PERF: Add GlobalSetting to redirect avatars instead of proxying (#19281)

When uploads are stored on S3, by default Discourse will fetch the avatars and proxy them through to the requesting client. This is simple, but it can lead to significant inbound/outbound network load in the hosting environment.

This commit adds an optional redirect_avatar_requests GlobalSetting. When enabled, requests for user avatars will be redirected to the S3 asset instead of being proxied. This adds an extra round-trip for clients, but it should significantly reduce server load. To mitigate that extra round-trip for clients, a CDN with 'follow redirect' capability could be used.

app/controllers/user_avatars_controller.rb

@@ -125,6 +125,8 @@ class UserAvatarsController < ApplicationController
       if optimized.local?
         optimized_path = Discourse.store.path_for(optimized)
         image = optimized_path if File.exist?(optimized_path)
+      elsif GlobalSetting.redirect_avatar_requests
+        return redirect_s3_avatar(Discourse.store.cdn_url(optimized.url))
       else
         return proxy_avatar(Discourse.store.cdn_url(optimized.url), upload.created_at)
       end
@@ -179,6 +181,11 @@ class UserAvatarsController < ApplicationController
     send_file path, disposition: nil
   end
 
+  def redirect_s3_avatar(url)
+    immutable_for 1.hour
+    redirect_to url, allow_other_host: true
+  end
+
   # this protects us from a DoS
   def render_blank
     path = Rails.root + "public/images/avatar.png"

config/discourse_defaults.conf

@@ -359,3 +359,6 @@ long_polling_interval =
 
 # Moves asset preloading from tags in the response document head to response headers
 preload_link_header = false
+
+# When using an external upload store, redirect `user_avatar` requests instead of proxying
+redirect_avatar_requests = false

spec/requests/user_avatars_controller_spec.rb

@@ -100,6 +100,33 @@ RSpec.describe UserAvatarsController do
       expect(response).to redirect_to("http://awesome.com/boom/user_avatar/default/#{user.encoded_username(lower: true)}/98/#{upload.id}_#{OptimizedImage::VERSION}.png")
     end
 
+    it "redirects to external store when enabled" do
+      global_setting :redirect_avatar_requests, true
+      setup_s3
+      SiteSetting.avatar_sizes = "100|49"
+      SiteSetting.s3_cdn_url = "https://s3-cdn.example.com"
+      set_cdn_url("https://app-cdn.example.com")
+
+      upload = Fabricate(:upload, url: "//#{SiteSetting.s3_upload_bucket}.s3.dualstack.us-west-1.amazonaws.com/upload/path")
+
+      optimized_image = Fabricate(:optimized_image,
+        sha1: SecureRandom.hex << "A" * 8,
+        upload: upload,
+        width: 98,
+        height: 98,
+        url: "//#{SiteSetting.s3_upload_bucket}.s3.dualstack.us-west-1.amazonaws.com/optimized/path",
+        version: OptimizedImage::VERSION
+      )
+
+      user = Fabricate(:user, uploaded_avatar_id: upload.id)
+
+      get "/user_avatar/default/#{user.username}/98/#{upload.id}.png"
+
+      expect(response.status).to eq(302)
+      expect(response.location).to eq("https://s3-cdn.example.com/optimized/path")
+      expect(response.headers["Cache-Control"]).to eq('max-age=31556952, public, immutable')
+    end
+
     it 'serves new version for old urls' do
       user = Fabricate(:user)
       SiteSetting.avatar_sizes = "45"