Use the directory locking for the filecache

CHANGES.txt

@@ -59,6 +59,9 @@
 * Allowing using extras when installing from a file path without requiring the
   use of an editable (:pull`2785`).
 
+* Fix an infinite loop when the cache directory is stored on a file system
+  which does not support hard links (:pull:`2796`).
+
 
 **6.1.1 (2015-04-07)**
 

pip/download.py

@@ -342,7 +342,7 @@ class PipSession(requests.Session):
         # require manual evication from the cache to fix it.
         if cache:
             secure_adapter = CacheControlAdapter(
-                cache=SafeFileCache(cache),
+                cache=SafeFileCache(cache, use_dir_lock=True),
                 max_retries=retries,
             )
         else: