gdb_server: avoid gdb server for virtual targets

Virtual targets, like mem_ap, do not or cannot implement the required
functionality to accept a GDB connection. In the case of mem_ap, the
method get_gdb_reg_list() is missing and a following connection from
gdb causes OpenOCD to segfault.
OpenOCD opens a GDB port for each target; it's always possible to
connect, by mistake, GDB to one virtual target.

Add a method to check if the target supports GDB connections (for the
moment just checking if get_gdb_reg_list is implemented).
Skip opening a gdb server for every targets that don't support GDB
connections.

Change-Id: Ia439a43efe1a9adbb1771cd9d252db8ffa32eb9d
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4676
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>

src/server/gdb_server.c

@@ -3404,6 +3404,12 @@ static int gdb_target_add_one(struct target *target)
 	if ((target->smp) && (target->gdb_service))
 		return ERROR_OK;
 
+	/* skip targets that cannot handle a gdb connections (e.g. mem_ap) */
+	if (!target_supports_gdb_connection(target)) {
+		LOG_DEBUG("skip gdb server for target %s", target_name(target));
+		return ERROR_OK;
+	}
+
 	if (target->gdb_port_override) {
 		if (strcmp(target->gdb_port_override, "disabled") == 0) {
 			LOG_INFO("gdb port disabled");

src/target/target.c

@@ -1205,6 +1205,16 @@ int target_get_gdb_reg_list(struct target *target,
 {
 	return target->type->get_gdb_reg_list(target, reg_list, reg_list_size, reg_class);
 }
+
+bool target_supports_gdb_connection(struct target *target)
+{
+	/*
+	 * based on current code, we can simply exclude all the targets that
+	 * don't provide get_gdb_reg_list; this could change with new targets.
+	 */
+	return !!target->type->get_gdb_reg_list;
+}
+
 int target_step(struct target *target,
 		int current, target_addr_t address, int handle_breakpoints)
 {

src/target/target.h

@@ -481,6 +481,13 @@ int target_get_gdb_reg_list(struct target *target,
 		struct reg **reg_list[], int *reg_list_size,
 		enum target_register_class reg_class);
 
+/**
+ * Check if @a target allows GDB connections.
+ *
+ * Some target do not implement the necessary code required by GDB.
+ */
+bool target_supports_gdb_connection(struct target *target);
+
 /**
  * Step the target.
  *