"""Monitors static_url_for calls and inserts HTTP Link rel=preload headers

for them.

If the "nopush" option is not set, the upstream NGINX server will use

HTTP/2 server push to send these assets along with the page, which is

a significant performance improvement on first page load. However, it's

wasteful to push the same assets on subsequent page loads.

Otherwise, the browser still knows about the asset earlier and can start

loading them before it's parsed the HTML.

"""

from flask import g, session

from main import static_digest

# If this file changes its hash, content will be pushed.

PUSH_TRIGGER_FILE = "css/main.css"

def get_link_type(url):

ext = url.split(".")[-1]

return {

"css": "style",

"js": "script",

"jpg": "image",

"png": "image",

"webp": "image",

}.get(ext)

def static_url_for(endpoint, **values):

"""Intercept static_url_for calls and store them in the

request context to allow preload header to be added for HTTP/2 push.

"""

if "static_urls" not in g:

g.static_urls = {}

result = static_digest.static_url_for(endpoint, **values)

if endpoint == "static":

g.static_urls[values["filename"]] = result

return result

def init_preload(app_obj):

@app_obj.context_processor

def static_url_for_processor():

return {"static_url_for": static_url_for}

@app_obj.after_request

def static_urls_to_preload(response):

"""Collect static URLs and send in Link header for preloading and HTTP/2 push"""

if "static_urls" not in g:

return response

do_push = False

# Has the hashed URL of the push trigger file changed?

if PUSH_TRIGGER_FILE in g.static_urls and session.get("push") != g.static_urls[PUSH_TRIGGER_FILE]:

do_push = True

session["push"] = g.static_urls[PUSH_TRIGGER_FILE]

links = []

for filename, url in g.static_urls.items():

link_as = get_link_type(filename)

if link_as is None:

continue

nopush = ""

if do_push is False:

nopush = "; nopush"

links.append(f"<{url}>; as={link_as}; rel=preload{nopush}")

if len(links) > 0:

response.headers.add("Link", ", ".join(links))

return response