Backport of c120884 from trunk

CHANGES.txt

@@ -4,6 +4,8 @@ Change History
 1.4.5 (unreleased)
 ==================
 
+- Introduce a cache for the expensive `buildout._dir_hash` function.
+
 - Remove duplicate path from script's sys.path setup.
 
 Bugs fixed:

src/zc/buildout/buildout.py

@@ -1371,7 +1371,11 @@ def _open(base, filename, seen, dl_options, override):
 
 
 ignore_directories = '.svn', 'CVS'
+_dir_hashes = {}
 def _dir_hash(dir):
+    dir_hash = _dir_hashes.get(dir, None)
+    if dir_hash is not None:
+        return dir_hash
     hash = md5()
     for (dirpath, dirnames, filenames) in os.walk(dir):
         dirnames[:] = [n for n in dirnames if n not in ignore_directories]
@@ -1383,7 +1387,8 @@ def _dir_hash(dir):
         hash.update(' '.join(filenames))
         for name in filenames:
             hash.update(open(os.path.join(dirpath, name)).read())
-    return hash.digest().encode('base64').strip()
+    _dir_hashes[dir] = dir_hash = hash.digest().encode('base64').strip()
+    return dir_hash
 
 def _dists_sig(dists):
     result = []