Bug 1380123 - mitigate duplicate nightly graphs off the same revision…

…. r=taskgraph-reviewers,jmaher

We can still get multiple nightly graphs off the same revision, if
the 2nd cron task runs before the 1st one finishes. However, once
the 1st finishes, we will prevent a 2nd nightly graph. (This has the
side effect of not being able to trigger a 2nd nightly graph off the
same revision, even if the first nightly graph failed, but I think
that's still an improvement. We can work around this with a new push,
or triggering the platform-specific nightly graphs.)

Differential Revision: https://phabricator.services.mozilla.com/D118543

taskcluster/taskgraph/target_tasks.py

@@ -7,13 +7,16 @@
 from __future__ import absolute_import, print_function, unicode_literals
 
 import copy
+import os
 import re
 
 import six
+from redo import retry
 from taskgraph import try_option_syntax
 from taskgraph.parameters import Parameters
 from taskgraph.util.attributes import match_run_on_projects, match_run_on_hg_branches
 from taskgraph.util.platforms import platform_family
+from taskgraph.util.taskcluster import find_task_id
 
 _target_task_methods = {}
 
@@ -57,6 +60,17 @@ def get_method(method):
     return _target_task_methods[method]
 
 
+def index_exists(index_path, reason=""):
+    print(f"Looking for existing index {index_path} {reason}...")
+    try:
+        task_id = find_task_id(index_path)
+        print(f"Index {index_path} exists: taskId {task_id}")
+        return True
+    except KeyError:
+        print(f"Index {index_path} doesn't exist.")
+        return False
+
+
 def filter_out_shipping_phase(task, parameters):
     return (
         # nightly still here because of geckodriver
@@ -908,6 +922,19 @@ def filter(task):
 def target_tasks_nightly_desktop(full_task_graph, parameters, graph_config):
     """Select the set of tasks required for a nightly build of linux, mac,
     windows."""
+    index_path = (
+        f"{graph_config['trust-domain']}.v2.{parameters['project']}.revision."
+        f"{parameters['head_rev']}.taskgraph.decision-nightly-desktop"
+    )
+    if os.environ.get("MOZ_AUTOMATION") and retry(
+        index_exists,
+        args=(index_path,),
+        kwargs={
+            "reason": "to avoid triggering multiple nightlies off the same revision",
+        },
+    ):
+        return []
+
     # Tasks that aren't platform specific
     release_filter = make_desktop_nightly_filter({None})
     release_tasks = [