Handle unresolved Sentinel maste rerror

redis-rb#178 introduced
a regression that caused a ConnectionError to be thrown to the
caller if the Sentinel master or replica could not be resolved.

When a ConnectionError is thrown, the error message handler
would attempt to retrieve `config.server_url`, but this in turn
causes another Sentinel resolution to be attempted.

We avoid this by adding a `resolved?` method that will indicate
whether the config can be used. The error handler won't attempt
to provide more details if the config has yet to be resolved.

Closes redis-rb#182

Author: stanhu

lib/redis_client.rb

@@ -85,7 +85,7 @@ def _set_config(config)
     end
 
     def message
-      return super unless config
+      return super unless config&.resolved?
 
       "#{super} (#{config.server_url})"
     end

lib/redis_client/config.rb

@@ -91,6 +91,10 @@ def username
         @username || DEFAULT_USERNAME
       end
 
+      def resolved?
+        true
+      end
+
       def sentinel?
         false
       end

lib/redis_client/sentinel_config.rb

@@ -112,6 +112,12 @@ def check_role!(role)
       end
     end
 
+    def resolved?
+      @mutex.synchronize do
+        !!@config
+      end
+    end
+
     private
 
     def sentinels_to_configs(sentinels)

test/sentinel/sentinel_test.rb

@@ -83,6 +83,22 @@ def test_unknown_master
       end
     end
 
+    def test_unresolved_config
+      client = @config.new_client
+
+      @config.stub(:server_url, -> { raise ConnectionError.with_config('this should not be called', @config) }) do
+        @config.stub(:resolved?, false) do
+          client.stub(:call, ->(_) { raise ConnectionError.with_config('call error', @config) }) do
+            error = assert_raises ConnectionError do
+              client.call('PING')
+            end
+
+            assert_equal 'call error', error.message
+          end
+        end
+      end
+    end
+
     def test_master_failover_not_ready
       sentinel_client_mock = SentinelClientMock.new([
         [["SENTINEL", "get-master-addr-by-name", "cache"], [Servers::REDIS_REPLICA.host, Servers::REDIS_REPLICA.port.to_s]],