Don't fail config loading on missing min. ltime

If the list of branches for a project includes items that are not (yet)
in the min. ltimes for a layout state we can end up in a situation where
a scheduler is unable to start.

2022-05-16 17:34:50,895 ERROR zuul.Scheduler: Error starting Zuul:
Traceback (most recent call last):
  File "/opt/zuul/lib/python3.8/site-packages/zuul/cmd/scheduler.py", line 98, in run
    self.sched.prime(self.config)
  File "/opt/zuul/lib/python3.8/site-packages/zuul/scheduler.py", line 931, in prime
    tenant = loader.loadTenant(
  File "/opt/zuul/lib/python3.8/site-packages/zuul/configloader.py", line 2530, in loadTenant
    new_tenant = self.tenant_parser.fromYaml(
  File "/opt/zuul/lib/python3.8/site-packages/zuul/configloader.py", line 1631, in fromYaml
    self._cacheTenantYAML(abide, tenant, loading_errors, min_ltimes,
  File "/opt/zuul/lib/python3.8/site-packages/zuul/configloader.py", line 1896, in _cacheTenantYAML
    pb_ltime = min_ltimes[project.canonical_name][branch]
KeyError: 'new_branch'

The difference could be due to a missed branch creation event or a
simple race condition. The later case might fix itself after the
reconfig triggered by the branch creation event was processed.

Change-Id: I1838e66bc5296f153aa4c7a83ac0addb6c4db1aa

tests/unit/test_configloader.py

@@ -211,6 +211,31 @@ class TestTenantSimple(TenantParserTestCase):
                 r'review.example.com/org/project2 @master.*',
                 update_logs.output)
 
+    def test_cache_new_branch(self):
+        first = self.scheds.first
+        lock1 = first.sched.layout_update_lock
+        lock2_ = first.sched.run_handler_lock
+        with lock1, lock2_:
+            self.create_branch('org/project1', 'stable')
+            self.fake_gerrit.addEvent(
+                self.fake_gerrit.getFakeBranchCreatedEvent(
+                    'org/project1', 'stable'))
+
+            second = self.createScheduler()
+            second.start()
+            self.assertEqual(len(self.scheds), 2)
+            for _ in iterate_timeout(10, "until priming is complete"):
+                state_one = first.sched.local_layout_state.get("tenant-one")
+                if state_one:
+                    break
+
+            for _ in iterate_timeout(
+                    10, "all schedulers to have the same layout state"):
+                if (second.sched.local_layout_state.get(
+                        "tenant-one") == state_one):
+                    break
+        self.waitUntilSettled()
+
     def test_variant_description(self):
         tenant = self.scheds.first.sched.abide.tenants.get('tenant-one')
         job = tenant.layout.jobs.get("project2-job")

zuul/configloader.py

@@ -1907,7 +1907,12 @@ class TenantParser(object):
                         project.canonical_name, branch)
                     branch_cache = abide.getUnparsedBranchCache(
                         project.canonical_name, branch)
-                    pb_ltime = min_ltimes[project.canonical_name][branch]
+                    try:
+                        pb_ltime = min_ltimes[project.canonical_name][branch]
+                    except KeyError:
+                        self.log.exception(
+                            "Min. ltime missing for project/branch")
+                        pb_ltime = -1
 
                     # If our unparsed branch cache is valid for the
                     # time, then we don't need to do anything else.