runner: never embed a fossil screenshot from a collided run dir (#45)

tests/unit/test_screenshot.py

@@ -184,3 +184,78 @@ def test_screenshot_reachable_through_real_load_path(tmp_path):
     hook = S._load_screenshot_hook(meta)
     assert callable(hook), "SCREENSHOT hook did not survive the orchestrator load path (R2)"
     assert S._load_screenshot_hook(meta_mod.load("no-such", tests_dir=str(tmp_path))) is None
+
+
+# ---- capture() fossil guard (2026-10-05) -----------------------------------------------------
+# Drone build numbers restarted when its DB was re-created while /var/lib/cc-ci-runs still holds
+# old-era dirs numbered into the thousands; run 35 (!testme gitea #10) collided with a May-31 run
+# 35 and embedded custom-html-tiny's fossil screenshot.png because the hook branch SKIPPED the
+# snap when out_path already existed. capture() must always snap fresh.
+
+
+class _FakeLaunchable:
+    def __init__(self, page):
+        self._page = page
+        self.chromium = self
+
+    def launch(self, args=None):
+        return self
+
+    def new_context(self, **kw):
+        return self
+
+    def new_page(self):
+        return self._page
+
+    def close(self):
+        pass
+
+
+class _FakeSyncPlaywright:
+    """sync_playwright() replacement: context-manager factory handing out one fake browser."""
+
+    def __init__(self, page):
+        self._page = page
+
+    def __call__(self):
+        return self
+
+    def __enter__(self):
+        return _FakeLaunchable(self._page)
+
+    def __exit__(self, *exc):
+        return False
+
+
+def _install_fake_playwright(monkeypatch, page):
+    import types
+
+    fake_api = types.ModuleType("playwright.sync_api")
+    fake_api.sync_playwright = _FakeSyncPlaywright(page)
+    fake = types.ModuleType("playwright")
+    fake.sync_api = fake_api
+    monkeypatch.setitem(sys.modules, "playwright", fake)
+    monkeypatch.setitem(sys.modules, "playwright.sync_api", fake_api)
+
+
+def test_capture_overwrites_fossil_screenshot(tmp_path, monkeypatch):
+    """A pre-existing out_path in a recycled run artifact dir must never be embedded: the hook
+    branch snaps fresh regardless (the old exists-guard returned the fossil as 'captured')."""
+    out_path = str(tmp_path / "screenshot.png")
+    with open(out_path, "wb") as f:
+        f.write(b"STALE-FOSSIL-FROM-ANOTHER-RECIPE" * 400)
+
+    fired = {}
+    page = _FakePage([35707])  # one real-sized frame, like a painted page
+
+    def hook(p, ctx):
+        fired["yes"] = True  # navigation only — hooks never save the PNG themselves
+
+    _install_fake_playwright(monkeypatch, page)
+    got = S.capture("gitea.example.org", out_path, recipe_meta={"SCREENSHOT": hook})
+
+    assert got == out_path, "capture must report success after snapping fresh"
+    assert fired.get("yes"), "SCREENSHOT hook must run"
+    assert page.shots == 1, "capture must snap exactly once — the fossil is not a captured frame"
+    with open(out_path, "rb") as f:
+        assert f.read(4) == b"\x89PNG", "out_path still holds the fossil — snap did not overwrite"