fix(ci): reserve the room a squash merge takes

.github/workflows/commits.yml

@@ -23,6 +23,9 @@ jobs:
       - name: Validate Conventional Commit PR title
         env:
           PR_TITLE: ${{ github.event.pull_request.title }}
+          # The squash subject is "<title> (#<number>)", so the title budget
+          # depends on how long the number is.
+          PR_NUMBER: ${{ github.event.pull_request.number }}
         run: make check-pr-title
 
   messages:

scripts/check_pr_title.py

@@ -28,14 +28,43 @@ def _load_commit_policy() -> ModuleType:
 MAX_TITLE_LENGTH = _POLICY.MAX_TITLE_LENGTH
 TITLE_RE = _POLICY.TITLE_RE
 
+# Room for the longest suffix a squash merge is likely to append, used when the
+# PR number is not known (a local ``make check-pr-title``). CI passes the real
+# number and gets an exact budget.
+FALLBACK_SUFFIX_LENGTH = len(" (#0000)")
 
-def validate_title(title: str) -> list[str]:
+
+def squash_suffix(pr_number: str | int | None) -> str | None:
+    """What a squash merge appends to the title, if the PR number is known.
+
+    This repository merges by squash, and GitHub builds that commit's subject
+    as ``<PR title> (#<number>)``. The subject that lands on ``main`` is
+    therefore longer than the title the author typed -- which is the only
+    thing this check used to measure. v1.4.2 shipped a 73-char subject that
+    way, from a title that was a perfectly legal 66, and every open PR then
+    failed ``commit messages`` for a commit none of them wrote.
+    """
+    if pr_number is None or not str(pr_number).strip():
+        return None
+    return f" (#{str(pr_number).strip()})"
+
+
+def validate_title(title: str, *, pr_number: str | int | None = None) -> list[str]:
     title = title.strip()
     if not title:
         return ["missing PR title"]
 
-    if len(title) > MAX_TITLE_LENGTH:
-        return [f"PR title is {len(title)} chars; max is {MAX_TITLE_LENGTH}: {title}"]
+    suffix = squash_suffix(pr_number)
+    reserved = len(suffix) if suffix is not None else FALLBACK_SUFFIX_LENGTH
+    budget = MAX_TITLE_LENGTH - reserved
+
+    if len(title) > budget:
+        appended = f'"{suffix}"' if suffix is not None else f"{reserved} chars"
+        return [
+            f"PR title is {len(title)} chars; max is {budget}: {title}\n"
+            f"  a squash merge appends {appended}, and the commit subject it "
+            f"produces may not exceed {MAX_TITLE_LENGTH}"
+        ]
 
     if not TITLE_RE.match(title):
         allowed = ", ".join(ALLOWED_TYPES)
@@ -56,7 +85,7 @@ def _title_from_args_or_env(argv: list[str]) -> str:
 
 def main(argv: list[str] | None = None) -> int:
     title = _title_from_args_or_env(sys.argv[1:] if argv is None else argv)
-    failures = validate_title(title)
+    failures = validate_title(title, pr_number=os.getenv("PR_NUMBER"))
     if failures:
         print("Pull request title check failed:")
         print("\n".join(failures))

tests/unit/test_scripts/test_check_pr_title.py

@@ -46,11 +46,63 @@ def test_long_pr_title_is_blocked() -> None:
     failures = checker.validate_title(title)
 
     assert len(failures) == 1
-    assert "max is 72" in failures[0]
+    assert "max is 64" in failures[0]
+
+
+def test_title_that_only_fits_before_the_squash_suffix_is_blocked() -> None:
+    """The v1.4.2 regression: a legal title that squashes into an illegal one.
+
+    ``fix(lancedb): cap both caches, give queries their own embed budget`` is
+    66 characters and passed this check. GitHub appended `` (#496)`` on squash
+    and put a 73-character subject on ``main``, which then failed
+    ``commit messages`` on every open PR until each one rebased past it.
+    """
+    checker = _load_checker()
+    title = "fix(lancedb): cap both caches, give queries their own embed budget"
+    assert len(title) == 66
+
+    assert checker.validate_title(title, pr_number=None) != []
+
+    failures = checker.validate_title(title, pr_number=496)
+
+    assert len(failures) == 1
+    assert "max is 65" in failures[0]
+    assert '" (#496)"' in failures[0]
+
+
+def test_title_that_still_fits_with_the_suffix_is_allowed() -> None:
+    checker = _load_checker()
+    title = "fix(lancedb): cap both caches, give queries their own budget"
+    assert len(title) + len(" (#496)") <= checker.MAX_TITLE_LENGTH
+
+    assert checker.validate_title(title, pr_number=496) == []
+
+
+def test_longer_pr_numbers_cost_more_budget() -> None:
+    """The reserve is the real suffix, not a guess."""
+    checker = _load_checker()
+    title = "fix(lancedb): cap both caches, give queries their own embe budget"
+    assert len(title) == 65
+
+    assert checker.validate_title(title, pr_number=496) == []
+    assert checker.validate_title(title, pr_number=4096) != []
 
 
 def test_main_reads_pr_title_environment(monkeypatch) -> None:
     checker = _load_checker()
     monkeypatch.setenv("PR_TITLE", "docs(readme): polish launch highlights")
 
     assert checker.main([]) == 0
+
+
+def test_main_reads_pr_number_environment(monkeypatch) -> None:
+    """CI passes the number; without it the check falls back to a reserve."""
+    checker = _load_checker()
+    monkeypatch.setenv(
+        "PR_TITLE", "fix(lancedb): cap both caches, give queries their own budget"
+    )
+    monkeypatch.setenv("PR_NUMBER", "496")
+    assert checker.main([]) == 0
+
+    monkeypatch.setenv("PR_NUMBER", "496000000")
+    assert checker.main([]) == 1