buildbot-effects: fix --branch not resolving rev to branch tip

When --branch was specified without --rev, effects_args() called
get_git_rev() which always resolves HEAD of the current checkout,
ignoring the requested branch entirely. This meant running:

  buildbot-effects --branch feature run my-effect

would use the rev from whatever branch was checked out, not from
'feature'.

Resolve the rev from the specified branch via git rev-parse when
--branch is given.

Closes: #583

Author: Mic92

buildbot_effects/buildbot_effects/__init__.py

@@ -69,8 +69,18 @@ def git_get_tag(path: Path, rev: str) -> str | None:
     return None
 
 
+def get_git_branch_rev(path: Path, branch: str) -> str:
+    """Resolve the tip commit of a given branch."""
+    return git_command(["rev-parse", "--verify", branch], path)
+
+
 def effects_args(opts: EffectsOptions) -> dict[str, Any]:
-    rev = opts.rev or get_git_rev(opts.path)
+    if opts.rev:
+        rev = opts.rev
+    elif opts.branch:
+        rev = get_git_branch_rev(opts.path, opts.branch)
+    else:
+        rev = get_git_rev(opts.path)
     short_rev = rev[:7]
     branch = opts.branch or get_git_branch(opts.path)
     repo = opts.repo or opts.path.name

buildbot_effects/tests/test_branch_rev.py

@@ -0,0 +1,61 @@
+"""Regression test for https://github.com/nix-community/buildbot-nix/issues/583
+
+When --branch is specified without --rev, effects_args() should resolve
+the rev from the tip of that branch, not from HEAD of the current checkout.
+"""
+
+from __future__ import annotations
+
+import subprocess
+from typing import TYPE_CHECKING
+
+from buildbot_effects import effects_args
+from buildbot_effects.options import EffectsOptions
+
+if TYPE_CHECKING:
+    from pathlib import Path
+
+
+def _git(repo: Path, *args: str) -> str:
+    return subprocess.run(
+        ["git", "-C", str(repo), *args],
+        check=True,
+        text=True,
+        capture_output=True,
+    ).stdout.strip()
+
+
+def test_branch_resolves_to_branch_tip(tmp_path: Path) -> None:
+    """--branch should resolve rev to the tip of that branch, not HEAD."""
+    repo = tmp_path / "repo"
+    repo.mkdir()
+    _git(repo, "init", "-b", "main")
+    _git(repo, "config", "user.name", "test")
+    _git(repo, "config", "user.email", "test@test")
+
+    # Initial commit on main
+    (repo / "file.txt").write_text("main")
+    _git(repo, "add", ".")
+    _git(repo, "commit", "-m", "initial")
+    main_rev = _git(repo, "rev-parse", "HEAD")
+
+    # Create a feature branch with a new commit
+    _git(repo, "checkout", "-b", "feature")
+    (repo / "file.txt").write_text("feature")
+    _git(repo, "add", ".")
+    _git(repo, "commit", "-m", "feature commit")
+    feature_rev = _git(repo, "rev-parse", "HEAD")
+
+    # Go back to main — HEAD is now main_rev
+    _git(repo, "checkout", "main")
+    assert _git(repo, "rev-parse", "HEAD") == main_rev
+
+    # Ask for --branch=feature without --rev: should get feature_rev, not main_rev
+    opts = EffectsOptions(path=repo, branch="feature")
+    result = effects_args(opts)
+
+    assert result["rev"] == feature_rev, (
+        f"Expected rev from 'feature' branch ({feature_rev[:7]}), "
+        f"got HEAD ({result['rev'][:7]})"
+    )
+    assert result["branch"] == "feature"