[1288] catch exception from _check_xfrout_available and return SERVFAIL

in that case.  also log that event.  added a test case for it.

src/bin/xfrout/tests/xfrout_test.py.in

@@ -656,6 +656,13 @@ class TestXfroutSession(TestXfroutSessionBase):
         get_msg = self.sock.read_msg()
         self.assertEqual(get_msg.get_rcode().to_text(), "NOTAUTH")
 
+    def test_dns_xfrout_start_datasrc_servfail(self):
+        def internal_raise(x, y):
+            raise isc.datasrc.Error('exception for the sake of test')
+        self.xfrsess.ClientClass = internal_raise
+        self.xfrsess.dns_xfrout_start(self.sock, self.mdata)
+        self.assertEqual(self.sock.read_msg().get_rcode(), Rcode.SERVFAIL())
+
     def test_dns_xfrout_start_noerror(self):
         self.xfrsess._get_query_zone_name = self.default
         def noerror(form):

src/bin/xfrout/xfrout.py.in

@@ -363,7 +363,12 @@ class XfroutSession():
         zone_str = format_zone_str(zone_name, zone_class) # for logging
 
         # TODO: we should also include class in the check
-        rcode_ = self._check_xfrout_available(zone_name)
+        try:
+            rcode_ = self._check_xfrout_available(zone_name)
+        except Exception as ex:
+            logger.error(XFROUT_XFR_TRANSFER_CHECK_ERROR, self._request_type,
+                         format_addrinfo(self._remote), zone_str, ex)
+            rcode_ = Rcode.SERVFAIL()
         if rcode_ != Rcode.NOERROR():
             logger.info(XFROUT_AXFR_TRANSFER_FAILED, self._request_type,
                         format_addrinfo(self._remote), zone_str, rcode_)

src/bin/xfrout/xfrout_messages.mes

@@ -25,6 +25,13 @@ an AXFR query. The error message of the exception is included in the
 log message, but this error most likely points to incomplete exception
 handling in the code.
 
+% XFROUT_XFR_TRANSFER_CHECK_ERROR %1 client %2: check for transfer of %3 failed: %4
+Pre-response check for an incomding XFR request failed unexpectedly.
+The most likely cause of this is that some low level error in the data
+source, but it may also be other general (more unlikely) errors such
+as memory shortage.  Some detail of the error is also included in the
+message.  The xfrout server tries to return a SERVFAIL response in this case.
+
 % XFROUT_AXFR_TRANSFER_FAILED %1 client %2: transfer of %3 failed, rcode: %4
 A transfer out for the given zone failed. An error response is sent
 to the client. The given rcode is the rcode that is set in the error