Fix a regression in determining when to collect static files.

A commit made in 1.6.10 (ae6b3f8e)
changed a storage lookup to use the prefixed_path variable instead of
the path variable when looking up modification times in order to
determine if an existing collected file should be deleted and
re-collected. By using the wrong path variable, the storage lookups
always failed, resulting in every single file being re-collected every
single time.

This reduces performance considerably, particularly with large codebases.
Since every file ends up copied, they appear as new and are recompiled.
This can result in very long page reloads on a development server,
especially if working on an older machine, in a VM, on a shared
filesystem, or on the Linux subsystem for Windows.

This is a simple change that fixes the lookup to use the correct
variable. Unit tests were added to ensure this continues to work as
expected and does not regress.

pipeline/collector.py

@@ -78,7 +78,7 @@ class Collector(object):
             else:
                 try:
                     # When was the source file modified last time?
-                    source_last_modified = self._get_modified_time(source_storage, prefixed_path)
+                    source_last_modified = self._get_modified_time(source_storage, path)
                 except (OSError, NotImplementedError, AttributeError):
                     pass
                 else:

tests/tests/test_collector.py

@@ -3,12 +3,17 @@ from __future__ import unicode_literals
 import os
 
 from django.contrib.staticfiles import finders
+from django.core.files.storage import FileSystemStorage
 from django.test import TestCase
 
 from pipeline.collector import default_collector
 from pipeline.finders import PipelineFinder
 
 
+def local_path(path):
+    return os.path.abspath(os.path.join(os.path.dirname(__file__), '..', path))
+
+
 class CollectorTest(TestCase):
     def tearDown(self):
         super(CollectorTest, self).tearDown()
@@ -31,6 +36,24 @@ class CollectorTest(TestCase):
                 'pipeline/js/second.js',
             ]))
 
+    def test_delete_file_with_modified(self):
+        list(default_collector.collect())
+
+        storage = FileSystemStorage(local_path('assets'))
+        new_mtime = os.path.getmtime(storage.path('js/first.js')) - 1000
+        os.utime(default_collector.storage.path('pipeline/js/first.js'),
+                 (new_mtime, new_mtime))
+
+        self.assertTrue(default_collector.delete_file(
+            'js/first.js', 'pipeline/js/first.js', storage))
+
+    def test_delete_file_with_unmodified(self):
+        list(default_collector.collect(files=['pipeline/js/first.js']))
+
+        self.assertFalse(default_collector.delete_file(
+            'js/first.js', 'pipeline/js/first.js',
+            FileSystemStorage(local_path('assets'))))
+
     def _get_collectable_files(self):
         for finder in finders.get_finders():
             if not isinstance(finder, PipelineFinder):