add cachebusting for static files

app.py

@@ -3,6 +3,7 @@ from flask_sqlalchemy import SQLAlchemy
 from sqlalchemy import MetaData
 from flask_migrate import Migrate
 import version
+from lib import cachebust
 
 app = Flask(__name__)
 
@@ -36,9 +37,10 @@ if 'SENTRY_DSN' in app.config:
     app.config['SENTRY_CONFIG']['release'] = version.version
     sentry = Sentry(app, dsn=app.config['SENTRY_DSN'])
 
+url_for = cachebust(app)
+
 @app.context_processor
 def inject_static():
-    from flask import url_for
     def static(filename, **kwargs):
         return url_for('static', filename=filename, **kwargs)
     return {'st': static}

lib/__init__.py

@@ -1,3 +1,4 @@
 from .auth import require_auth
 from .interval import decompose_interval
 from .interval import SCALES as interval_scales
+from .cachebust import cachebust

lib/cachebust.py

@@ -0,0 +1,28 @@
+from flask import url_for, abort
+import os
+def cachebust(app):
+    @app.route('/static-<int:timestamp>/<path:filename>')
+    def static_cachebust(timestamp, filename):
+        path = os.path.join(app.static_folder, filename)
+        mtime = os.stat(path).st_mtime
+        if abs(mtime - timestamp) > 1:
+            abort(404)
+        else:
+            resp = app.view_functions['static'](filename=filename)
+            resp.headers.set('cache-control', 'public, immutable, max-age=%s' % (60*60*24*365,))
+            if 'expires' in resp.headers:
+                resp.headers.remove('expires')
+            return resp
+
+    @app.context_processor
+    def replace_url_for():
+        return dict(url_for = cachebust_url_for)
+
+    def cachebust_url_for(endpoint, **kwargs):
+        if endpoint == 'static':
+            endpoint = 'static_cachebust'
+            path = os.path.join(app.static_folder, kwargs.get('filename'))
+            kwargs['timestamp'] = int(os.stat(path).st_mtime)
+        return url_for(endpoint, **kwargs)
+
+    return cachebust_url_for