improve old entity daisy chaining

digital_land/phase/lookup.py

@@ -152,18 +152,29 @@ def _check_entity_range(self, entity, line_number=0):
             )
 
     def redirect_entity(self, entity):
-        """
-        Given an entity number can check the redirect lookups to see if the entity
-        has been removed or redirected.
-        """
-        if self.redirect_lookups:
-            redirect_entity = self.redirect_lookups.get(entity, "")
-            if redirect_entity:
-                if redirect_entity["status"] == "301":
-                    return redirect_entity["entity"]
-                elif redirect_entity["status"] == "410":
-                    return ""
-        return entity
+        """Follow redirects until the entity has no further redirect."""
+        current_entity = entity
+        visited_entities = set()
+
+        while current_entity in self.redirect_lookups:
+            if current_entity in visited_entities:
+                raise ValueError(
+                    f"Cycle found while following entity redirects from {entity}"
+                )
+
+            visited_entities.add(current_entity)
+            redirect = self.redirect_lookups[current_entity]
+            status = redirect["status"]
+
+            if status == "410":
+                return ""
+
+            if status != "301":
+                return current_entity
+
+            current_entity = redirect["entity"]
+
+        return current_entity
 
     def _log_unknown_entity(self, reference, curie, line_number):
         """

tests/unit/phase/test_lookup.py

@@ -49,6 +49,7 @@ def get_lookup():
 
 
 class TestLookupPhase:
+
     def test_redirect_entity_entity_is_not_redirected(self):
         redirects = {"101": {"entity": "101", "status": "310"}}
         phase = LookupPhase(redirect_lookups=redirects)
@@ -258,6 +259,24 @@ def test_lookup_rule_fallback_in_get_entity(self):
         }
         assert phase.get_entity(block) == "1100"
 
+    def test_redirect_entity_follows_multiple_redirects(self):
+        redirects = {
+            "101": {"entity": "202", "status": "301"},
+            "202": {"entity": "303", "status": "301"},
+        }
+        phase = LookupPhase(redirect_lookups=redirects)
+
+        assert phase.redirect_entity("101") == "303"
+
+    def test_redirect_entity_returns_empty_when_chain_ends_in_410(self):
+        redirects = {
+            "101": {"entity": "202", "status": "301"},
+            "202": {"entity": "", "status": "410"},
+        }
+        phase = LookupPhase(redirect_lookups=redirects)
+
+        assert phase.redirect_entity("101") == ""
+
 
 class TestPrintLookupPhase:
     def test_process_produces_lookup_without_organisation(self):