Add Vary: Accept-Language to all pages using that information

It was found during testing that some user agents cache aggressively
even between switches of the UI language. To properly indicate that the
pages actually depend on that information, we add the correct Vary
header.

Fixes #106.

snikket_web/infra.py

@@ -8,6 +8,7 @@ import quart.flask_patch  # noqa:F401
 from quart import (
     current_app,
     request,
+    g,
 )
 
 import flask_babel
@@ -34,6 +35,9 @@ BYTE_UNIT_SCALE_MAP = [
 
 @babel.localeselector  # type:ignore
 def selected_locale() -> str:
+    # Needs mypy ignore because this is a free-for-all object and has no
+    # publicly known attributes.
+    g.language_header_accessed = True  # type: ignore
     selected = request.accept_languages.best_match(
         current_app.config['LANGUAGES']
     ) or current_app.config['LANGUAGES'][0]
@@ -68,6 +72,12 @@ def format_bytes(n: float) -> str:
     return "{} {}".format(n, unit)
 
 
+def add_vary_language_header(resp: quart.Response) -> quart.Response:
+    if getattr(g, "language_header_accessed", False):
+        resp.vary.add("Accept-Language")
+    return resp
+
+
 def init_templating(app: quart.Quart) -> None:
     app.template_filter("repr")(repr)
     app.template_filter("format_datetime")(flask_babel.format_datetime)
@@ -78,6 +88,7 @@ def init_templating(app: quart.Quart) -> None:
     app.template_filter("format_bytes")(format_bytes)
     app.template_filter("flatten")(flatten)
     app.template_filter("circle_name")(circle_name)
+    app.after_request(add_vary_language_header)
 
 
 def generate_error_id() -> str: