rewrite converter to avoid using attic code

the unit tests themselves still use attic to generate an attic
repository for testing, but the converter code should now be
standalone

borg/testsuite/convert.py

@@ -1,4 +1,4 @@
-import binascii
+from binascii import hexlify
 import os
 import pytest
 import shutil
@@ -20,6 +20,37 @@ class AtticKeyfileKey(KeyfileKey):
     '''backwards compatible Attick key file parser'''
     FILE_ID = 'ATTIC KEY'
 
+    # verbatim copy from attic
+    @staticmethod
+    def get_keys_dir():
+        """Determine where to repository keys and cache"""
+        return os.environ.get('ATTIC_KEYS_DIR',
+                              os.path.join(os.path.expanduser('~'), '.attic', 'keys'))
+
+    @classmethod
+    def find_key_file(cls, repository):
+        '''copy of attic's `find_key_file`_
+
+        this has two small modifications:
+
+        1. it uses the above `get_keys_dir`_ instead of the global one,
+           assumed to be borg's
+
+        2. it uses `repository.path`_ instead of
+           `repository._location.canonical_path`_ because we can't
+           assume the repository has been opened by the archiver yet
+        '''
+        get_keys_dir = cls.get_keys_dir
+        id = hexlify(repository.id).decode('ascii')
+        keys_dir = get_keys_dir()
+        for name in os.listdir(keys_dir):
+            filename = os.path.join(keys_dir, name)
+            with open(filename, 'r') as fd:
+                line = fd.readline().strip()
+                if line and line.startswith(cls.FILE_ID) and line[10:] == id:
+                    return filename
+        raise KeyfileNotFoundError(repository.path, get_keys_dir())
+
 class ConversionTestCase(BaseTestCase):
 
     class MockArgs:
@@ -116,11 +147,10 @@ class ConversionTestCase(BaseTestCase):
         no key is found. whether that exception is from Borg or Attic
         is unclear.
 
-        this is split in a separate function in case we want to
-        reimplement the attic code here.
-        '''
-        self.repository._location = attic.helpers.Location(self.tmppath)
-        return attic.key.KeyfileKey().find_key_file(self.repository)
+        this is split in a separate function in case we want to use
+        the attic code here directly, instead of our local
+        implementation.'''
+        return AtticKeyfileKey.find_key_file(self.repository)
 
     def convert_keyfiles(self, keyfile):