xC/xP: finalize and implement Event.PING

xC-proto

@@ -8,11 +8,12 @@ struct CommandMessage {
 	u32 command_seq;
 	union CommandData switch (enum Command {
 		HELLO,
-		PING,
 		ACTIVE,
-		BUFFER_COMPLETE,
 		BUFFER_INPUT,
 		BUFFER_ACTIVATE,
+		PING_RESPONSE,
+		PING,
+		BUFFER_COMPLETE,
 		BUFFER_LOG,
 	} command) {
 	case HELLO:
@@ -20,19 +21,24 @@ struct CommandMessage {
 		// If the version check succeeds, the client will receive
 		// an initial stream of BUFFER_UPDATE, BUFFER_LINE,
 		// and finally a BUFFER_ACTIVATE message.
-	case PING:
-		void;
 	case ACTIVE:
 		void;
-	case BUFFER_COMPLETE:
-		string buffer_name;
-		string text;
-		u32 position;
 	case BUFFER_INPUT:
 		string buffer_name;
 		string text;
 	case BUFFER_ACTIVATE:
 		string buffer_name;
+	case PING_RESPONSE:
+		u32 event_seq;
+
+	// Only these commands may produce Event.RESPONSE, as below,
+	// but any command may produce an error.
+	case PING:
+		void;
+	case BUFFER_COMPLETE:
+		string buffer_name;
+		string text;
+		u32 position;
 	case BUFFER_LOG:
 		string buffer_name;
 	} data;