fix: let two generators purge the coverage cache

scripts/generate_readme.py

@@ -219,13 +219,16 @@ def platform_coverages(
             except FileNotFoundError as exc:
                 print(f"  {name}: skipped ({exc})", file=sys.stderr)
         COVERAGE_CACHE_DIR.mkdir(parents=True, exist_ok=True)
-        for stale in COVERAGE_CACHE_DIR.glob("*.pickle"):
-            stale.unlink()
         scratch = cache.with_suffix(f".{os.getpid()}.tmp")
         with scratch.open("wb") as handle:
             pickle.dump({"platforms": names, "coverages": coverages}, handle,
                         protocol=pickle.HIGHEST_PROTOCOL)
         scratch.replace(cache)
+        # generate_site and generate_readme both purge here. One unlinking
+        # what the other had just listed ended it on FileNotFoundError.
+        for stale in COVERAGE_CACHE_DIR.glob("*.pickle"):
+            if stale != cache:
+                stale.unlink(missing_ok=True)
     ordered = {name: coverages[name] for name in platform_names if name in coverages}
     if report:
         for name in names:

tests/test_coverage_cache.py

@@ -95,6 +95,23 @@ class CoverageCache(unittest.TestCase):
         self._run()
         self.assertEqual(len(list((self.work / "cache").glob("*.pickle"))), 1)
 
+    def test_a_purge_racing_another_generator_does_not_fail(self):
+        """generate_site and generate_readme purge the same directory: one
+        unlinking a pickle the other had listed ended on FileNotFoundError."""
+        cache_dir = self.work / "cache"
+        cache_dir.mkdir()
+        ghost = cache_dir / "gone.pickle"
+        real_glob = Path.glob
+
+        def listing_a_vanished_file(path, pattern):
+            found = list(real_glob(path, pattern))
+            return [*found, ghost] if path == cache_dir else found
+
+        with mock.patch.object(Path, "glob", listing_a_vanished_file):
+            coverages = self._run()
+        self.assertEqual(sorted(coverages), ["alpha", "beta"])
+        self.assertEqual(len(list(cache_dir.glob("*.pickle"))), 1)
+
     def test_a_missing_input_directory_stops_the_run(self):
         """A missing --emulators-dir skipped every platform as if it had no
         YAML, then replaced the cache with the empty result."""