fix(vcs): support finding merge-base in git shallow clones

src/taskgraph/util/vcs.py

@@ -388,6 +388,10 @@ def base_rev(self):
     def branch(self):
         return self.run("branch", "--show-current").strip() or None
 
+    @property
+    def is_shallow(self):
+        return self.run("rev-parse", "--is-shallow-repository").strip() == "true"
+
     @property
     def all_remote_names(self):
         remotes = self.run("remote").splitlines()
@@ -546,10 +550,36 @@ def update(self, ref):
         self.run("checkout", ref)
 
     def find_latest_common_revision(self, base_ref_or_rev, head_rev):
-        try:
-            return self.run("merge-base", base_ref_or_rev, head_rev).strip()
-        except subprocess.CalledProcessError:
-            return self.NULL_REVISION
+        def run_merge_base():
+            try:
+                return self.run("merge-base", base_ref_or_rev, head_rev).strip()
+            except subprocess.CalledProcessError:
+                return None
+
+        if not self.is_shallow:
+            return run_merge_base() or self.NULL_REVISION
+
+        rev = run_merge_base()
+        deepen = 10
+        while not rev:
+            self.run("fetch", "--deepen", str(deepen), self.remote_name)
+            rev = run_merge_base()
+            deepen = deepen * 10
+
+            try:
+                self.run("rev-list", "--max-parents=0", head_rev)
+                # We've reached a root commit, stop trying to deepen.
+                break
+            except subprocess.CalledProcessError:
+                pass
+
+        if not rev:
+            # If we still haven't found a merge base, unshallow the repo and
+            # try one last time.
+            self.run("fetch", "--unshallow", self.remote_name)
+            rev = run_merge_base()
+
+        return rev or self.NULL_REVISION
 
     def does_revision_exist_locally(self, revision):
         try:

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
@@ -495,6 +496,58 @@ def test_find_latest_common_revision(repo_with_remote):
         )
 
 
+def test_find_latest_common_revision_shallow_clone(
+    tmpdir, git_repo, default_git_branch
+):
+    """Test finding common revision in a shallow clone that requires deepening."""
+    remote_path = str(tmpdir / "remote_repo")
+    shutil.copytree(git_repo, remote_path)
+
+    # Add several commits to the remote repository to create depth
+    remote_repo = get_repository(remote_path)
+
+    # Create multiple commits to establish depth
+    for i in range(5):
+        test_file = os.path.join(remote_path, f"file_{i}.txt")
+        with open(test_file, "w") as f:
+            f.write(f"content {i}")
+        remote_repo.run("add", test_file)
+        remote_repo.run("commit", "-m", f"Commit {i}")
+
+    # Store the head revision of remote for comparison
+    remote_head = remote_repo.head_rev
+
+    # Create a shallow clone with depth 1
+    # Need to use file:// protocol for --depth to work with local repos
+    shallow_clone_path = str(tmpdir / "shallow_clone")
+    subprocess.check_call(
+        ["git", "clone", "--depth", "1", f"file://{remote_path}", shallow_clone_path]
+    )
+
+    shallow_repo = get_repository(shallow_clone_path)
+    assert shallow_repo.is_shallow
+
+    remote_name = "origin"
+
+    # Create a new commit in the shallow clone to diverge from remote
+    new_file = os.path.join(shallow_clone_path, "local_file.txt")
+    with open(new_file, "w") as f:
+        f.write("local content")
+    shallow_repo.run("add", new_file)
+    shallow_repo.run("commit", "-m", "Local commit")
+
+    # Now try to find the common revision - this should trigger deepening
+    # because the shallow clone doesn't have enough history
+    base_ref = f"{remote_name}/{default_git_branch}"
+    result = shallow_repo.find_latest_common_revision(base_ref, shallow_repo.head_rev)
+
+    # The result should be the remote's head (the common ancestor)
+    assert result == remote_head
+
+    # Verify the repository has been deepened
+    assert shallow_repo.does_revision_exist_locally(result)
+
+
 def test_does_revision_exist_locally(repo):
     first_revision = repo.head_rev