fix: keep a mister entry whole without a size

Downloader rejects an entry without an integer size (db_entity.py:187)
and compares it after each fetch (fetch_file_worker.py:127). A new hash
the truth gives no size for is withheld and named, not written beside
the old size nor reported as a size removal.

scripts/exporter/misterfpga_exporter.py

@@ -113,6 +113,7 @@ class Exporter(BaseExporter):
         database = json.loads(original, object_pairs_hook=OrderedDict)
         indexed = self._by_path(systems)
         self._written = set()
+        self._unsized: list[str] = []
 
         for path, entry in database.get("files", {}).items():
             fe = indexed.get(path)
@@ -124,9 +125,17 @@ class Exporter(BaseExporter):
                 # entry without one, a new size beside the old hash describes
                 # no file, and the entry is left as MiSTer wrote it.
                 continue
+            size = fe.size()
+            if md5 != entry.get("hash") and not size:
+                # Downloader refuses an entry without an integer size
+                # (db_entity.py:187) and compares it after each fetch
+                # (fetch_file_worker.py:127). The old size beside the new
+                # hash describes the dump the truth rejects, so the entry
+                # stays as MiSTer wrote it.
+                self._unsized.append(path)
+                continue
             self._written.add(id(fe))
             entry["hash"] = md5
-            size = fe.size()
             if size:
                 entry["size"] = size
 
@@ -143,6 +152,14 @@ class Exporter(BaseExporter):
             produced[_ARCHIVE] = archive if unchanged else rebuild_archive(archive, _DB_NAME, text)
         return produced
 
+    def withheld(self) -> list[str]:
+        """Each hash correction left out for want of the new dump's size."""
+        return [
+            f"{path} keeps its hash: the truth states no size for the new "
+            "dump, and the Downloader checks every entry by size"
+            for path in getattr(self, "_unsized", [])
+        ]
+
     def validate(
         self,
         systems: dict[str, NativeSystem],
@@ -169,6 +186,9 @@ class Exporter(BaseExporter):
         for path, fe in indexed.items():
             declared = files.get(path)
             md5 = fe.hash("md5")
-            if declared is not None and md5 and declared.get("hash") != md5:
+            if (
+                declared is not None and md5 and declared.get("hash") != md5
+                and path not in getattr(self, "_unsized", [])
+            ):
                 issues.append(f"hash not applied: {path}")
         return issues

tests/test_exporters.py

@@ -1670,6 +1670,31 @@ class MisterDatabaseComesBackAsWritten(unittest.TestCase):
             export_native._write_produced(Path(tmp), {"a.zip": b"PK\x03\x04\xff"}, {})
             self.assertEqual((Path(tmp) / "a.zip").read_bytes(), b"PK\x03\x04\xff")
 
+    def test_a_new_hash_without_a_size_leaves_the_entry_whole(self):
+        """The Downloader checks each entry by size (fetch_file_worker.py:127):
+        the old size beside the new hash described no file, and the report
+        announced a size removal the database did not carry."""
+        from export_native import landed_changes  # noqa: PLC0415
+
+        database = {"db_id": "bios_db", "files": {
+            "games/X/boot.rom": {"hash": "a" * 32, "size": 100, "url": "u"},
+        }}
+        original = json.dumps(database)
+        scraped = {"systems": {"x": {"files": [{
+            "name": "boot.rom", "destination": "X/boot.rom",
+            "md5": "a" * 32, "size": 100,
+        }]}}}
+        truth = {"systems": {"x": {"files": [{
+            "name": "boot.rom", "path": "X/boot.rom", "md5": "b" * 32, "_cores": ["x"],
+        }]}}}
+        exporter = discover_exporters()["misterfpga"]()
+        systems, report = build_native_model(truth, scraped)
+        produced = exporter.render(systems, report, {"bios_db.json": original}, scraped)
+        self.assertEqual(produced["bios_db.json"], original)
+        self.assertEqual(landed_changes(exporter, systems).records, [])
+        self.assertEqual(len(exporter.withheld()), 1)
+        self.assertEqual(exporter.validate(systems, produced), [])
+
     def test_an_empty_truth_hands_back_the_database(self):
         archive = UPSTREAM_CACHE / "misterfpga" / "bios_db.json.zip"
         if not archive.exists():