fix(run-task)!: stop fetching / checking out 'base_ref'

BREAKING CHANGE: `base_ref` will no longer be fetched or checked out by
run-task

Taskgraph uses base_rev anyway for computing files changed, so there's
no need to additionally fetch base_ref. Some tasks may need to be
updated to not rely on base_ref being present in the local clone.

Author: ahal

src/taskgraph/run-task/run-task

@@ -609,7 +609,6 @@ def git_checkout(
     destination_path: str,
     head_repo: str,
     base_repo: Optional[str],
-    base_ref: Optional[str],
     base_rev: Optional[str],
     head_ref: Optional[str],
     head_rev: Optional[str],
@@ -664,23 +663,11 @@ def git_checkout(
 
         retry_required_command(b"vcs", args, extra_env=env)
 
-    if base_ref:
-        args = ["git", "fetch", "origin", base_ref]
-
-        retry_required_command(b"vcs", args, cwd=destination_path, extra_env=env)
-
-        # Create local branch so that taskgraph is able to compute differences
-        # between the head branch and the base one, if needed
-        args = ["git", "checkout", base_ref]
-
-        retry_required_command(b"vcs", args, cwd=destination_path, extra_env=env)
-
-    # When commits are force-pushed (like on a testing branch), base_rev doesn't
-    # exist on base_ref. Fetching it allows taskgraph to compute differences
-    # between the previous state before the force-push and the current state.
-    #
-    # Unlike base_ref just above, there is no need to checkout the revision:
-    # it's immediately available after the fetch.
+    # For Github based repos, base_rev often doesn't refer to an ancestor of
+    # head_rev simply due to Github not providing that information in their
+    # webhook events. Therefore we fetch it independently from `head_rev` so
+    # that consumers can compute the merge-base or files modified between the
+    # two as needed.
     if base_rev and base_rev != NULL_REVISION:
         args = ["git", "fetch", "origin", base_rev]
 
@@ -900,7 +887,6 @@ def collect_vcs_options(args, project, name):
 
     repo_type = os.environ.get(f"{env_prefix}_REPOSITORY_TYPE")
     base_repo = os.environ.get(f"{env_prefix}_BASE_REPOSITORY")
-    base_ref = os.environ.get(f"{env_prefix}_BASE_REF")
     base_rev = os.environ.get(f"{env_prefix}_BASE_REV")
     head_repo = os.environ.get(f"{env_prefix}_HEAD_REPOSITORY")
     head_ref = os.environ.get(f"{env_prefix}_HEAD_REF")
@@ -933,7 +919,6 @@ def collect_vcs_options(args, project, name):
         "checkout": checkout,
         "sparse-profile": sparse_profile,
         "base-repo": base_repo,
-        "base-ref": base_ref,
         "base-rev": base_rev,
         "head-repo": head_repo,
         "head-ref": head_ref,
@@ -984,7 +969,6 @@ def vcs_checkout_from_args(options):
                 options["checkout"],
                 options["head-repo"],
                 options["base-repo"],
-                options["base-ref"],
                 options["base-rev"],
                 head_ref,
                 head_rev,

test/test_scripts_run_task.py

@@ -183,7 +183,6 @@ def test_collect_vcs_options(monkeypatch, run_task_mod, env, extra_expected):
 
     expected = {
         "base-repo": env.get("BASE_REPOSITORY"),
-        "base-ref": env.get("BASE_REF"),
         "base-rev": env.get("BASE_REV"),
         "checkout": os.path.join(os.getcwd(), "checkout"),
         "env-prefix": name.upper(),
@@ -354,7 +353,7 @@ def _commit_file(message, filename):
 
 
 @pytest.mark.parametrize(
-    "base_ref,head_ref,files,hash_key",
+    "base_rev,head_ref,files,hash_key",
     [
         (None, None, ["mainfile"], "main"),
         (None, "main", ["mainfile"], "main"),
@@ -367,7 +366,7 @@ def test_git_checkout(
     mock_stdin,
     run_task_mod,
     mock_git_repo,
-    base_ref,
+    base_rev,
     head_ref,
     files,
     hash_key,
@@ -378,8 +377,7 @@ def test_git_checkout(
             destination_path=destination,
             head_repo=mock_git_repo["path"],
             base_repo=mock_git_repo["path"],
-            base_ref=base_ref,
-            base_rev=None,
+            base_rev=base_rev,
             head_ref=head_ref,
             head_rev=None,
             ssh_key_file=None,
@@ -414,7 +412,6 @@ def test_git_checkout_with_commit(
             destination_path=destination,
             head_repo=mock_git_repo["path"],
             base_repo=mock_git_repo["path"],
-            base_ref="mybranch",
             base_rev=mock_git_repo["main"],
             head_ref=mock_git_repo["branch"],
             head_rev=mock_git_repo["branch"],