fix(test): empty --changed-files is uncertainty (run all), not skip-all

Author: sbryngelson

toolchain/mfc/test/coverage.py

@@ -144,7 +144,13 @@ def get_changed_files(root_dir, compare_branch="master", explicit: Optional[str]
     Otherwise use git merge-base, self-healing a shallow clone with a deepen+retry.
     """
     if explicit is not None:
-        return {f for f in explicit.replace(",", " ").split() if f.strip()}
+        files = {f for f in explicit.replace(",", " ").split() if f.strip()}
+        if files:
+            return files
+        # explicit was given but empty/whitespace -> ambiguous (a paths-filter/env failure vs
+        # genuinely nothing). Per the soundness invariant, uncertainty must run, not skip:
+        # fall through to git detection, ultimately None -> rung 1 (run all). Never return
+        # an empty set here, which would be read as "nothing changed -> skip all".
     try:
         base = _merge_base(root_dir, compare_branch)
         if base is None:

toolchain/mfc/test/test_coverage_unit.py

@@ -267,3 +267,15 @@ def test_sim_include_fpp_forces_all():
     # gcov can't reliably attribute #:include'd files; any src include change runs all.
     assert is_always_run_all({"src/simulation/include/inline_riemann.fpp"})
     assert is_always_run_all({"src/pre_process/include/2dHardcodedIC.fpp"})
+
+
+def test_empty_explicit_is_uncertainty_not_skipall():
+    # An empty/whitespace --changed-files must NOT become an empty set (skip-all under
+    # enforce). It falls through to git detection -> None when that fails -> run all.
+    def fail_git(cmd, **kw):
+        rc = 1 if (len(cmd) > 1 and cmd[1] == "merge-base") else 0
+        return _types.SimpleNamespace(returncode=rc, stdout="", stderr="x")
+
+    with patch("subprocess.run", fail_git):
+        assert get_changed_files("/r", "master", explicit="") is None
+        assert get_changed_files("/r", "master", explicit="  ,  ") is None