Allow watch_assets observer to use custom timeout value

This has the effect of making the observer poll less regularly, which
allows developers to choose between rapid asset updates and resource
usage.

pavelib/assets.py

@@ -15,6 +15,7 @@ from threading import Timer
 from paver import tasks
 from paver.easy import call_task, cmdopts, consume_args, needs, no_help, path, sh, task
 from watchdog.events import PatternMatchingEventHandler
+from watchdog.observers.api import DEFAULT_OBSERVER_TIMEOUT
 from watchdog.observers.polling import PollingObserver
 
 from openedx.core.djangoapps.theming.paver_helpers import get_theme_paths
@@ -782,6 +783,7 @@ def listfy(data):
     ('background', 'b', 'Background mode'),
     ('theme-dirs=', '-td', 'The themes dir containing all themes (defaults to None)'),
     ('themes=', '-t', 'The themes to add sass watchers for (defaults to None)'),
+    ('wait=', '-w', 'How long to pause between filesystem scans.')
 ])
 @timed
 def watch_assets(options):
@@ -795,6 +797,10 @@ def watch_assets(options):
     themes = get_parsed_option(options, 'themes')
     theme_dirs = get_parsed_option(options, 'theme_dirs', [])
 
+    # wait comes in as a list of strings, define the default value similarly for convenience.
+    default_wait = [unicode(DEFAULT_OBSERVER_TIMEOUT)]
+    wait = float(get_parsed_option(options, 'wait', default_wait)[0])
+
     if not theme_dirs and themes:
         # We can not add theme sass watchers without knowing the directory that contains the themes.
         raise ValueError('theme-dirs must be provided for watching theme sass.')
@@ -802,7 +808,7 @@ def watch_assets(options):
         theme_dirs = [path(_dir) for _dir in theme_dirs]
 
     sass_directories = get_watcher_dirs(theme_dirs, themes)
-    observer = PollingObserver()
+    observer = PollingObserver(timeout=wait)
 
     CoffeeScriptWatcher().register(observer)
     SassWatcher().register(observer, sass_directories)
@@ -869,6 +875,10 @@ def update_assets(args):
         '--collect-log', dest=COLLECTSTATIC_LOG_DIR_ARG, default=None,
         help="When running collectstatic, direct output to specified log directory",
     )
+    parser.add_argument(
+        '--wait', type=float, default=0.0,
+        help="How long to pause between filesystem scans"
+    )
     args = parser.parse_args(args)
     collect_log_args = {}
 
@@ -894,5 +904,10 @@ def update_assets(args):
     if args.watch:
         call_task(
             'pavelib.assets.watch_assets',
-            options={'background': not args.debug, 'theme_dirs': args.theme_dirs, 'themes': args.themes},
+            options={
+                'background': not args.debug,
+                'theme_dirs': args.theme_dirs,
+                'themes': args.themes,
+                'wait': float(args.wait)
+            },
         )

requirements/edx/paver.txt

@@ -3,6 +3,6 @@ Paver==1.2.4
 psutil==1.2.1
 lazy==1.1
 path.py==8.2.1
-watchdog==0.7.1
+watchdog==0.8.3
 python-memcached
 libsass==0.10.0