gitaly_client: Fix force-routing to primary with empty hook env

With commit edab619a (gitaly: Fix access checks with transactions and
quarantine environments, 2021-02-05), we started injecting a flag into
Gitaly requests to force-route to the primary Gitaly node in case a hook
environment is set in order to not break access to quarantined objects.
Turns out that this change breaks read distribution though as now all
requests are force-routed to the primary.

The cause of this is trivial enough: the SafeRequestStore returns an
empty hash if it wasn't set up to contain anything. Given that the
checks whether a HookEnv was set up only checked whether there was
something in the SafeRequestStore, they thus always thought that
requests were running in the context of a HookEnv.

Fix the issue by checking that the returned value is non-empty.

changelogs/unreleased/pks-gitaly-fix-force-routing.yml

+---
+title: Fix force-routing to Gitaly primary with empty hook env
+merge_request: 54317
+author:
+type: fixed

lib/gitlab/gitaly_client.rb

@@ -248,6 +248,8 @@ module Gitlab
 
       return {} unless Gitlab::SafeRequestStore[:gitlab_git_env]
 
+      return {} if Gitlab::SafeRequestStore[:gitlab_git_env].empty?
+
       { 'gitaly-route-repository-accessor-policy' => 'primary-only' }
     end
     private_class_method :route_to_primary

spec/lib/gitlab/gitaly_client_spec.rb

@@ -311,9 +311,21 @@ RSpec.describe Gitlab::GitalyClient do
         end
       end
 
-      context 'when RequestStore is enabled with git_env', :request_store do
+      context 'when RequestStore is enabled with empty git_env', :request_store do
         before do
-          Gitlab::SafeRequestStore[:gitlab_git_env] = true
+          Gitlab::SafeRequestStore[:gitlab_git_env] = {}
+        end
+
+        it 'disables force-routing to primary' do
+          expect(described_class.request_kwargs('default', timeout: 1)[:metadata][policy]).to be_nil
+        end
+      end
+
+      context 'when RequestStore is enabled with populated git_env', :request_store do
+        before do
+          Gitlab::SafeRequestStore[:gitlab_git_env] = {
+            "GIT_OBJECT_DIRECTORY_RELATIVE" => "foo/bar"
+          }
         end
 
         it 'enables force-routing to primary' do