stop triggering multiple nightlies off the same revision

.taskcluster.yml

@@ -185,6 +185,7 @@ tasks:
                               MOBILE_HEAD_REV: '${head_sha}'
                               MOBILE_REPOSITORY_TYPE: git
                               MOBILE_PIP_REQUIREMENTS: taskcluster/requirements.txt
+                              MOZ_AUTOMATION: "1"
                               REPOSITORIES: {$json: {mobile: "Fenix"}}
                               HG_STORE_PATH: /builds/worker/checkouts/hg-store
                               ANDROID_SDK_ROOT: /builds/worker/android-sdk

taskcluster/fenix_taskgraph/target_tasks.py

@@ -2,7 +2,22 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
+import os
+from redo import retry
+
 from taskgraph.target_tasks import _target_task
+from taskgraph.util.taskcluster import find_task_id
+
+
+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
 
 
 @_target_task("release")
@@ -22,6 +37,19 @@ def target_tasks_nightly(full_task_graph, parameters, graph_config):
     def filter(task, parameters):
         return task.attributes.get("nightly", False)
 
+    index_path = (
+        f"{graph_config['trust-domain']}.v2.{parameters['project']}.branch."
+        f"{parameters['head_ref']}.revision.{parameters['head_rev']}.taskgraph.decision-nightly"
+    )
+    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 []
+
     return [l for l, t in full_task_graph.tasks.items() if filter(t, parameters)]
 
 

taskcluster/requirements.in

@@ -3,3 +3,4 @@
 
 taskcluster-taskgraph>=1.3.1
 mozilla-version
+redo