Gracefully handle case where current instance is not a Geo Node

If the primary Geo Node were deleted or not configured properly,
attempts to retrieve the status of secondaries would fail with an
Error 500.

app/services/geo/node_status_service.rb

@@ -39,6 +39,8 @@ module Geo
 
             Array([message, details].compact.join("\n"))
           end
+        rescue Gitlab::Geo::GeoNodeNotFoundError
+          ['This GitLab instance does not appear to be configured properly as a Geo node. Make sure the URLs are using the correct fully-qualified domain names.']
         rescue OpenSSL::Cipher::CipherError
           ['Error decrypting the Geo secret from the database. Check that the primary uses the correct db_key_base.']
         rescue HTTParty::Error, Timeout::Error, SocketError, Errno::ECONNRESET, Errno::ECONNREFUSED => e

lib/gitlab/geo.rb

 module Gitlab
   module Geo
     OauthApplicationUndefinedError = Class.new(StandardError)
+    GeoNodeNotFoundError = Class.new(StandardError)
 
     CACHE_KEYS = %i(
       geo_primary_node

lib/gitlab/geo/base_request.rb

@@ -9,6 +9,7 @@ module Gitlab
         @request_data = request_data
       end
 
+      # Raises GeoNodeNotFoundError if current node is not a Geo node
       def headers
         {
           'Authorization' => geo_auth_token(request_data)
@@ -19,7 +20,7 @@ module Gitlab
 
       def geo_auth_token(message)
         geo_node = requesting_node
-        return unless geo_node
+        raise GeoNodeNotFoundError unless geo_node
 
         payload = { data: message.to_json, iat: Time.now.to_i }
         token = JWT.encode(payload, geo_node.secret_access_key, 'HS256')

spec/services/geo/node_status_service_spec.rb

@@ -66,5 +66,13 @@ describe Geo::NodeStatusService do
 
       expect(status.health).to eq('Error decrypting the Geo secret from the database. Check that the primary uses the correct db_key_base.')
     end
+
+    it 'gracefully handles case when primary is deleted' do
+      primary.destroy
+
+      status = subject.call(secondary)
+
+      expect(status.health).to eq('This GitLab instance does not appear to be configured properly as a Geo node. Make sure the URLs are using the correct fully-qualified domain names.')
+    end
   end
 end