Merge ~blake-rouse/maas:fix-1754493 into maas:master

[Blake Rouse]

Fixes LP: #1754493 - Call import boot images again if trigger for import when an import is already running.

This fixes an issue where the rack could still be performing an update with old data when the region triggers the rack to import again. This allows up to 1 import to run after the current import is finished. This will ensure that the rack controller will get always match the regiond images.

[MAAS Lander (maas-lander)]

UNIT TESTS
-b fix-1754493 lp:~blake-rouse/maas/+git/maas into -b master lp:~maas-committers/maas

STATUS: SUCCESS
COMMIT: 5b917d8b9d56202b56194b325668abef07d20abb

review: Approve

[Blake Rouse (blake-rouse)]

Passed in the CI twice, latest run:

http://162.213.35.104:8080/job/maas-git-manual/192/console

[Mike Pontillo (mpontillo)]

Interesting and subtle problem. Fix looks good!

review: Approve

[MAAS Lander (maas-lander)]

LANDING
-b fix-1754493 lp:~blake-rouse/maas/+git/maas into -b master lp:~maas-committers/maas

STATUS: FAILED BUILD
LOG: http://maas-ci-jenkins.internal:8080/job/maas/job/branch-tester/1985/consoleText

[MAAS Lander (maas-lander)]

LANDING
-b fix-1754493 lp:~blake-rouse/maas/+git/maas into -b master lp:~maas-committers/maas

STATUS: FAILED BUILD
LOG: http://maas-ci-jenkins.internal:8080/job/maas/job/branch-tester/1986/consoleText

diff --git a/src/provisioningserver/rpc/boot_images.py b/src/provisioningserver/rpc/boot_images.py
index 317d01f..e1ebdf4 100644
--- a/src/provisioningserver/rpc/boot_images.py
+++ b/src/provisioningserver/rpc/boot_images.py
@@ -129,7 +129,13 @@ def _run_import(sources, http_proxy=None, https_proxy=None):
 def import_boot_images(sources, http_proxy=None, https_proxy=None):
     """Imports the boot images from the given sources."""
     lock = concurrency.boot_images
-    if not lock.locked:
+    # This checks if any other defer is already waiting. If nothing is waiting
+    # then add the _import again. If its already waiting nothing is added.
+    #
+    # This is important to how this functions. If the rackd is already
+    # importing images and the regiond triggers another import then after the
+    # original import another will be fired.
+    if not lock.waiting:
         return lock.run(
             _import_boot_images, sources, http_proxy=http_proxy,
             https_proxy=https_proxy)
diff --git a/src/provisioningserver/rpc/tests/test_boot_images.py b/src/provisioningserver/rpc/tests/test_boot_images.py
index 5cbc734..0d45b79 100644
--- a/src/provisioningserver/rpc/tests/test_boot_images.py
+++ b/src/provisioningserver/rpc/tests/test_boot_images.py
@@ -272,20 +272,29 @@ class TestImportBootImages(MAASTestCase):
     run_tests_with = MAASTwistedRunTest.make_factory(timeout=5)
 
     @defer.inlineCallbacks
-    def test__does_not_run_if_lock_taken(self):
+    def test__add_to_waiting_if_lock_already_held(self):
         yield concurrency.boot_images.acquire()
-        self.addCleanup(concurrency.boot_images.release)
         deferToThread = self.patch(boot_images, 'deferToThread')
         deferToThread.return_value = defer.succeed(None)
-        yield import_boot_images(sentinel.sources)
+        d = import_boot_images(sentinel.sources)
+        self.assertEqual(1, len(concurrency.boot_images.waiting))
+        concurrency.boot_images.release()
+        yield d
         self.assertThat(
-            deferToThread, MockNotCalled())
+            deferToThread, MockCalledOnceWith(
+                _run_import, sentinel.sources,
+                http_proxy=None, https_proxy=None))
 
     @defer.inlineCallbacks
-    def test__calls__run_import_using_deferToThread(self):
+    def test__never_more_than_one_waiting(self):
+        yield concurrency.boot_images.acquire()
         deferToThread = self.patch(boot_images, 'deferToThread')
         deferToThread.return_value = defer.succeed(None)
-        yield import_boot_images(sentinel.sources)
+        d = import_boot_images(sentinel.sources)
+        self.assertIsNone(import_boot_images(sentinel.sources))
+        self.assertEqual(1, len(concurrency.boot_images.waiting))
+        concurrency.boot_images.release()
+        yield d
         self.assertThat(
             deferToThread, MockCalledOnceWith(
                 _run_import, sentinel.sources,