fix: reject mode-only changes instead of silently dropping them

pr_split/exceptions.py

@@ -121,6 +121,10 @@ class ErrorMsg(StrEnum):
         "Diff contains binary files, which cannot be split into hunks: {files}."
         " Commit them separately and re-run"
     )
+    HUNKLESS_FILES_UNSUPPORTED = (
+        "Diff contains files with no text hunks (mode-only or empty changes), which"
+        " cannot be assigned to a sub-PR: {files}. Commit them separately and re-run"
+    )
     GH_STACK_MISSING = (
         "The gh-stack extension is required for stacked PRs;"
         " run 'gh extension install github/gh-stack'"

pr_split/planner/validator.py

@@ -13,15 +13,20 @@
 
 
 def validate_no_binary_files(parsed_diff: ParsedDiff) -> None:
-    """Refuse diffs with binary files.
+    """Refuse diffs with binary files or hunk-less entries.
 
-    Binary patches carry no hunks, so coverage validation would pass
-    without any group claiming them and the change would silently be
-    dropped from every sub-PR.
+    Binary patches and mode-only changes carry no hunks, so coverage
+    validation would pass without any group claiming them and the change
+    would silently be dropped from every sub-PR.
     """
     binary = [pf.path for pf in parsed_diff.patch_set if pf.is_binary_file]
     if binary:
         raise PlanValidationError(ErrorMsg.BINARY_FILES_UNSUPPORTED(files=", ".join(binary)))
+    # Mode-only changes (chmod +x) and other hunk-less entries have the same
+    # failure mode: nothing for a group to claim, so the change vanishes.
+    hunkless = [pf.path for pf in parsed_diff.patch_set if len(pf) == 0]
+    if hunkless:
+        raise PlanValidationError(ErrorMsg.HUNKLESS_FILES_UNSUPPORTED(files=", ".join(hunkless)))
 
 
 def validate_coverage(groups: list[Group], parsed_diff: ParsedDiff) -> None:

tests/test_validator.py

@@ -275,3 +275,29 @@ def test_stale_whole_file_index_does_not_create_a_phantom_conflict(self) -> None
             _make_group("g2", [_ga("b.py", WHOLE, [0]), _ga("a.py", PARTIAL, [99])], 4),
         ]
         validate_no_conflicts(groups, PlanDAG(groups), {"a.py": 1, "b.py": 1})
+
+
+MODE_ONLY_DIFF = """\
+diff --git a/run.sh b/run.sh
+old mode 100644
+new mode 100755
+diff --git a/a.py b/a.py
+--- a/a.py
++++ b/a.py
+@@ -1 +1 @@
+-x
++y
+"""
+
+
+class TestValidateNoHunklessFiles:
+    def test_mode_only_change_raises(self) -> None:
+        parsed = parse_diff(MODE_ONLY_DIFF)
+        with pytest.raises(PlanValidationError, match=r"no text hunks.*run\.sh"):
+            validate_no_binary_files(parsed)
+
+    def test_validate_plan_rejects_mode_only_even_when_text_hunks_are_covered(self) -> None:
+        parsed = parse_diff(MODE_ONLY_DIFF)
+        groups = [_make_group("g1", [_ga("a.py", WHOLE, [0])], 2)]
+        with pytest.raises(PlanValidationError, match="no text hunks"):
+            validate_plan(groups, parsed, PlanDAG(groups), max_loc=400)