Add preload Link header only to HTML pages

weasyl/middleware.py

@@ -298,52 +298,47 @@ def cleanup(request):
     return database_session_cleanup_tween
 
 
-def _generate_http2_server_push_headers():
-    """
-    Generates the Link headers to load HTTP/2 Server Push resources which are needed on each pageload. Written
-    as a separate function to only execute this code a single time, since we just need to generate this each
-    time the code is relaunched (e.g., each time the web workers are kicked to a new version of the code).
-
-    A component of ``http2_server_push_tween_factory``
-    :return: An ASCII encoded string to be loaded into the Link header set inside of ``http2_server_push_tween_factory``
-    """
-    css_preload = [
-        '<' + item + '>; rel=preload; as=style' for item in [
-            d.get_resource_path('css/site.css'),
+# The value of the `Link` header that will be set in the `preload_tween_factory` function below.
+# Only constructed once on application load; not affected by `WEASYL_RELOAD_ASSETS`.
+_WEBPAGE_PRELOADS_LINK = ", ".join([
+    # CSS
+    *(
+        f"<{d.get_resource_path(item)}>;rel=preload;as=style" for item in [
+            "css/site.css",
         ]
-    ]
+    ),
 
-    js_preload = [
-        '<' + item + '>; rel=preload; as=script' for item in [
-            d.get_resource_path('js/jquery-2.2.4.min.js'),
-            d.get_resource_path('js/scripts.js'),
+    # JavaScript
+    *(
+        f"<{d.get_resource_path(item)}>;rel=preload;as=script" for item in [
+            "js/jquery-2.2.4.min.js",
+            "js/scripts.js",
         ]
-    ]
+    ),
 
-    esm_preload = [
-        '<' + item + '>; rel=modulepreload' for item in [
-            d.get_resource_path('js/main.js'),
+    # ES modules
+    *(
+        f"<{d.get_resource_path(item)}>;rel=modulepreload" for item in [
+            "js/main.js",
         ]
-    ]
-
-    return ", ".join(css_preload + js_preload + esm_preload)
+    ),
+])
 
 
-# Part of the `Link` header that will be set in the `http2_server_push_tween_factory` function, below
-HTTP2_LINK_HEADER_PRELOADS = _generate_http2_server_push_headers()
-
-
-def http2_server_push_tween_factory(handler, registry):
+def preload_tween_factory(handler, registry):
     """
-    Add the 'Link' header to outgoing responses to HTTP/2 Server Push render-blocking resources
+    Add the `Link` header to outgoing responses to preload resources needed on every webpage, which is served ahead of time by Cloudflare as an HTTP 103 Early Hints message.
     """
-    def http2_server_push(request):
+    def preload_tween(request):
         resp = handler(request)
 
-        # Combined HTTP/2 headers indicating which resources to server push
-        resp.headers['Link'] = HTTP2_LINK_HEADER_PRELOADS
+        content_type = resp.headers.get('Content-Type')
+
+        if content_type is not None and content_type.startswith("text/html"):
+            resp.headers['Link'] = _WEBPAGE_PRELOADS_LINK
+
         return resp
-    return http2_server_push
+    return preload_tween
 
 
 # Properties and methods to enhance the pyramid `request`.

weasyl/wsgi.py

@@ -27,7 +27,7 @@
 config.add_tween("weasyl.middleware.db_timer_tween_factory")
 config.add_tween("weasyl.middleware.cache_clear_tween_factory")
 config.add_tween("weasyl.middleware.database_session_cleanup_tween_factory")
-config.add_tween("weasyl.middleware.http2_server_push_tween_factory")
+config.add_tween("weasyl.middleware.preload_tween_factory")
 config.add_tween("weasyl.middleware.query_debug_tween_factory")
 config.add_tween("pyramid.tweens.excview_tween_factory")  # Required to catch exceptions thrown in tweens.
 config.add_tween("weasyl.middleware.utf8_path_tween_factory")