Merge ~r00ta/maas:lp-2034014-add-retry-header-for-409-responses-3.3 into maas:3.3

[Jacopo Rota]

add retry-after header in 409 responses - cherry pick e8aa5d01a5272c536a2aa3e23bd3fc11b580bd90

cherry pick of e8aa5d01a5272c536a2aa3e23bd3fc11b580bd90. See https://bugs.launchpad.net/maas/+bug/2034014/comments/6 for more details about this change.

[Jacopo Rota (r00ta)]

review: Approve

[Jacopo Rota (r00ta)]

jenkins: !test

[MAAS Lander (maas-lander)]

UNIT TESTS
-b lp-2034014-add-retry-header-for-409-responses-3.3 lp:~r00ta/maas/+git/maas into -b 3.3 lp:~maas-committers/maas

STATUS: FAILED
LOG: http://maas-ci.internal:8080/job/maas-tester/3520/console
COMMIT: 5c16f5e62b7459b6495270e16d3328f9e44d49e5

review: Needs Fixing

[Jacopo Rota (r00ta)]

jenkins: !test

[MAAS Lander (maas-lander)]

UNIT TESTS
-b lp-2034014-add-retry-header-for-409-responses-3.3 lp:~r00ta/maas/+git/maas into -b 3.3 lp:~maas-committers/maas

STATUS: SUCCESS
COMMIT: 5c16f5e62b7459b6495270e16d3328f9e44d49e5

review: Approve

diff --git a/src/maasserver/utils/tests/test_views.py b/src/maasserver/utils/tests/test_views.py
index e456a1e..f53dfd4 100644
--- a/src/maasserver/utils/tests/test_views.py
+++ b/src/maasserver/utils/tests/test_views.py
@@ -382,6 +382,7 @@ class TestWebApplicationHandler(SerializationFailureTestCase):
             MockCallsMatch(call(request), call(request), call(request)),
         )
         self.assertIsInstance(response, HttpResponseConflict)
+        self.expectThat(response["Retry-After"], Equals("5"))
         self.expectThat(response.status_code, Equals(http.client.CONFLICT))
         self.expectThat(
             response.reason_phrase,
diff --git a/src/maasserver/utils/views.py b/src/maasserver/utils/views.py
index 18b67b9..e20c5e1 100644
--- a/src/maasserver/utils/views.py
+++ b/src/maasserver/utils/views.py
@@ -38,6 +38,8 @@ from provisioningserver.utils.twisted import retries
 
 logger = logging.getLogger(__name__)
 
+RETRY_AFTER_CONFLICT = 5
+
 
 class InternalErrorResponse(BaseException):
     """Exception raised to exit the transaction context.
@@ -149,6 +151,9 @@ class HttpResponseConflict(MAASDjangoTemplateResponse):
 
     def __init__(self, response=None, exc_info=None):
         super().__init__(response=response)
+        # Responses with status code 409 should be retried by the clients
+        # see https://bugs.launchpad.net/maas/+bug/2034014/comments/6 for more information
+        self["Retry-After"] = RETRY_AFTER_CONFLICT
         self.exc_info = exc_info