Merge pull request rails#48405 from Shopify/refactor-preload-link-header

Stop appending preload links once the head is already 1kB

Author: byroot

actionview/CHANGELOG.md

@@ -1,3 +1,13 @@
+*   Stop generating `Link preload` headers once it has reached 1KB.
+
+    Some proxies have trouble handling large headers, but more importantly preload links
+    have diminishing returns so it's preferable not to go overboard with them.
+
+    If tighter control is needed, it's recommended to disable automatic generation of preloads
+    and to generate them manually from the controller or from a middleware.
+
+    *Jean Boussier*
+
 *   `simple_format` helper now handles a `:sanitize_options` - any extra options you want appending to the sanitize.
 
     Before:

actionview/lib/action_view/helpers/asset_tag_helper.rb

@@ -633,7 +633,10 @@ def resolve_link_as(extname, mime_type)
           end
         end
 
-        MAX_HEADER_SIZE = 8_000 # Some HTTP client and proxies have a 8kiB header limit
+        # Some HTTP client and proxies have a 4kiB header limit, but more importantly
+        # including preload links has diminishing returns so it's best to not go overboard
+        MAX_HEADER_SIZE = 1_000 # :nodoc:
+
         def send_preload_links_header(preload_links, max_header_size: MAX_HEADER_SIZE)
           return if preload_links.empty?
           response_present = respond_to?(:response) && response
@@ -644,30 +647,15 @@ def send_preload_links_header(preload_links, max_header_size: MAX_HEADER_SIZE)
           end
 
           if response_present
-            header = response.headers["Link"]
-            header = header ? header.dup : +""
-
-            # rindex count characters not bytes, but we assume non-ascii characters
-            # are rare in urls, and we have a 192 bytes margin.
-            last_line_offset = header.rindex("\n")
-            last_line_size = if last_line_offset
-              header.bytesize - last_line_offset
-            else
-              header.bytesize
-            end
-
+            header = +response.headers["Link"].to_s
             preload_links.each do |link|
-              if link.bytesize + last_line_size + 1 < max_header_size
-                unless header.empty?
-                  header << ","
-                  last_line_size += 1
-                end
+              break if header.bytesize + link.bytesize > max_header_size
+
+              if header.empty?
+                header << link
               else
-                header << "\n"
-                last_line_size = 0
+                header << "," << link
               end
-              header << link
-              last_line_size += link.bytesize
             end
 
             response.headers["Link"] = header

actionview/test/template/asset_tag_helper_test.rb

@@ -648,8 +648,8 @@ def test_should_generate_links_under_the_max_size
         stylesheet_link_tag("http://example.com/style.css?#{i}")
         javascript_include_tag("http://example.com/all.js?#{i}")
       end
-      lines = @response.headers["Link"].split("\n")
-      assert_equal 2, lines.size
+      links = @response.headers["Link"].count(",")
+      assert_equal 14, links
     end
   end