Disable query cache when outside Rails executor

When outside of the Rails executor context, we do not want to enable the
query cache because the connection may never get released until the
thread terminates.

For example, this happens on console sessions or in long-running threads
that do not work in a request context.

Changelog: fixed

lib/gitlab/database/load_balancing/load_balancer.rb

@@ -266,7 +266,10 @@ module Gitlab
         private
 
         def ensure_caching!
-          host.enable_query_cache! unless host.query_cache_enabled
+          return unless Rails.application.executor.active?
+          return if host.query_cache_enabled
+
+          host.enable_query_cache!
         end
 
         def request_cache

spec/lib/gitlab/database/load_balancing/load_balancer_spec.rb

@@ -89,6 +89,7 @@ RSpec.describe Gitlab::Database::LoadBalancing::LoadBalancer, :request_store do
       host = double(:host)
 
       allow(lb).to receive(:host).and_return(host)
+      allow(Rails.application.executor).to receive(:active?).and_return(true)
       allow(host).to receive(:query_cache_enabled).and_return(false)
       allow(host).to receive(:connection).and_return(connection)
 
@@ -97,6 +98,20 @@ RSpec.describe Gitlab::Database::LoadBalancing::LoadBalancer, :request_store do
       lb.read { 10 }
     end
 
+    it 'does not enable query cache when outside Rails executor context' do
+      connection = double(:connection)
+      host = double(:host)
+
+      allow(lb).to receive(:host).and_return(host)
+      allow(Rails.application.executor).to receive(:active?).and_return(false)
+      allow(host).to receive(:query_cache_enabled).and_return(false)
+      allow(host).to receive(:connection).and_return(connection)
+
+      expect(host).not_to receive(:enable_query_cache!)
+
+      lb.read { 10 }
+    end
+
     it 'marks hosts that are offline' do
       allow(lb).to receive(:connection_error?).and_return(true)