Optimize read-only middleware so that it does not consume as much CPU

In !15082, we changed the behavior of the middleware to call
`Rails.application.routes.recognize_path` whenever a new route arrived.
However, this can be a CPU-intensive task because Rails needs to allocate
memory and compile 850+ different regular expressions, which are complicated
in GitLab.

As a short-term fix, we can do a lightweight string match before
we do the heavier comparison.

Closes #40185, gitlab-com/infrastructure#3240

lib/gitlab/middleware/read_only.rb

@@ -74,10 +74,16 @@ module Gitlab
       end
 
       def grack_route
+        # Calling route_hash may be expensive. Only do it if we think there's a possible match
+        return false unless request.path.end_with?('.git/git-upload-pack')
+
         route_hash[:controller] == 'projects/git_http' && route_hash[:action] == 'git_upload_pack'
       end
 
       def lfs_route
+        # Calling route_hash may be expensive. Only do it if we think there's a possible match
+        return false unless request.path.end_with?('/info/lfs/objects/batch')
+
         route_hash[:controller] == 'projects/lfs_api' && route_hash[:action] == 'batch'
       end
     end

spec/lib/gitlab/middleware/read_only_spec.rb

@@ -84,6 +84,7 @@ describe Gitlab::Middleware::ReadOnly do
     end
 
     it 'expects POST of new file that looks like an LFS batch url to be disallowed' do
+      expect(Rails.application.routes).to receive(:recognize_path).and_call_original
       response = request.post('/root/gitlab-ce/new/master/app/info/lfs/objects/batch')
 
       expect(response).to be_a_redirect
@@ -92,6 +93,8 @@ describe Gitlab::Middleware::ReadOnly do
 
     context 'whitelisted requests' do
       it 'expects a POST internal request to be allowed' do
+        expect(Rails.application.routes).not_to receive(:recognize_path)
+
         response = request.post("/api/#{API::API.version}/internal")
 
         expect(response).not_to be_a_redirect
@@ -99,6 +102,7 @@ describe Gitlab::Middleware::ReadOnly do
       end
 
       it 'expects a POST LFS request to batch URL to be allowed' do
+        expect(Rails.application.routes).to receive(:recognize_path).and_call_original
         response = request.post('/root/rouge.git/info/lfs/objects/batch')
 
         expect(response).not_to be_a_redirect
@@ -106,6 +110,7 @@ describe Gitlab::Middleware::ReadOnly do
       end
 
       it 'expects a POST request to git-upload-pack URL to be allowed' do
+        expect(Rails.application.routes).to receive(:recognize_path).and_call_original
         response = request.post('/root/rouge.git/git-upload-pack')
 
         expect(response).not_to be_a_redirect