Add "forever" option to tools/compile-handlebars-templates.

The "forever" option causes the tool to continue looking for
template changes and, when they happen, to recompile them.

(imported from commit 2fa719a205f02c7c90cc071f99252148a888654f)

tools/compile-handlebars-templates

@@ -5,6 +5,7 @@ import os
 import glob
 import subprocess
 import sys
+import time
 
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), '..'))
 os.environ['DJANGO_SETTINGS_MODULE'] = 'zproject.settings'
@@ -13,11 +14,37 @@ from django.conf import settings
 os.chdir(settings.DEPLOY_ROOT)
 STATIC_PATH = 'static/'
 
+def get_templates():
+    return glob.glob(os.path.join(STATIC_PATH, 'templates/*.handlebars'))
+
 def run():
     subprocess.check_call(['tools/node', 'node_modules/.bin/handlebars']
-        + glob.glob(os.path.join(STATIC_PATH, 'templates/*.handlebars'))
+        + get_templates()
         + ['--output', os.path.join(STATIC_PATH, 'templates/compiled.js'),
            '--known', 'if,unless,each,with'])
 
+def run_forever():
+    # Keep polling for file changes, similar to how Django does it in
+    # django/utils/autoreload.py.  If any of our templates change, rebuild
+    # compiled.js
+    mtimes = {}
+    while True:
+        changed = False
+        for fn in get_templates():
+            new_mtime = os.stat(fn).st_mtime
+            if new_mtime != mtimes.get(fn, None):
+                changed = True
+            mtimes[fn] = new_mtime
+        if changed:
+            print 'Recompiling templates'
+            try:
+                run()
+                print 'done'
+            except:
+                print '\n\n\nPLEASE FIX!!\n\n'
+        time.sleep(0.200)
+
 if __name__ == '__main__':
+    if len(sys.argv) == 2 and sys.argv[1] == 'forever':
+        run_forever()
     run()