gdb_server: drop useless check in gdb_keep_client_alive()

OpenOCD can send it's log to gdb, and gdb replies with 'OK'.
Calls to LOG_XXX() are also present in the code that communicates
with gdb. This can cause infinite nested calls.

OpenOCD uses the flag 'gdb_con->busy' to protect the communication
with gdb and prevent nested calls.

There is no reason to check for 'gdb_con->busy' in the code for
keep-alive, as keep_alive() is never called in this gdb server;
the flag would eventually be set if the current keep_alive() will
send something to gdb.

Drop the flag 'gdb_con->busy' in gdb_keep_client_alive().
While there, document the use of 'gdb_con->busy'.

Change-Id: I1ea20bf96abb5d2f1fcdba1e3861df257c396bb6
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8166
Tested-by: jenkins

src/server/gdb_server.c

@@ -73,6 +73,8 @@ struct gdb_connection {
 	enum target_state frontend_state;
 	struct image *vflash_image;
 	bool closed;
+	/* set to prevent re-entrance from log messages during gdb_get_packet()
+	 * and gdb_put_packet(). */
 	bool busy;
 	int noack_mode;
 	/* set flag to true if you want the next stepi to return immediately.
@@ -3794,11 +3796,6 @@ static void gdb_keep_client_alive(struct connection *connection)
 {
 	struct gdb_connection *gdb_con = connection->priv;
 
-	if (gdb_con->busy) {
-		/* do not send packets, retry asap */
-		return;
-	}
-
 	switch (gdb_con->output_flag) {
 	case GDB_OUTPUT_NO:
 		/* no need for keep-alive */