location-header: Be careful with headers_out->location

Only set headers_out->location when the upstream originally did
as well. If the Location: header value involved starts with "/"
nginx will absolutify it, ignoring any X-Forwarded-Proto header
in the process.

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/819
(Confirmed: https://github.com/pagespeed/ngx_pagespeed/issues/1029)
Hopefully fixes https://github.com/pagespeed/ngx_pagespeed/issues/711

test/nginx_system_test.sh

@@ -322,8 +322,8 @@ fi
 #
 # TODO(sligicki): When the prioritize critical css race condition is fixed, the
 # two prioritize_critical_css tests no longer need to be listed here.
-# TODO(oschaaf): Now that we wait after we send a SIGHUP for the new worker 
-# process to handle requests, check if we can remove more from the expected 
+# TODO(oschaaf): Now that we wait after we send a SIGHUP for the new worker
+# process to handle requests, check if we can remove more from the expected
 # failures here under valgrind.
 if $USE_VALGRIND; then
     PAGESPEED_EXPECTED_FAILURES+="
@@ -514,6 +514,17 @@ check $WGET_DUMP -O $FETCHED $HEADERS $URL
 # When enabled, we respect X-Forwarded-Proto and thus list base as https.
 check fgrep -q '<base href="https://' $FETCHED
 
+start_test Relative redirects starting with a forward slash survive.
+URL=http://xfp.example.com/redirect
+# wget seems a bit hairy here, when it comes to handling (relative) redirects.
+# I could not get this test going with wget, and that is why curl is used here.
+# TODO(oschaaf): debug wget some more and swap out curl here.
+OUT=$(curl -v --proxy $SECONDARY_HOSTNAME $URL 2>&1)
+check_from "$OUT" egrep -q '301 Moved Permanently'
+# The important part is that we don't end up with an absolute location here.
+check_from "$OUT" grep -q 'Location: /mod_pagespeed_example'
+check_not_from "$OUT" grep -q 'Location: http'
+
 # Test that loopback route fetcher works with vhosts not listening on
 # 127.0.0.1
 start_test IP choice for loopback fetches.
@@ -566,7 +577,7 @@ check test $(scrape_stat image_rewrite_total_original_bytes) -ge 10000
 start_test "Reload config"
 
 # Fire up some heavy load if ab is available to test a stressed reload.
-# TODO(oschaaf): make sure we wait for the new worker to get ready to accept 
+# TODO(oschaaf): make sure we wait for the new worker to get ready to accept
 # requests.
 fire_ab_load
 

test/pagespeed_test.conf.template

@@ -621,8 +621,23 @@ http {
     listen [::]:@@SECONDARY_PORT@@;
     server_name xfp.example.com;
     pagespeed FileCachePath "@@FILE_CACHE@@";
-
     pagespeed RespectXForwardedProto on;
+
+    location /redirecting_origin {
+      pagespeed off;
+      # Hack: we clear the response headers using headers_more.
+      # If we don't, nginx will add an extra empty Location: headers here.
+      # It is kind of hard to get nginx to generate a relative location header
+      # that starts with "/".
+      more_clear_headers 'Location';
+      add_header Location /mod_pagespeed_example;
+      return 301;
+    }
+    location /redirect {
+      proxy_method GET;
+      proxy_pass http://127.0.0.1:@@SECONDARY_PORT@@/redirecting_origin;
+      proxy_set_header "Host" "xfp.example.com";
+    }
   }
 
   server {