Fix schema base paths (#4611)

rest_framework/schemas.py

-import os
 import re
 from collections import OrderedDict
 from importlib import import_module
@@ -37,6 +36,18 @@ types_lookup = ClassLookupDict({
 })
 
 
+def common_path(paths):
+    split_paths = [path.strip('/').split('/') for path in paths]
+    s1 = min(split_paths)
+    s2 = max(split_paths)
+    common = s1
+    for i, c in enumerate(s1):
+        if c != s2[i]:
+            common = s1[:i]
+            break
+    return '/' + '/'.join(common)
+
+
 def get_pk_name(model):
     meta = model._meta.concrete_model._meta
     return _get_pk(meta).name
@@ -292,7 +303,7 @@ class SchemaGenerator(object):
                 # one URL that doesn't have a path prefix.
                 return '/'
             prefixes.append('/' + prefix + '/')
-        return os.path.commonprefix(prefixes)
+        return common_path(prefixes)
 
     def create_view(self, callback, method, request=None):
         """

tests/test_schemas.py

@@ -335,3 +335,14 @@ class TestSchemaGeneratorNotAtRoot(TestCase):
             }
         )
         self.assertEqual(schema, expected)
+
+
+@unittest.skipUnless(coreapi, 'coreapi is not installed')
+class Test4605Regression(TestCase):
+    def test_4605_regression(self):
+        generator = SchemaGenerator()
+        prefix = generator.determine_path_prefix([
+            '/api/v1/items/',
+            '/auth/convert-token/'
+        ])
+        assert prefix == '/'