index: Expand load() to handle arbitrary module names

It would be nice if users could point to their own index
implementations if a suitable backend is not supplied by
docker-registry.  This commit adds support for loading Index
implementations from arbitrary modules.

Reported-by: Lee Trout <[email protected]>

Author: wking

README.md

@@ -227,6 +227,16 @@ backend-specific configuration options are:
       `sqlalchemy_index_database`, which is passed through to
       [create_engine][].
 
+If `search_backend` is neither empty nor one of the above backends, it
+should point to a module:
+
+```yaml
+search_backend: foo.registry.index.xapian
+```
+
+In this case, the module is imported, and an instance of it's `Index`
+class is used as the search backend.
+
 ### Email options
 
 Settings these options makes the Registry send an email on each code Exception:

lib/index/__init__.py

@@ -1,6 +1,7 @@
 """Index backends for the search endpoint
 """
 
+import importlib
 import signals
 
 import config
@@ -76,5 +77,10 @@ def load(kind=None):
     if kind == 'sqlalchemy':
         from . import db
         return db.SQLAlchemyIndex()
+    try:
+        module = importlib.import_module(kind)
+    except ImportError:
+        pass
     else:
-        raise NotImplementedError('Unknown index type {0!r}'.format(kind))
+        return module.Index()
+    raise NotImplementedError('Unknown index type {0!r}'.format(kind))