add stale-while-revalidate and stale-if-error headers to cpan.mc.org

modules/metacpan/templates/web/metacpan-cpan-static/fastly.erb

@@ -28,12 +28,13 @@ location ~ /authors/id/[^/]+$ {
 location ~ /authors/.*/.*$ {
   # files in author directories, basically the
   # other regexes are above are the exceptions for this rule
-  add_header Surrogate-Control max-age=31104000;
+  # See: https://www.fastly.com/blog/stale-while-revalidate/
+  add_header Surrogate-Control max-age=31104000, stale-while-revalidate=31104000, stale-if-error=31104000;
 }
 location ~ /robots.txt$ {
   # robots gets requested a lot - let fastly cache
-  # for an hour
-  add_header Surrogate-Control max-age=3600;
+  # for an hour, revalidate in the background (up to a day) and serve from cache if error revalidating
+  add_header Surrogate-Control max-age=3600, stale-while-revalidate=86400, stale-if-error=31104000;
 }
 location ~ /authors/ {
   # anything else