FEATURE: introduce additional request limit buckets keyed on cloud or crawler identifiers

If we have additional information about the semantic source IP of a request
(i.e. a public cloud or a crawler) then we can group those requests together
under the same bucket.

lib/middleware/request_tracker.rb

@@ -439,12 +439,34 @@ class Middleware::RequestTracker
     return :skip if @@ip_skipper&.call(request.ip)
     return :skip if STATIC_IP_SKIPPER&.any? { |entry| entry.include?(request.ip) }
 
-    if cookie && cookie[:user_id] && cookie[:trust_level] &&
-         cookie[:trust_level] >= GlobalSetting.skip_per_ip_rate_limit_trust_level
-      [cookie[:user_id], false, "id", "user"]
-    else
-      [request.ip, true, "ip", "IP address"]
+    # authenticated user?
+    if cookie && cookie[:user_id] && cookie[:trust_level]
+      if cookie[:trust_level] >= GlobalSetting.skip_per_ip_rate_limit_trust_level
+        return "id/#{cookie[:user_id]}", false, "id", "user"
+      else
+        # if the authenticated user comes from another bucketed source
+        # (e.g. public cloud) but does not meet skip_per_ip_rate_limit_trust_level,
+        # we don't want to bucket them along with the rest of the cloud,
+        # so they still get bucketed by IP
+        return "ip/#{request.ip}", true, "ip", "IP address"
+      end
     end
+
+    # TODO: authenticated user via API?
+
+    src_tag_info = SrcTagInfo.new(request)
+    # known crawler?
+    if verified_crawler = src_tag_info.verified_crawler
+      return "crawler/#{verified_crawler}", false, "crawler", "crawler (#{verified_crawler})"
+    end
+
+    # public cloud?
+    if verified_cloud = src_tag_info.verified_cloud
+      return "cloud/#{verified_cloud}", false, "cloud", "public cloud (#{verified_cloud})"
+    end
+
+    # fallback choice: IP
+    ["ip/#{request.ip}", true, "ip", "IP address"]
   end
 
   def rate_limit(request, cookie)

spec/lib/middleware/request_tracker_spec.rb

@@ -747,6 +747,45 @@ RSpec.describe Middleware::RequestTracker do
       expect(status).to eq(200)
     end
 
+    context "with src-tag headers" do
+      ENV["DISCOURSE_HTTP_SRC_TAG_HEADER"] = "src-tag"
+      ENV["DISCOURSE_HTTP_SRC_TAG_SUPPORTED_HEADER"] = "src-tag-lists"
+
+      it "does bucket a public cloud" do
+        global_setting :max_reqs_per_ip_per_10_seconds, 1
+        global_setting :max_reqs_per_ip_mode, "block"
+
+        env1 = env("REMOTE_ADDR" => "1.1.1.1", "HTTP_SRC_TAG" => "cloud-aws-ec2")
+        env2 = env("REMOTE_ADDR" => "1.1.1.2", "HTTP_SRC_TAG" => "cloud-aws-ec2")
+        env3 = env("REMOTE_ADDR" => "1.1.1.3")
+
+        status, _ = middleware.call(env1)
+        expect(status).to eq(200)
+
+        status, headers = middleware.call(env2)
+        expect(status).to eq(429)
+        expect(headers["Retry-After"]).to eq("10")
+        expect(headers["Discourse-Rate-Limit-Error-Code"]).to eq("cloud_10_secs_limit")
+
+        status, _ = middleware.call(env3)
+        expect(status).to eq(200)
+      end
+
+      it "does bucket public clouds separately" do
+        global_setting :max_reqs_per_ip_per_10_seconds, 1
+        global_setting :max_reqs_per_ip_mode, "block"
+
+        env1 = env("REMOTE_ADDR" => "1.1.1.1", "HTTP_SRC_TAG" => "cloud-aws-ec2")
+        env2 = env("REMOTE_ADDR" => "1.1.1.2", "HTTP_SRC_TAG" => "cloud-gcp")
+
+        status, _ = middleware.call(env1)
+        expect(status).to eq(200)
+
+        status, _ = middleware.call(env2)
+        expect(status).to eq(200)
+      end
+    end
+
     describe "diagnostic information" do
       it "is included when the requests-per-10-seconds limit is reached" do
         global_setting :max_reqs_per_ip_per_10_seconds, 1