test(roms): add fixture-driven multi-file ROM download test

Add a shared `multi_file_rom` fixture (a game folder with multiple
RomFile rows) and an endpoint-level test that downloads it via
`GET /api/roms/{id}/content/{file_name}`. This exercises the multi-file
download path end-to-end, which builds each mod_zip manifest entry from
`file.rom.full_path` after the handler session has closed — the exact
path that 500'd with `DetachedInstanceError` before the backref fix.

The download endpoint had no test coverage for multi-file ROMs (the
`rom` fixture has no RomFile rows), which is why the regression slipped
through. Reuse the new fixture in the handler-level regression test too.

https://claude.ai/code/session_01PSXKmejPRzdxLFMN6P2QQ4

backend/tests/conftest.py

@@ -23,7 +23,7 @@ from models.device import Device
 from models.device_save_sync import DeviceSaveSync
 from models.platform import Platform
 from models.play_session import PlaySession
-from models.rom import Rom
+from models.rom import Rom, RomFile
 from models.sync_session import SyncSession
 from models.user import Role, User
 
@@ -47,6 +47,7 @@ def clear_database():
         s.query(Save).delete(synchronize_session="evaluate")
         s.query(State).delete(synchronize_session="evaluate")
         s.query(Screenshot).delete(synchronize_session="evaluate")
+        s.query(RomFile).delete(synchronize_session="evaluate")
         s.query(Rom).delete(synchronize_session="evaluate")
         s.query(Platform).delete(synchronize_session="evaluate")
         s.query(User).delete(synchronize_session="evaluate")
@@ -88,6 +89,41 @@ def rom(admin_user: User, platform: Platform):
     return rom
 
 
+@pytest.fixture
+def multi_file_rom(admin_user: User, platform: Platform):
+    """A ROM stored as a game folder with multiple files (e.g. multi-disc).
+
+    Exercises the multi-file download path, where each entry's download name is
+    derived from `file.rom.full_path` — the back-reference that must remain
+    usable after the handler session closes.
+    """
+    rom = Rom(
+        platform_id=platform.id,
+        name="test_multi_file_rom",
+        slug="test_multi_file_rom_slug",
+        fs_name="test_multi_file_rom",
+        fs_name_no_tags="test_multi_file_rom",
+        fs_name_no_ext="test_multi_file_rom",
+        fs_extension="",
+        fs_path=f"{platform.slug}/roms",
+    )
+    rom = db_rom_handler.add_rom(rom)
+    db_rom_handler.add_rom_user(rom_id=rom.id, user_id=admin_user.id)
+
+    folder_path = f"{rom.fs_path}/{rom.fs_name}"
+    for file_name in ("disc1.bin", "disc2.bin"):
+        db_rom_handler.add_rom_file(
+            RomFile(
+                rom_id=rom.id,
+                file_name=file_name,
+                file_path=folder_path,
+                file_size_bytes=1,
+            )
+        )
+
+    return db_rom_handler.get_rom(rom.id)
+
+
 @pytest.fixture
 def save(rom: Rom, platform: Platform, admin_user: User):
     """Slot-bound save (the canonical device-uploaded shape).

backend/tests/endpoints/roms/test_rom.py

@@ -38,6 +38,29 @@ def test_get_rom(client: TestClient, access_token: str, rom: Rom):
     assert body["id"] == rom.id
 
 
+def test_download_multi_file_rom_content(
+    client: TestClient, access_token: str, multi_file_rom: Rom
+):
+    """Downloading a multi-file (game folder) ROM must not 500.
+
+    The download endpoint builds each manifest entry's name from
+    `file.rom.full_path` after the handler session has closed; a missing
+    `RomFile.rom` back-reference previously raised `DetachedInstanceError`.
+    """
+    response = client.get(
+        f"/api/roms/{multi_file_rom.id}/content/{multi_file_rom.fs_name}",
+        headers={"Authorization": f"Bearer {access_token}"},
+    )
+
+    assert response.status_code == status.HTTP_200_OK
+    # mod_zip manifest: one line per file, plus a generated .m3u playlist.
+    assert response.headers["X-Archive-Files"] == "zip"
+    body = response.text
+    assert "disc1.bin" in body
+    assert "disc2.bin" in body
+    assert f"{multi_file_rom.fs_name}.m3u" in body
+
+
 def test_get_all_roms(
     client: TestClient, access_token: str, rom: Rom, platform: Platform
 ):

backend/tests/handler/test_db_handler.py

@@ -14,7 +14,7 @@ from handler.database import (
 )
 from models.assets import Save, Screenshot, State
 from models.platform import Platform
-from models.rom import Rom, RomFile
+from models.rom import Rom
 from models.user import Role, User
 
 
@@ -73,35 +73,24 @@ def test_roms(rom: Rom, platform: Platform):
     assert len(roms) == 1
 
 
-def test_multi_file_rom_backref_survives_session_close(rom: Rom, platform: Platform):
+def test_multi_file_rom_backref_survives_session_close(multi_file_rom: Rom):
     """Multi-file ROM downloads read `file.rom.full_path` after the handler
     session closes. The detail loaders eager-load `Rom.files` but not the
     reverse `RomFile.rom` relationship, so without the backref being populated
     this raises `DetachedInstanceError` (a 500 on the download endpoint).
     """
-    folder_path = f"{rom.fs_path}/{rom.fs_name}"
-    for file_name in ("disc1.bin", "disc2.bin"):
-        db_rom_handler.add_rom_file(
-            RomFile(
-                rom_id=rom.id,
-                file_name=file_name,
-                file_path=folder_path,
-                file_size_bytes=1,
-            )
-        )
+    folder_path = f"{multi_file_rom.fs_path}/{multi_file_rom.fs_name}"
 
-    fetched = db_rom_handler.get_rom(rom.id)
-    assert fetched is not None
-    assert len(fetched.files) == 2
-
-    # These all dereference `file.rom` on a now-detached instance.
-    for file in fetched.files:
+    # `multi_file_rom` is returned by `get_rom`, with the session already closed.
+    assert len(multi_file_rom.files) == 2
+    for file in multi_file_rom.files:
+        # All of these dereference `file.rom` on a now-detached instance.
         assert file.rom.full_path == folder_path
         assert file.is_top_level
         assert file.file_name_for_download() == file.file_name
 
     # `get_roms_by_ids` (used by the bulk zip download) must behave the same.
-    by_ids = db_rom_handler.get_roms_by_ids([rom.id])
+    by_ids = db_rom_handler.get_roms_by_ids([multi_file_rom.id])
     assert len(by_ids) == 1
     for file in by_ids[0].files:
         assert file.rom.full_path == folder_path