Serve redirects in deploy previews from the Hugo manifest

grafana/website#32512 replaces client-side alias pages with a
generated redirects manifest (dist/redirects.txt). Previews serve dist
directly, so without rewrites alias URLs would 404 once that change
ships in docs-base.

Generate dist/preview-redirects.conf after the Hugo build with the
redirects binary already present in the docs-base image, and include
it in the preview nginx server block. Older images have a redirects
binary without the --manifest option, so fall back to scraping dist
for alias pages, which preserves current behaviour.

The temporary alias in whats-new.md triggers the preview workflow and
exercises the redirect end to end. Remove it before merge.

Signed-off-by: Jack Baldry <jack.baldry@grafana.com>

Author: jdbaldry

deploy-preview/Dockerfile

@@ -8,4 +8,8 @@ RUN rm -rf /etc/nginx/sites-enabled && \
 COPY deploy-preview-files/deploy-preview/nginx.conf /etc/nginx/nginx.conf
 COPY build.conf /etc/nginx/build.conf
 
+# The build script writes redirect rewrites into dist. Ensure the include
+# target exists even when the build script predates redirects generation.
+RUN touch /usr/share/nginx/dist/preview-redirects.conf
+
 RUN nginx -t

deploy-preview/build

@@ -51,6 +51,14 @@ EOF
 fi
 cat "/hugo/content/docs/${REPO}/_index.md"
 HUGO_SSI=false hugo --environment=docs --destination=dist/ --baseURL= --minify
+
+# Generate nginx rewrites for the preview server from the Hugo redirects
+# manifest. Older docs-base images have a redirects binary without the
+# --manifest option, so fall back to scraping dist for alias pages.
+touch dist/preview-redirects.conf
+redirects --base-url "" --manifest dist/redirects.txt dist dist/preview-redirects.conf \
+  || redirects --base-url "" dist dist/preview-redirects.conf \
+  || true
 '
 # End legacy input.
 else
@@ -245,6 +253,14 @@ EOINDEX
 done
 
 HUGO_SSI=false hugo --environment=docs --destination=dist/ --baseURL= --minify
+
+# Generate nginx rewrites for the preview server from the Hugo redirects
+# manifest. Older docs-base images have a redirects binary without the
+# --manifest option, so fall back to scraping dist for alias pages.
+touch dist/preview-redirects.conf
+redirects --base-url "" --manifest dist/redirects.txt dist dist/preview-redirects.conf \
+  || redirects --base-url "" dist dist/preview-redirects.conf \
+  || true
 EOSCRIPT
     chmod +x "${tempfile}"
   }

deploy-preview/nginx.conf

@@ -39,6 +39,7 @@ http {
     listen 80;
 
     include /etc/nginx/build.conf;
+    include /usr/share/nginx/dist/preview-redirects.conf;
 
     add_header "X-UA-Compatible" "IE=Edge,chrome=1";
     add_header "Strict-Transport-Security" "max-age=31536000; includeSubDomains; preload";

docs/sources/whats-new.md

@@ -1,4 +1,6 @@
 ---
+aliases:
+  - /docs/writers-toolkit/test-preview-redirect/ # TODO: remove before merge, exercises preview redirects.
 date: 2022-08-27
 description: |
   A summary of notable changes to Writers' Toolkit guidance.