Bring back the assertion

Author: jklukas

python/runfiles/runfiles.py

@@ -129,6 +129,14 @@ def lookup(self, source_repo: str, target_apparent: str) -> Optional[str]:
         # No mapping found
         return None
 
+    def is_empty(self) -> bool:
+        """Check if this repository mapping is empty (no exact or prefixed mappings).
+
+        Returns:
+            True if there are no mappings, False otherwise
+        """
+        return len(self._exact_mappings) == 0 and len(self._prefixed_mappings) == 0
+
 
 class _ManifestBased:
     """`Runfiles` strategy that parses a runfiles-manifest to look up runfiles."""
@@ -286,7 +294,7 @@ def Rlocation(self, path: str, source_repo: Optional[str] = None) -> Optional[st
         if os.path.isabs(path):
             return path
 
-        if source_repo is None and self._repo_mapping is not None:
+        if source_repo is None and not self._repo_mapping.is_empty():
             # Look up runfiles using the repository mapping of the caller of the
             # current method. If the repo mapping is empty, determining this
             # name is not necessary.
@@ -295,11 +303,20 @@ def Rlocation(self, path: str, source_repo: Optional[str] = None) -> Optional[st
         # Split off the first path component, which contains the repository
         # name (apparent or canonical).
         target_repo, _, remainder = path.partition("/")
-        if not remainder:
-            # path did not contain a slash and referred to a root symlink,
-            # which should not be mapped.
+        if not remainder or self._repo_mapping.lookup(source_repo, target_repo) is None:
+            # One of the following is the case:
+            # - not using Bzlmod, so the repository mapping is empty and
+            #   apparent and canonical repository names are the same
+            # - target_repo is already a canonical repository name and does not
+            #   have to be mapped.
+            # - path did not contain a slash and referred to a root symlink,
+            #   which also should not be mapped.
             return self._strategy.RlocationChecked(path)
 
+        assert (
+            source_repo is not None
+        ), "BUG: if the `source_repo` is None, we should never go past the `if` statement above"
+
         # Look up the target repository using the repository mapping
         if source_repo is not None:
             target_canonical = self._repo_mapping.lookup(source_repo, target_repo)

tests/runfiles/runfiles_test.py

@@ -683,6 +683,10 @@ def testRepositoryMappingLookup(self) -> None:
         # Test empty mapping
         empty_mapping = _RepositoryMapping({}, {})
         self.assertIsNone(empty_mapping.lookup("any", "repo"))
+        
+        # Test is_empty() method
+        self.assertFalse(repo_mapping.is_empty())  # Should have mappings
+        self.assertTrue(empty_mapping.is_empty())  # Should be empty
 
     def testCurrentRepository(self) -> None:
         # Under bzlmod, the current repository name is the empty string instead