dhcp-server: T3544: test newly rendered configuration before restarting daemon

The current DHCP server implementation comes with options (see below) which
allow the user to pass in any arbitrary option(s) in a verbatim way which will
manifest in dhcpd.conf.

The options are:

* set service dhcp-server global-parameters
* set service dhcp-server shared-network-name foo shared-network-parameters
* set service dhcp-server shared-network-name foo subnet 192.0.2.0/25
  subnet-parameters
* set service dhcp-server shared-network-name foo subnet 192.0.2.0/25
  static-mapping ff static-mapping-parameters

Having an invalid configuration will yield:

vyos@vyos# commit
[ service dhcp-server ]
Configuration file errors encountered - check your options!

[[service dhcp-server]] failed

1 files changed, 20 insertions, 3 deletions

diff --git a/src/conf_mode/dhcp_server.py b/src/conf_mode/dhcp_server.py
index 84a8736e8..47df12804 100755
--- a/src/conf_mode/dhcp_server.py
+++ b/src/conf_mode/dhcp_server.py
@@ -18,6 +18,7 @@ import os
 
 from ipaddress import ip_address
 from ipaddress import ip_network
+from shutil import move
 from sys import exit
 
 from vyos.config import Config
@@ -25,6 +26,7 @@ from vyos.configdict import dict_merge
 from vyos.template import render
 from vyos.util import call
 from vyos.util import dict_search
+from vyos.util import run
 from vyos.validate import is_subnet_connected
 from vyos.validate import is_addr_assigned
 from vyos.xml import defaults
@@ -272,10 +274,25 @@ def generate(dhcp):
     if not dhcp or 'disable' in dhcp:
         return None
 
-    # Please see: https://phabricator.vyos.net/T1129 for quoting of the raw parameters
-    # we can pass to ISC DHCPd
-    render(config_file, 'dhcp-server/dhcpd.conf.tmpl', dhcp,
+    # Please see: https://phabricator.vyos.net/T1129 for quoting of the raw
+    # parameters we can pass to ISC DHCPd
+    tmp_file = '/tmp/dhcpd.conf'
+    render(tmp_file, 'dhcp-server/dhcpd.conf.tmpl', dhcp,
            formater=lambda _: _.replace(""", '"'))
+    # XXX: as we have the ability for a user to pass in "raw" options via VyOS
+    # CLI (see T3544) we now ask ISC dhcpd to test the newly rendered
+    # configuration
+    tmp = run(f'/usr/sbin/dhcpd -t -cf {tmp_file}')
+    if tmp > 0:
+        if os.path.exists(tmp_file):
+            os.unlink(tmp_file)
+        raise ConfigError('Configuration file errors encountered - check your options!')
+
+    # Now that we know that the newly rendered configuration is "good" we can
+    # move the temporary configuration to the "real" configuration - we could
+    # also render it two times but that would not be as fast as a move operation
+    move(tmp_file, config_file)
+
     return None
 
 def apply(dhcp):