fix: #506 copy extend namespace will mounting files to temporary pack

examples/nosnap_extend_namespace/beet.yml

@@ -0,0 +1,4 @@
+
+
+pipeline:
+  - main

examples/nosnap_extend_namespace/main.py

@@ -0,0 +1,17 @@
+from beet import Context, BinaryFile, NamespaceFileScope
+from beet.contrib.vanilla import Vanilla
+from typing import ClassVar
+
+
+class UnihexZip(BinaryFile):
+    scope: ClassVar[NamespaceFileScope] = ()
+    extension: ClassVar[str] = ".zip"
+
+
+def beet_default(ctx: Context):
+    vanilla = ctx.inject(Vanilla)
+    vanilla.assets.extend_namespace.append(UnihexZip)
+    unihex = vanilla.mount("assets/minecraft/font", fetch_objects=True).assets[
+        UnihexZip
+    ]
+    print(unihex)

src/beet/contrib/vanilla.py

@@ -94,6 +94,8 @@ def mount(
                 # triggering merge policies that might try to deserialize
                 # files before they're fully retrieved.
                 temp = ResourcePack()
+                temp.extend_namespace.extend(pack.extend_namespace)
+                temp.extend_namespace_extra.update(pack.extend_namespace_extra)
                 with self.cache.parallel_downloads():
                     temp.mount(prefix, object_mapping.with_prefix(prefix))
                 pack.merge(temp)