,7 +205,7 @@ func (fs *FS) Mknod(path string, mode uint32, dev uint32, context *fuse.Context)
 		}
 
 		// Create device node
-		err = syscall.Mknodat(int(dirfd.Fd()), cName, uint32(mode), int(dev))
+		err = syscallcompat.Mknodat(int(dirfd.Fd()), cName, uint32(mode), int(dev))
 		if err != nil {
 			nametransform.DeleteLongName(dirfd, cName)
 		}

internal/syscallcompat/sys_darwin.go

@@ -87,6 +87,24 @@ func Unlinkat(dirfd int, path string) (err error) {
 	return syscall.Unlink(path)
 }
 
+// Poor man's Mknodat
+func Mknodat(dirfd int, path string, mode uint32, dev int) (err error) {
+	chdirMutex.Lock()
+	defer chdirMutex.Unlock()
+	if !filepath.IsAbs(path) {
+		oldWd, err := os.Getwd()
+		if err != nil {
+			return err
+		}
+		defer os.Chdir(oldWd)
+	}
+	path, err = dirfdAbs(dirfd, path)
+	if err != nil {
+		return err
+	}
+	return syscall.Mknod(path, mode, dev)
+}
+
 // dirfdAbs transforms the dirfd-relative "path" to an absolute one. If the
 // path is not already absolute, this function will change the working
 // directory. The caller has to chdir back.

internal/syscallcompat/sys_linux.go

@@ -52,3 +52,7 @@ func Renameat(olddirfd int, oldpath string, newdirfd int, newpath string) (err e
 func Unlinkat(dirfd int, path string) error {
 	return syscall.Unlinkat(dirfd, path)
 }
+
+func Mknodat(dirfd int, path string, mode uint32, dev int) (err error) {
+	return syscall.Mknodat(dirfd, path, mode, dev)
+}