Avoid sorting the working set and requirements when it won't be logged.

When profiling a simple buildout with 10 parts with identical and
large working sets, this resulted in a decrease of run time from
93.411 to 15.068 seconds, about a 6 fold improvement.  To see the
benefit be sure to run without any increase in verbosity ("-v"
option).

git-svn-id: http://svn.zope.org/repos/main/zc.buildout/trunk@124059 62d5b8a3-27da-0310-9561-8e5933582275

CHANGES.txt

@@ -4,6 +4,13 @@ Change History
 1.6.0 (unreleased)
 ==================
 
+- Avoid sorting the working set and requirements when it won't be
+  logged.  When profiling a simple buildout with 10 parts with
+  identical and large working sets, this resulted in a decrease of run
+  time from 93.411 to 15.068 seconds, about a 6 fold improvement.  To
+  see the benefit be sure to run without any increase in verbosity
+  ("-v" option).  (rossp)
+
 - The buildout init command now accepts distribution requirements and
   paths to set up a custom interpreter part that has the distributions
   or parts in the path. For example::

src/zc/buildout/easy_install.py

@@ -1850,6 +1850,15 @@ class MissingDistribution(zc.buildout.UserError):
         return "Couldn't find a distribution for %r." % str(req)
 
 def _log_requirement(ws, req):
+    if not logger.isEnabledFor(logging.DEBUG):
+        # Sorting the working set and iterating over it's requirements
+        # is expensive, so short cirtuit the work if it won't even be
+        # logged.  When profiling a simple buildout with 10 parts with
+        # identical and large working sets, this resulted in a
+        # decrease of run time from 93.411 to 15.068 seconds, about a
+        # 6 fold improvement.
+        return
+    
     ws = list(ws)
     ws.sort()
     for dist in ws: