Make memoized zip manifests opt-in using the PKG_RESOURCES_CACHE_ZIP_MANIFESTS...

Make memoized zip manifests opt-in using the PKG_RESOURCES_CACHE_ZIP_MANIFESTS environment variable. Ref #154.

CHANGES.txt

@@ -6,8 +6,12 @@ CHANGES
 5.4
 ---
 
-* Issue #154: Cache the reading of the zip file index for cases where the
-  same zip-file is used for multiple packages (like PEX).
+* Issue #154: ``pkg_resources`` will now cache the zip manifests rather than
+  re-processing the same file from disk multiple times, but only if the
+  environment variable ``PKG_RESOURCES_CACHE_ZIP_MANIFESTS`` is set. Clients
+  that package many modules in the same zip file will see some improvement
+  in startup time by enabling this feature. This feature is not enabled by
+  default because it causes a substantial increase in memory usage.
 
 ---
 5.3

pkg_resources.py

@@ -1602,7 +1602,11 @@ class ZipProvider(EggProvider):
     """Resource support for zips and eggs"""
 
     eagers = None
-    _zip_manifests = ZipManifests()
+    _zip_manifests = (
+        MemoizedZipManifests()
+        if os.environ.get('PKG_RESOURCES_CACHE_ZIP_MANIFESTS') else
+        ZipManifests()
+    )
 
     def __init__(self, module):
         EggProvider.__init__(self, module)