Yet another stab at anchor lookup from non-canonical URIs.

This fixes looking up anchors in a subresource which changes the base
URI (e.g. the location-independent identifier with base URI change tests
in the official suite).

Refs: f520b40

docs/changes.rst

@@ -2,6 +2,11 @@
 Changelog
 =========
 
+v0.27.2
+-------
+
+* Another fix for looking up anchors from non-canonical URIs, now when they're inside a subresource which has a relative ``$id``.
+
 v0.27.1
 -------
 

referencing/_core.py

@@ -364,20 +364,23 @@ def anchor(self, uri: URI, name: str):
         """
         Retrieve a given anchor from a resource which must already be crawled.
         """
-        resource = self.get(uri)
-        if resource is None:
-            canonical_uri = uri
-        else:
-            canonical_uri = resource.id() or uri
-
-        value = self._anchors.get((canonical_uri, name))
+        value = self._anchors.get((uri, name))
         if value is not None:
             return Retrieved(value=value, registry=self)
 
         registry = self.crawl()
-        value = registry._anchors.get((canonical_uri, name))
+        value = registry._anchors.get((uri, name))
         if value is not None:
             return Retrieved(value=value, registry=registry)
+
+        resource = self.get(uri)
+        if resource is not None:
+            canonical_uri = resource.id()
+            if canonical_uri is not None:
+                value = registry._anchors.get((canonical_uri, name))
+                if value is not None:
+                    return Retrieved(value=value, registry=registry)
+
         if "/" in name:
             raise exceptions.InvalidAnchor(
                 ref=uri,