fix: keep a held file in emulator region packs

Emulator mode reduced region groups without knowing which files the
collection holds, so a requested-region image nobody had collected
withdrew the held one and left the slot empty. Platform mode already
kept it; both emulator readers now pass the database.

scripts/generate_pack.py

@@ -1351,7 +1351,7 @@ def generate_emulator_pack(
     os.makedirs(output_dir, exist_ok=True)
 
     region_drops: set[str] = (
-        emulator_region_drops(selected, standalone, regions, required_only)
+        emulator_region_drops(selected, standalone, regions, required_only, db)
         if regions
         else set()
     )

scripts/packextras.py

@@ -1028,6 +1028,7 @@ def emulator_region_drops(
     standalone: bool,
     regions: list[str],
     required_only: bool = False,
+    db: dict | None = None,
 ) -> set[str]:
     """Destinations an emulator-mode `--region` withdraws.
 
@@ -1035,6 +1036,11 @@ def emulator_region_drops(
     a multi-system core such as O2EM keeps a fallback BIOS for every system
     instead of letting one matching system empty another. The emulator pack
     and `verify --emulator` both read this, keyed by pack destination.
+
+    With `db`, a requested-region file the collection lacks never displaces
+    a held one, as in platform mode: a held scph5501.bin was withdrawn for a
+    Japanese image nobody had collected, and the pack shipped no PlayStation
+    BIOS at all.
     """
     import region as region_mod
     from packpaths import _resolve_destination
@@ -1042,6 +1048,7 @@ def emulator_region_drops(
 
     region_index = region_mod.build_region_index(dict(selected))
     region_groups: dict[str, list[tuple[str, str]]] = {}
+    held: set[str] | None = set() if db is not None else None
     for emu_name, profile in sorted(selected):
         # Decided over what the pack keeps, as on the platform side: an
         # optional winner removed afterwards left its slot empty.
@@ -1053,7 +1060,11 @@ def emulator_region_drops(
                 region_groups.setdefault(group_id, []).append(
                     (dest, fe.get("name", ""))
                 )
-    return region_mod.resolve_region_drops(region_groups, region_index, regions)
+                if held is not None and resolve_local_file(
+                    dict(fe, source_profile=emu_name), db, dest_hint=dest
+                )[0]:
+                    held.add(dest)
+    return region_mod.resolve_region_drops(region_groups, region_index, regions, held)
 
 
 def _emulator_region_group(emu_name: str, profile: dict, file_entry: dict) -> str:

scripts/verify.py

@@ -1519,7 +1519,7 @@ def verify_emulator(
     if regions:
         from packextras import emulator_region_drops
 
-        region_drops = emulator_region_drops(selected, standalone, regions)
+        region_drops = emulator_region_drops(selected, standalone, regions, db=db)
 
     # The emulator pack walks profiles in name order and keeps the first
     # file at a path, folding case: a later profile's different file there

tests/test_region.py

@@ -877,5 +877,38 @@ class FontRomsCarryNoTerritory(unittest.TestCase):
         ]
         self.assertEqual(tagged, [])
 
+class EmulatorModeKeepsAHeldFile(unittest.TestCase):
+    """Platform mode never lets a missing file displace a held one; emulator
+    mode did. A held scph5501.bin (north-america) was withdrawn under
+    --region jp for a Japanese image the collection lacks, and the pack
+    shipped no PlayStation BIOS."""
+
+    def test_a_missing_match_does_not_withdraw_the_held_file(self):
+        import hashlib  # noqa: PLC0415
+        import shutil  # noqa: PLC0415
+        import tempfile  # noqa: PLC0415
+
+        root = Path(tempfile.mkdtemp(dir=Path(__file__).resolve().parents[1] / "tmp"))
+        self.addCleanup(shutil.rmtree, root, True)
+        held = root / "scph5501.bin"
+        held.write_bytes(b"us bios")
+        sha1 = hashlib.sha1(b"us bios").hexdigest()
+        db = {"files": {sha1: {"path": str(held), "name": held.name, "sha1": sha1}},
+              "indexes": {"by_name": {held.name: [sha1]}, "by_md5": {},
+                          "by_crc32": {}, "by_path_suffix": {}}}
+        profile = {"systems": ["sony-playstation"], "files": [
+            {"name": "scph5501.bin", "region": ["north-america"]},
+            {"name": "scph5500.bin", "region": ["japan"]},
+        ]}
+        selected = [("psx", profile)]
+        self.assertEqual(
+            emulator_region_drops(selected, False, ["japan"], db=db), set()
+        )
+        # Without the collection to ask, the old reading stands.
+        self.assertEqual(
+            emulator_region_drops(selected, False, ["japan"]), {"scph5501.bin"}
+        )
+
+
 if __name__ == "__main__":
     unittest.main()