build:none label: a PR that changes no packages only runs the checks

README.md

@@ -110,7 +110,8 @@ of a distribution uses `full_rebuild: true` there. A new build number goes into
 and all of them as soon as it touches anything outside `distros/<distro>/`. To limit
 that (e.g. a humble rebuild that also changes `shared/` or `tools/`), add the label
 `build:humble` (one per distribution to build), then push or re-run the build: adding a
-label doesn't start one by itself. Nothing has to be reset before merging.
+label doesn't start one by itself. Nothing has to be reset before merging. A PR that
+changes no packages (tooling, CI, docs) gets `build:none`: only the checks run.
 
 ### CI
 

tools/robostack.py

@@ -481,10 +481,15 @@ def generate_gha(distro: str, args: list[str]) -> int:
     return 0
 
 
+NO_BUILD_LABEL = "build:none"  # only the checks (tooling, CI or docs changes)
+
+
 def build_distros(changed: list[str], labels: list[str]) -> list[str]:
-    """The distributions a pull request builds: those of its build:<distro> labels when it
-    has any (e.g. a rebuild of one distribution that also touches shared files), else the
-    changed ones."""
+    """The distributions a pull request builds: none with the build:none label, those of
+    its build:<distro> labels when it has any (e.g. a rebuild of one distribution that also
+    touches shared files), else the changed ones."""
+    if NO_BUILD_LABEL in labels:
+        return []
     selected = sorted({label.split(":", 1)[1] for label in labels if label.startswith("build:")})
     unknown = sorted(set(selected) - set(distros()))
     if unknown:

tools/tests/test_bot_commands.py

@@ -230,6 +230,9 @@ def test_build_distros_labels_override_the_changed_ones():
     assert rs.build_distros(["humble", "jazzy"], ["automerge"]) == ["humble", "jazzy"]
     assert rs.build_distros(rs.distros(), ["build:humble", "automerge"]) == ["humble"]
     assert rs.build_distros([], ["build:rolling", "build:humble"]) == ["humble", "rolling"]
+    # only the checks
+    assert rs.build_distros(rs.distros(), ["build:none"]) == []
+    assert rs.build_distros(rs.distros(), ["build:none", "build:humble"]) == []
     with pytest.raises(SystemExit):
         rs.build_distros([], ["build:kinetic"])