[04/15] Add gdb::task_group

diff --git a/gdbsupport/Makefile.am b/gdbsupport/Makefile.am
index 00524e9a566..1871b96f49b 100644
--- a/gdbsupport/Makefile.am
+++ b/gdbsupport/Makefile.am
@@ -74,6 +74,7 @@  libgdbsupport_a_SOURCES = \
     search.cc \
     signals.cc \
     signals-state-save-restore.cc \
+    task-group.cc \
     tdesc.cc \
     thread-pool.cc \
     xml-utils.cc \
diff --git a/gdbsupport/Makefile.in b/gdbsupport/Makefile.in
index 50a5a55e19d..bbbbbd57282 100644
--- a/gdbsupport/Makefile.in
+++ b/gdbsupport/Makefile.in
@@ -160,8 +160,8 @@  am_libgdbsupport_a_OBJECTS = agent.$(OBJEXT) btrace-common.$(OBJEXT) \
 	ptid.$(OBJEXT) rsp-low.$(OBJEXT) run-time-clock.$(OBJEXT) \
 	safe-strerror.$(OBJEXT) scoped_mmap.$(OBJEXT) search.$(OBJEXT) \
 	signals.$(OBJEXT) signals-state-save-restore.$(OBJEXT) \
-	tdesc.$(OBJEXT) thread-pool.$(OBJEXT) xml-utils.$(OBJEXT) \
-	$(am__objects_1) $(am__objects_2)
+	task-group.$(OBJEXT) tdesc.$(OBJEXT) thread-pool.$(OBJEXT) \
+	xml-utils.$(OBJEXT) $(am__objects_1) $(am__objects_2)
 libgdbsupport_a_OBJECTS = $(am_libgdbsupport_a_OBJECTS)
 AM_V_P = $(am__v_P_@AM_V@)
 am__v_P_ = $(am__v_P_@AM_DEFAULT_V@)
@@ -401,6 +401,7 @@  libgdbsupport_a_SOURCES = \
     search.cc \
     signals.cc \
     signals-state-save-restore.cc \
+    task-group.cc \
     tdesc.cc \
     thread-pool.cc \
     xml-utils.cc \
@@ -511,6 +512,7 @@  distclean-compile:
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/selftest.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/signals-state-save-restore.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/signals.Po@am__quote@
+@AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/task-group.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/tdesc.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/thread-pool.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/xml-utils.Po@am__quote@
diff --git a/gdbsupport/task-group.cc b/gdbsupport/task-group.cc
new file mode 100644
index 00000000000..e6c8a6f2e31
--- /dev/null
+++ b/gdbsupport/task-group.cc
@@ -0,0 +1,97 @@ 
+/* 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/>.  */
+
+#include "common-defs.h"
+#include "task-group.h"
+#include "thread-pool.h"
+
+namespace gdb
+{
+
+class task_group::impl : public std::enable_shared_from_this<task_group::impl>
+{
+public:
+
+  explicit impl (std::function<void ()> &&done)
+    : m_completed (0),
+      m_done (std::move (done))
+  { }
+  DISABLE_COPY_AND_ASSIGN (impl);
+
+  ~impl ()
+  {
+    if (m_started)
+      m_done ();
+  }
+
+  /* Add a task to the task group.  */
+  void add_task (std::function<void ()> &&task)
+  {
+    m_tasks.push_back (std::move (task));
+  };
+
+  /* Start this task group.  */
+  void start ();
+
+  /* True if started.  */
+  bool m_started = false;
+  /* The tasks.  */
+  std::vector<std::function<void ()>> m_tasks;
+  /* The number of tasks that have completed.  */
+  std::atomic<int> m_completed;
+  /* The 'done' function.  */
+  std::function<void ()> m_done;
+};
+
+void
+task_group::impl::start ()
+{
+  std::shared_ptr<impl> shared_this = shared_from_this ();
+  m_started = true;
+  for (size_t i = 0; i < m_tasks.size (); ++i)
+    {
+      gdb::thread_pool::g_thread_pool->post_task ([=] ()
+      {
+	/* Be sure to capture a shared reference here.  */
+	shared_this->m_tasks[i] ();
+      });
+    }
+}
+
+task_group::task_group (std::function<void ()> &&done)
+  : m_task (new impl (std::move (done)))
+{
+}
+
+void
+task_group::add_task (std::function<void ()> &&task)
+{
+  gdb_assert (m_task != nullptr);
+  m_task->add_task (std::move (task));
+}
+
+void
+task_group::start ()
+{
+  gdb_assert (m_task != nullptr);
+  m_task->start ();
+  m_task.reset ();
+}
+
+} /* namespace gdb */
diff --git a/gdbsupport/task-group.h b/gdbsupport/task-group.h
new file mode 100644
index 00000000000..c11b5ccf158
--- /dev/null
+++ b/gdbsupport/task-group.h
@@ -0,0 +1,64 @@ 
+/* 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.  Before 'start' is called, the impl is owned by this
+     task_group, but after 'start' it is owned jointly by the running
+     tasks, and m_task is cleared.  */
+  // FIXME
+  std::shared_ptr<impl> m_task;
+};
+
+} /* namespace gdb */
+
+#endif /* GDBSUPPORT_TASK_GROUP_H */