Merge branch 'baserock/adamcoldrick/build-graph-speedups-v2'

Reviewed-By: Sam Thursfield <sam.thursfield@codethink.co.uk>
Reviewed-By: Richard Ipsum <richard.ipsum@codethink.co.uk>

1 files changed, 25 insertions, 1 deletions

diff --git a/morphlib/cachedrepo.py b/morphlib/cachedrepo.py
index aa2b5af1..8b38c5c9 100644
--- a/morphlib/cachedrepo.py
+++ b/morphlib/cachedrepo.py
@@ -1,4 +1,4 @@
-# Copyright (C) 2012-2014  Codethink Limited
+# Copyright (C) 2012-2015  Codethink Limited
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -15,7 +15,9 @@
 
 
 import cliapp
+
 import os
+import tempfile
 
 import morphlib
 
@@ -169,6 +171,28 @@ class CachedRepo(object):
 
         self._checkout_ref_in_clone(ref, target_dir)
 
+    def extract_commit(self, ref, target_dir):
+        '''Extract files from a given commit into target_dir.
+
+        This is different to a 'checkout': a checkout assumes a working tree
+        associated with a repository. Here, the repository is immutable (it's
+        in the cache) and we just want to look at the files in a quick way
+        (quicker than going 'git cat-file everything').
+
+        This seems marginally quicker than doing a shallow clone. Running
+        `morph list-artifacts` 10 times gave an average time of 1.334s
+        using `git clone --depth 1` and an average time of 1.261s using
+        this code.
+
+        '''
+        if not os.path.exists(target_dir):
+            os.makedirs(target_dir)
+
+        with tempfile.NamedTemporaryFile() as index_file:
+            index = self._gitdir.get_index(index_file=index_file.name)
+            index.set_to_tree(ref)
+            index.checkout(working_tree=target_dir)
+
     def requires_update_for_ref(self, ref):
         '''Returns False if there's no need to update this cached repo.