Implement hash-based cachebusting module.

Adds the files.helpers.assetcache module, which runs on worker startup
to compute CRC digests of files in given asset directories and provides
a Jinja filter to create cachebusted URLs based on those hashes.
This alleviates the need to increment cachebusting counters in query
strings entirely for files it is applied to.

files/helpers/assetcache.py

@@ -0,0 +1,32 @@
+import os
+import zlib
+from collections import defaultdict
+
+ASSET_DIR = 'files/assets'
+ASSET_SUBDIRS = ['/css', '/js']
+ASSET_URL = '/assets/'
+ASSET_CACHE = defaultdict(lambda: None)
+
+def assetcache_build(asset_dir, subdirs):
+	for subdir in subdirs:
+		for root, dirs, files in os.walk(asset_dir + subdir):
+			for fname in files:
+				fpath = root + '/' + fname
+				relpath = fpath[len(asset_dir) + 1:].replace('\\', '/')
+				with open(fpath, 'rb') as f:
+					fhash = zlib.crc32(f.read())
+					ASSET_CACHE[relpath] = '%x' % fhash
+
+def assetcache_hash(asset_path):
+	return ASSET_CACHE[asset_path]
+
+def assetcache_path(asset_path):
+	cachehash = assetcache_hash(asset_path)
+
+	url = ASSET_URL + asset_path
+	if cachehash:
+		url += '?v=' + cachehash
+
+	return url
+
+assetcache_build(ASSET_DIR, ASSET_SUBDIRS)

files/helpers/jinja2.py

@@ -3,6 +3,7 @@ from .get import *
 from os import listdir, environ
 from .const import * 
 import time
+from files.helpers.assetcache import assetcache_path
 
 @app.template_filter("post_embed")
 def post_embed(id, v):
@@ -16,6 +17,11 @@ def post_embed(id, v):
 	return ''
 
 
+@app.template_filter("asset")
+def template_asset(asset_path):
+	return assetcache_path(asset_path)
+
+
 @app.template_filter("timestamp")
 def timestamp(timestamp):