refactor: compute slot choices once per manifest

generate_manifest called slot_choices, and pack_overrides called it again with the same five arguments. pack_overrides takes the choices when the caller holds them. Measured on generate_pack.py --all --manifest --offline: 48 calls and 7.0 s in slot_choices before, 24 calls and 4.3 s after (wall 45.7 s and 42.4 s, one run each on a loaded host). The manifests written are identical.

scripts/generate_pack.py

@@ -4609,16 +4609,18 @@ def generate_manifest(
     # (cache file, size). Settled once the trees are walked.
     cache_held: dict[str, tuple[str, int]] = {}
     rename_notes: set[str] = set()
+    # The full manifest records every core's answer at a contested slot, so a
+    # target narrowed from it gets what that target's own pack serves.
+    whole = target_cores is None and source != "platform" and bool(emu_profiles)
+    slot_choices = (
+        slots.slot_choices(config, emu_profiles, db, zip_contents, data_registry)
+        if whole else {}
+    )
+    # Derived from the same choices: computed twice, it was 8% of a run.
     slot_overrides = slots.pack_overrides(
         config, emu_profiles, db, zip_contents, data_registry,
         target_cores=target_cores, source=source,
-    )
-    # The full manifest records every core's answer at a contested slot, so a
-    # target narrowed from it gets what that target's own pack serves.
-    slot_choices = (
-        slots.slot_choices(config, emu_profiles, db, zip_contents, data_registry)
-        if target_cores is None and source != "platform" and emu_profiles
-        else {}
+        choices=slot_choices if whole else None,
     )
 
     def manifest_destination(full_destination: str) -> str:

scripts/slots.py

@@ -514,6 +514,7 @@ def pack_overrides(
     data_dir_registry: dict | None = None,
     target_cores: set[str] | None = None,
     source: str = "full",
+    choices: dict[str, list[tuple[str, str | None]]] | None = None,
 ) -> dict[str, str]:
     """Full pack destination -> the file a platform pack serves there instead.
 
@@ -522,7 +523,8 @@ def pack_overrides(
     existence mode the frontend never reads the bytes, so serving the
     emulator's file satisfies both. The ZIP builder, the install manifest and
     verify all read this: two of them deciding alone gave the one-line
-    installer different bytes than the ZIP.
+    installer different bytes than the ZIP. ``choices`` is slot_choices
+    for the same arguments when the caller already holds it.
     """
     # A baseline-only pack serves what the platform declares, and a target's
     # pack answers to the cores that hardware builds: DuckStation does not
@@ -531,10 +533,10 @@ def pack_overrides(
         return {}
     cores = set(resolve_platform_cores(config, profiles, target_cores=target_cores))
     overrides: dict[str, str] = {}
-    for destination, choices in slot_choices(
-        config, profiles, db, zip_contents, data_dir_registry
-    ).items():
-        path = next((path for emulator, path in choices if emulator in cores), None)
+    if choices is None:
+        choices = slot_choices(config, profiles, db, zip_contents, data_dir_registry)
+    for destination, ordered in choices.items():
+        path = next((path for emulator, path in ordered if emulator in cores), None)
         if path:
             overrides[destination] = path
     return overrides

tests/test_install_selection.py

@@ -1231,6 +1231,16 @@ class AnExtraBesideItsDeclarationBelowTheBiosFolder(unittest.TestCase):
         )
         return {m for m in record.members if m.startswith("bios/")}
 
+    def test_the_slot_choices_are_computed_once(self):
+        """pack_overrides recomputed the choices the manifest had just
+        computed with the same arguments: 8% of a manifest run."""
+        import slots
+
+        real = slots.slot_choices
+        with mock.patch.object(slots, "slot_choices", side_effect=real) as counted:
+            self._manifest(None)
+        self.assertEqual(counted.call_count, 1)
+
     def test_the_pack_and_the_installer_drop_the_same_files(self):
         full = self._manifest(None)
         files, _omitted = install.narrow(