rewire cache copy mechanisms

we separate the conversion and the copy in order to be able to copy
arbitrary files from attic without converting them. this allows us to
copy the config file cleanly without attempting to rewrite its magic
number

borg/converter.py

@@ -144,25 +144,49 @@ class AtticRepositoryConverter(Repository):
         # copy of attic's get_cache_dir()
         attic_cache_dir = os.environ.get('ATTIC_CACHE_DIR',
                           os.path.join(os.path.expanduser('~'), '.cache', 'attic'))
+        attic_cache_dir = os.path.join(attic_cache_dir, hexlify(self.id).decode('ascii'))
+        borg_cache_dir = os.path.join(get_cache_dir(), hexlify(self.id).decode('ascii'))
 
-        # XXX: untested, because generating cache files is a PITA, see
-        # Archiver.do_create() for proof
-        for cache in [ 'files', 'chunks', 'config' ]:
-            attic_cache = os.path.join(attic_cache_dir, hexlify(self.id).decode('ascii'), cache)
-            if os.path.exists(attic_cache):
-                borg_cache_dir = os.path.join(get_cache_dir(), hexlify(self.id).decode('ascii'))
-                if not os.path.exists(borg_cache_dir):
-                    os.makedirs(borg_cache_dir)
-                borg_cache = os.path.join(borg_cache_dir, cache)
-                if os.path.exists(borg_cache):
-                    print("borg cache already exists in %s, skipping conversion of %s" % (borg_cache, attic_cache))
+        def copy_cache_file(file):
+            """copy the given attic cache file into the borg directory
+
+            does nothing if dryrun is True. also expects
+            attic_cache_dir and borg_cache_dir to be set in the parent
+            scope, to the directories path including the repository
+            identifier.
+
+            :params file: the basename of the cache file to copy
+            (example: "files" or "chunks") as a string
+
+            :returns: the borg file that was created or None if non
+            was created.
+
+            """
+            attic_file = os.path.join(attic_cache_dir, file)
+            if os.path.exists(attic_file):
+                borg_file = os.path.join(borg_cache_dir, file)
+                if os.path.exists(borg_file):
+                    print("borg cache file already exists in %s, skipping conversion of %s" % (borg_file, attic_file))
                 else:
-                    print("copying attic cache from %s to %s" % (attic_cache, borg_cache))
+                    print("copying attic cache file from %s to %s" % (attic_file, borg_file))
                     if not dryrun:
-                        shutil.copyfile(attic_cache, borg_cache)
-                    caches += [borg_cache]
+                        shutil.copyfile(attic_file, borg_file)
+                    return borg_file
             else:
-                print("no %s cache found in %s" % (cache, attic_cache))
+                print("no %s cache file found in %s" % (file, attic_file))
+            return None
+
+        if os.path.exists(attic_cache_dir):
+            if not os.path.exists(borg_cache_dir):
+                os.makedirs(borg_cache_dir)
+            copy_cache_file('config')
+
+        # XXX: untested, because generating cache files is a PITA, see
+        # Archiver.do_create() for proof
+        for cache in [ 'files', 'chunks' ]:
+            copied = copy_cache_file(cache)
+            if copied:
+                caches += [copied]
 
         for cache in caches:
             print("converting cache %s" % cache)