Add a "Conflict" exception

Provider drivers may need to raise a "Conflict" exception. For example
when the requested VIP address is already in use as is the case in the
related bug.
This patch adds this exception for provider drivers to use.

Partial-Bug: #2052682
Change-Id: I42e14233eb040412774db61f211603f7b6528341

octavia_lib/api/drivers/exceptions.py

@@ -203,3 +203,29 @@ class NotFound(Exception):
         self.operator_fault_string = kwargs.pop('operator_fault_string',
                                                 self.operator_fault_string)
         super().__init__(self.user_fault_string, *args, **kwargs)
+
+
+class Conflict(Exception):
+    """Exception raised when the driver has a resource conflict.
+
+    This exception includes two strings: The user fault string and the
+    optional operator fault string. The user fault string,
+    "user_fault_string", will be provided to the API requester. The operator
+    fault string, "operator_fault_string",  will be logged in the Octavia API
+    log file for the operator to use when debugging.
+
+    :param user_fault_string: String provided to the API requester.
+    :type user_fault_string: string
+    :param operator_fault_string: Optional string logged by the Octavia API
+      for the operator to use when debugging.
+    :type operator_fault_string: string
+    """
+    user_fault_string = _("The provider driver had a resource conflict.")
+    operator_fault_string = _("The provider driver had a resource conflict.")
+
+    def __init__(self, *args, **kwargs):
+        self.user_fault_string = kwargs.pop('user_fault_string',
+                                            self.user_fault_string)
+        self.operator_fault_string = kwargs.pop('operator_fault_string',
+                                                self.operator_fault_string)
+        super().__init__(self.user_fault_string, *args, **kwargs)

octavia_lib/tests/unit/api/drivers/test_exceptions.py

@@ -99,3 +99,14 @@ class TestProviderExceptions(base.TestCase):
         self.assertEqual(self.operator_fault_string,
                          not_found_error.operator_fault_string)
         self.assertIsInstance(not_found_error, Exception)
+
+    def test_Conflict(self):
+        conflict_error = exceptions.Conflict(
+            user_fault_string=self.user_fault_string,
+            operator_fault_string=self.operator_fault_string)
+
+        self.assertEqual(self.user_fault_string,
+                         conflict_error.user_fault_string)
+        self.assertEqual(self.operator_fault_string,
+                         conflict_error.operator_fault_string)
+        self.assertIsInstance(conflict_error, Exception)

releasenotes/notes/Add-Conflict-exception-67bca85f9d660a36.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Adds a "Conflict" exception drivers can raise when a resource conflict
+    occurs. This is part of a fix for error reporting when the requested VIP
+    address is already in use on the subnet.