Add static asset cache busting

Calculates MD5 hash of static assets on server start. These are appended to the file name's when url_for is used, telling browsers to download the latest version of the files, e.g. style.css?q=81cf0ee.

Inspired by https://github.com/ChrisTM/Flask-CacheBust but uses query strings instead of rewriting the path because fonts inside CSS files break when the path is rewritten.

cps/cache_buster.py

@@ -0,0 +1,57 @@
+# Inspired by https://github.com/ChrisTM/Flask-CacheBust
+# Uses query strings so CSS font files are found without having to resort to absolute URLs
+
+import hashlib
+import os
+
+
+def init_cache_busting(app):
+    """
+    Configure `app` to so that `url_for` adds a unique query string to URLs generated
+    for the `'static'` endpoint.
+
+    This allows setting long cache expiration values on static resources
+    because whenever the resource changes, so does its URL.
+    """
+
+    static_folder = app.static_folder  # the rooted path to the static file folder
+
+    hash_table = {}  # map of file hashes
+
+    app.logger.debug('Computing cache-busting values...')
+    # compute file hashes
+    for dirpath, dirnames, filenames in os.walk(static_folder):
+        for filename in filenames:
+            # compute version component
+            rooted_filename = os.path.join(dirpath, filename)
+            with open(rooted_filename, 'r') as f:
+                version = hashlib.md5(f.read()).hexdigest()[:7]
+
+            # save version to tables
+            file_path = rooted_filename.replace(static_folder + "/", "")
+            hash_table[file_path] = version
+    app.logger.debug('Finished computing cache-busting values')
+
+    def bust_filename(filename):
+        return hash_table.get(filename, "")
+
+    def unbust_filename(filename):
+        return filename.split("?", 1)[0]
+
+    @app.url_defaults
+    def reverse_to_cache_busted_url(endpoint, values):
+        """
+        Make `url_for` produce busted filenames when using the 'static' endpoint.
+        """
+        if endpoint == 'static':
+            values["q"] = bust_filename(values['filename'])
+
+    def debusting_static_view(filename):
+        """
+        Serve a request for a static file having a busted name.
+        """
+        return original_static_view(filename=unbust_filename(filename))
+
+    # Replace the default static file view with our debusting view.
+    original_static_view = app.view_functions['static']
+    app.view_functions['static'] = debusting_static_view

cps/web.py

@@ -21,6 +21,7 @@ from flask import (Flask, render_template, request, Response, redirect,
                    url_for, send_from_directory, make_response, g, flash,
                    abort, Markup, stream_with_context)
 from flask import __version__ as flaskVersion
+import cache_buster
 import ub
 from ub import config
 import helper
@@ -200,6 +201,7 @@ mimetypes.add_type('image/vnd.djvu', '.djvu')
 
 app = (Flask(__name__))
 app.wsgi_app = ReverseProxied(app.wsgi_app)
+cache_buster.init_cache_busting(app)
 
 gevent_server = None