feat(decision): support extracting parameters from Git note

Bug: 2028208

Author: ahal

.taskcluster.yml

@@ -226,6 +226,7 @@ tasks:
                               - $if: 'isPullRequest'
                                 then:
                                     TASKGRAPH_PULL_REQUEST_NUMBER: '${event.pull_request.number}'
+                                    TASKGRAPH_EXTRA_REFS: {$json: ["refs/notes/decision-parameters"]}
                               - $if: 'tasks_for == "action" || tasks_for == "pr-action"'
                                 then:
                                     ACTION_TASK_GROUP_ID: '${action.taskGroupId}'  # taskGroupId of the target task
@@ -251,7 +252,11 @@ tasks:
                           - bash
                           - -cx
                           - $let:
-                                extraArgs: {$if: 'tasks_for == "cron"', then: '${cron.quoted_args}', else: ''}
+                                extraArgs:
+                                    $switch:
+                                        'tasks_for == "cron"': '${cron.quoted_args}'
+                                        'tasks_for == "github-pull-request"': '--allow-parameter-override'
+                                        $default: ''
                             in:
                                 $if: 'tasks_for == "action" || tasks_for == "pr-action"'
                                 then: >

src/taskgraph/decision.py

@@ -264,6 +264,20 @@ def get_decision_parameters(graph_config, options):
     ] == "github-pull-request":
         set_try_config(parameters, task_config_file)
 
+    # load extra parameters from vcs note if able
+    note_ref = "refs/notes/decision-parameters"
+    if options.get("allow_parameter_override") and (
+        note_params := repo.get_note(note_ref)
+    ):
+        try:
+            note_params = json.loads(note_params)
+            logger.info(f"Overriding parameters from {note_ref}:\n{json.dumps(note_params, indent=2)}")
+            parameters.update(note_params)
+        except ValueError as e:
+            raise Exception(
+                f"Failed to parse {note_ref} as JSON: {e}"
+            ) from e
+
     result = Parameters(**parameters)
     result.check()
     return result

src/taskgraph/main.py

@@ -933,6 +933,12 @@ def load_task(args):
 @argument(
     "--verbose", "-v", action="store_true", help="include debug-level logging output"
 )
+@argument(
+    "--allow-parameter-override",
+    default=False,
+    action="store_true",
+    help="Allow user to override computed decision task parameters.",
+)
 def decision(options):
     from taskgraph.decision import taskgraph_decision  # noqa: PLC0415
 

src/taskgraph/util/vcs.py

@@ -215,6 +215,14 @@ def does_revision_exist_locally(self, revision: str) -> bool:
         If this function returns an unexpected value, then make sure
         the revision was fetched from the remote repository."""
 
+    def get_note(self, note: str, revision: Optional[str] = None) -> Optional[str]:
+        """Read a note attached to the given revision (defaults to HEAD).
+
+        Returns the note content as a string, or ``None`` if no note exists.
+        Only supported by Git; returns ``None`` for all other VCS types.
+        """
+        return None
+
 
 class HgRepository(Repository):
     @property
@@ -586,6 +594,20 @@ def does_revision_exist_locally(self, revision):
                 return False
             raise
 
+    def get_note(self, note: str, revision: Optional[str] = None) -> Optional[str]:
+        if not note.startswith("refs/notes/"):
+            note = f"refs/notes/{note}"
+
+        revision = revision or "HEAD"
+        try:
+            return self.run(
+                "notes", f"--ref={note}", "show", revision
+            ).strip()
+        except subprocess.CalledProcessError as e:
+            if e.returncode == 1:
+                return None
+            raise
+
 
 def get_repository(path: str):
     """Get a repository object for the repository at `path`.

test/test_decision.py

@@ -10,6 +10,8 @@
 import unittest
 from pathlib import Path
 
+import pytest
+
 from taskgraph import decision
 from taskgraph.util.vcs import GitRepository, HgRepository
 from taskgraph.util.yaml import load_yaml
@@ -137,3 +139,62 @@ def test_dontbuild_commit_message_yields_default_target_tasks_method(
         self.options["tasks_for"] = "hg-push"
         params = decision.get_decision_parameters(FAKE_GRAPH_CONFIG, self.options)
         self.assertEqual(params["target_tasks_method"], "nothing")
+
+
+_BASE_OPTIONS = {
+    "base_repository": "https://hg.mozilla.org/mozilla-unified",
+    "base_rev": "aaaa",
+    "head_repository": "https://hg.mozilla.org/mozilla-central",
+    "head_rev": "bbbb",
+    "head_ref": "default",
+    "head_tag": "",
+    "project": "mozilla-central",
+    "pushlog_id": "1",
+    "pushdate": 0,
+    "repository_type": "git",
+    "owner": "nobody@mozilla.com",
+    "tasks_for": "github-push",
+    "level": "1",
+}
+
+
+@unittest.mock.patch.object(
+    GitRepository,
+    "get_note",
+    return_value=json.dumps({"build_number": 99}),
+)
+@unittest.mock.patch.object(GitRepository, "get_changed_files", return_value=[])
+def test_decision_parameters_note(mock_files_changed, mock_get_note):
+    options = {**_BASE_OPTIONS, "allow_parameter_override": True}
+    params = decision.get_decision_parameters(FAKE_GRAPH_CONFIG, options)
+    mock_get_note.assert_called_once_with("decision-parameters")
+    assert params["build_number"] == 99
+
+
+@unittest.mock.patch.object(
+    GitRepository,
+    "get_note",
+    return_value=json.dumps({"build_number": 99}),
+)
+@unittest.mock.patch.object(GitRepository, "get_changed_files", return_value=[])
+def test_decision_parameters_note_disallow_override(
+    mock_files_changed, mock_get_note
+):
+    options = {**_BASE_OPTIONS, "allow_parameter_override": False}
+    params = decision.get_decision_parameters(FAKE_GRAPH_CONFIG, options)
+    mock_get_note.assert_not_called()
+    assert params["build_number"] == 1
+
+
+@unittest.mock.patch.object(
+    GitRepository,
+    "get_note",
+    return_value="not valid json {",
+)
+@unittest.mock.patch.object(GitRepository, "get_changed_files", return_value=[])
+def test_decision_parameters_note_invalid_json(mock_files_changed, mock_get_note):
+    options = {**_BASE_OPTIONS, "allow_parameter_override": True}
+    with pytest.raises(
+        Exception, match="Failed to parse refs/notes/decision-parameters as JSON"
+    ):
+        decision.get_decision_parameters(FAKE_GRAPH_CONFIG, options)

test/test_util_vcs.py

@@ -3,6 +3,7 @@
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 import os
+import shutil
 import subprocess
 from pathlib import Path
 from textwrap import dedent
@@ -648,3 +649,23 @@ def test_get_changed_files_with_null_base_revision_shallow_clone(
     assert "first_file" in changed_files
     assert "file1.txt" in changed_files
     assert "file2.txt" in changed_files
+
+
+def test_get_note_git(git_repo, tmpdir):
+    """get_note returns note content when present, None otherwise."""
+    repo_path = tmpdir.join("git")
+    shutil.copytree(git_repo, repo_path)
+    repo = get_repository(str(repo_path))
+
+    # No note yet
+    assert repo.get_note("try-config") is None
+
+    rev = repo.head_rev
+    subprocess.check_call(
+        ["git", "notes", "--ref=refs/notes/try-config", "add", "-m", "test note", rev],
+        cwd=repo.path,
+    )
+
+    assert repo.get_note("try-config") == "test note"
+    assert repo.get_note("try-config", rev) == "test note"
+    assert repo.get_note("other") is None