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.

Dockerfile

@@ -17,8 +17,7 @@ COPY babel.cfg /opt/snikket-web-portal/babel.cfg
 
 WORKDIR /opt/snikket-web-portal
 
-RUN set -eu; \
+RUN pip3 install -r requirements.txt; \
-    pip3 install -r requirements.txt; \
     pip3 install -r build-requirements.txt; \
     make;
 
@@ -36,21 +35,20 @@ ENV SNIKKET_WEB_PROSODY_ENDPOINT=http://127.0.0.1:5280/
 
 HEALTHCHECK CMD nc -zv ${SNIKKET_TWEAK_PORTAL_INTERNAL_HTTP_INTERFACE:-127.0.0.1} ${SNIKKET_TWEAK_PORTAL_INTERNAL_HTTP_PORT:-5765}
 
-COPY requirements.txt /opt/snikket-web-portal/requirements.txt
-
-WORKDIR /opt/snikket-web-portal
-
 RUN set -eu; \
     export DEBIAN_FRONTEND=noninteractive ; \
     apt-get update ; \
     apt-get install -y --no-install-recommends \
-        python3 python3-pip python3-setuptools python3-wheel build-essential libpython3-dev; \
+        python3 python3-pip python3-setuptools python3-wheel; \
-    pip3 install -r requirements.txt; \
-    apt-get remove -y --autoremove build-essential libpython3-dev; \
     apt-get clean ; rm -rf /var/lib/apt/lists; \
     pip3 install hypercorn; \
     rm -rf /root/.cache;
 
+WORKDIR /opt/snikket-web-portal
+
+COPY requirements.txt /opt/snikket-web-portal/requirements.txt
+RUN pip3 install -r requirements.txt; rm -rf /root/.cache;
+
 COPY --from=build /opt/snikket-web-portal/snikket_web/ /opt/snikket-web-portal/snikket_web
 COPY babel.cfg /opt/snikket-web-portal/babel.cfg
 

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: