pw_presubmit: Default --base to tracking branch

Default 'pw presubmit' to using the current branch's tracking branch as
the base commit for finding changed files (instead of running on all
files).

Bug: 588
Change-Id: I2cdf2919e83b63b5b884cc1cb869e1cdda7a30e5
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/76102
Reviewed-by: John Wiederhirn <wiederhirn@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Commit-Queue: Rob Mohr <mohrr@google.com>

3 files changed, 53 insertions, 3 deletions

diff --git a/pw_presubmit/py/pw_presubmit/cli.py b/pw_presubmit/py/pw_presubmit/cli.py
index 0bb82f869..528143a9f 100644
--- a/pw_presubmit/py/pw_presubmit/cli.py
+++ b/pw_presubmit/py/pw_presubmit/cli.py
@@ -41,7 +41,9 @@ def add_path_arguments(parser) -> None:
         '-b',
         '--base',
         metavar='commit',
-        help='Git revision against which to diff for changed files.')
+        default=git_repo.USE_TRACKING_BRANCH,
+        help=('Git revision against which to diff for changed files. '
+              'Default is the tracking branch of the current branch.'))
     base.add_argument(
         '--full',
         dest='base',
diff --git a/pw_presubmit/py/pw_presubmit/git_repo.py b/pw_presubmit/py/pw_presubmit/git_repo.py
index 5ab5301ab..f2fb22d82 100644
--- a/pw_presubmit/py/pw_presubmit/git_repo.py
+++ b/pw_presubmit/py/pw_presubmit/git_repo.py
@@ -24,6 +24,8 @@ from pw_presubmit.tools import log_run, plural
 _LOG = logging.getLogger(__name__)
 PathOrStr = Union[Path, str]
 
+USE_TRACKING_BRANCH = object()
+
 
 def git_stdout(*args: PathOrStr,
                show_stderr=False,
@@ -55,6 +57,45 @@ def _diff_names(commit: str, pathspecs: Collection[PathOrStr],
         yield git_root / file
 
 
+def tracking_branch(repo_path: Path = None,
+                    suppress_exception: bool = True) -> Optional[str]:
+    """Returns the tracking branch of the current branch.
+
+    Since most callers of this function can safely handle a return value of
+    None, default to suppressing exceptions and returning None when they
+    happen.
+
+    Args:
+        repo_path: repo path from which to run commands; defaults to Path.cwd()
+        suppress_exception: Whether to suppress CalledProcessError exceptions
+
+    Raises:
+        CalledProcessError: HEAD does not point to a branch or the branch is not
+            tracking another branch.
+    """
+    if repo_path is None:
+        repo_path = Path.cwd()
+
+    # This command should raise an exception if repo_path is not a git
+    # repository. Always raise errors in that case.
+    _ = git_stdout('remote', '-v', repo=repo_path)
+
+    # This command should only error out if there's no upstream branch set.
+    try:
+        return git_stdout('rev-parse',
+                          '--abbrev-ref',
+                          '--symbolic-full-name',
+                          '@{u}',
+                          repo=repo_path)
+
+    except subprocess.CalledProcessError:
+        if not suppress_exception:
+            raise
+        _LOG.warning('Error retrieving remote tracking branch of %s',
+                     repo_path)
+        return None
+
+
 def list_files(commit: Optional[str] = None,
                pathspecs: Collection[PathOrStr] = (),
                repo_path: Optional[Path] = None) -> List[Path]:
@@ -71,6 +112,9 @@ def list_files(commit: Optional[str] = None,
     if repo_path is None:
         repo_path = Path.cwd()
 
+    if commit is USE_TRACKING_BRANCH:
+        commit = tracking_branch(repo_path)
+
     if commit:
         try:
             return sorted(_diff_names(commit, pathspecs, repo_path))
diff --git a/pw_presubmit/py/pw_presubmit/presubmit.py b/pw_presubmit/py/pw_presubmit/presubmit.py
index 599ef1736..e64ba3cc2 100644
--- a/pw_presubmit/py/pw_presubmit/presubmit.py
+++ b/pw_presubmit/py/pw_presubmit/presubmit.py
@@ -439,12 +439,16 @@ def run(program: Sequence[Callable],
     files: List[Path] = []
 
     for repo, pathspecs in pathspecs_by_repo.items():
+        repo_base = base
+        if repo_base is git_repo.USE_TRACKING_BRANCH:
+            repo_base = git_repo.tracking_branch(repo)
+
         files += tools.exclude_paths(
-            exclude, git_repo.list_files(base, pathspecs, repo), root)
+            exclude, git_repo.list_files(repo_base, pathspecs, repo), root)
 
         _LOG.info(
             'Checking %s',
-            git_repo.describe_files(repo, repo, base, pathspecs, exclude))
+            git_repo.describe_files(repo, repo, repo_base, pathspecs, exclude))
 
     if output_directory is None:
         output_directory = root / '.presubmit'