unbound-dhcp-leases-bridge: Implement atomic file replacement

This change no longer renames the file, but removes the old link and
creates a new link for the temporary file. That helps us to jump out of
the code at any point without worrying about cleaning up the temporary
file.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>

config/unbound/unbound-dhcp-leases-bridge

@@ -526,16 +526,22 @@ class UnboundConfigWriter(object):
 	def write_dhcp_leases(self, leases):
 		log.debug("Writing DHCP leases...")
 
-		with tempfile.NamedTemporaryFile(mode="w", delete=False) as f:
+		with tempfile.NamedTemporaryFile(mode="w") as f:
 			for l in leases:
 				for rr in l.rrset:
 					f.write("local-data: \"%s\"\n" % " ".join(rr))
 
+			# Flush the file
+			f.flush()
+
 			# Make file readable for everyone
 			os.fchmod(f.fileno(), stat.S_IRUSR|stat.S_IWUSR|stat.S_IRGRP|stat.S_IROTH)
 
+			# Remove the old file
+			os.unlink(self.path)
+
 			# Move the file to its destination
-			os.rename(f.name, self.path)
+			os.link(f.name, self.path)
 
 	def _control(self, *args):
 		command = ["unbound-control"]