Add gdb::task_group

This adds gdb::task_group, a convenient way to group background tasks
and then call a function when all the tasks have completed.

gdbsupport/task-group.h

@@ -0,0 +1,61 @@
+/* Task group
+
+   Copyright (C) 2023 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef GDBSUPPORT_TASK_GROUP_H
+#define GDBSUPPORT_TASK_GROUP_H
+
+#include <memory>
+
+namespace gdb
+{
+
+/* A task group is a collection of tasks.  Each task in the group is
+   submitted to the thread pool.  When all the tasks in the group have
+   finished, a final action is run.  */
+
+class task_group
+{
+public:
+
+  explicit task_group (std::function<void ()> &&done);
+  DISABLE_COPY_AND_ASSIGN (task_group);
+
+  /* Add a task to the task group.  All tasks must be added before the
+     group is started.  Note that a task may not throw an
+     exception.  */
+  void add_task (std::function<void ()> &&task);
+
+  /* Start this task group.  A task group may only be started once.
+     This will submit all the tasks to the global thread pool.  */
+  void start ();
+
+private:
+
+  class impl;
+
+  /* A task group is just a facade around an impl.  This is done
+     because the impl object must live as long as its longest-lived
+     task, so it is heap-allocated and destroyed when the last task
+     completes.  */
+  std::shared_ptr<impl> m_task;
+};
+
+} /* namespace gdb */
+
+#endif /* GDBSUPPORT_TASK_GROUP_H */