gdbsupport: rename source files to .cc

This patch renames the .c source files in gdbsupport to .cc.

In the gdb directory, there is an argument against renaming the source
files, which is that it makes using some git commands more difficult to
do archeology.  Some commands have some kind of "follow" option that
makes git try to follow renames, but it doesn't work in all situations.

Given that we have just moved the gdbsupport directory, that argument
doesn't hold for source files in that directory.  I therefore suggest
renaming them to .cc, so that they are automatically recognized as C++
by various tools and editors.

The original motivation behind this is that when building gdbsupport
with clang, I get:

      CC       agent.o
    clang: error: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Werror,-Wdeprecated]

In the gdb/ directory, we make clang happy by passing "-x c++".  We
could do this in gdbsupport too, but I think that renaming the files is
a better long-term solution.

gdbserver still does its own build of gdbsupport, so a few changes in
its Makefile are necessary.

gdbsupport/ChangeLog:

	* Makefile.am: Rename source files from .c to .cc.
	(CC, CFLAGS): Don't override.
	(AM_CFLAGS): Rename to ...
	(AM_CXXFLAGS): ... this.
	* Makefile.in: Re-generate.
	* %.c: Rename to %.cc.

gdbserver/ChangeLog:

	* Makefile.in: Rename gdbsupport source files from .c to .cc.

1 files changed, 175 insertions, 0 deletions

diff --git a/gdbsupport/thread-pool.cc b/gdbsupport/thread-pool.cc
new file mode 100644
index 00000000000..be9ca22682a
--- /dev/null
+++ b/gdbsupport/thread-pool.cc
@@ -0,0 +1,175 @@
+/* Thread pool
+
+   Copyright (C) 2019-2020 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"
+
+#if CXX_STD_THREAD
+
+#include "gdbsupport/thread-pool.h"
+#include "gdbsupport/alt-stack.h"
+#include "gdbsupport/block-signals.h"
+#include <algorithm>
+
+/* On the off chance that we have the pthread library on a Windows
+   host, but std::thread is not using it, avoid calling
+   pthread_setname_np on Windows.  */
+#ifndef _WIN32
+#ifdef HAVE_PTHREAD_SETNAME_NP
+#define USE_PTHREAD_SETNAME_NP
+#endif
+#endif
+
+#ifdef USE_PTHREAD_SETNAME_NP
+
+#include <pthread.h>
+
+/* Handle platform discrepancies in pthread_setname_np: macOS uses a
+   single-argument form, while Linux uses a two-argument form.  NetBSD
+   takes a printf-style format and an argument.  This wrapper handles the
+   difference.  */
+
+ATTRIBUTE_UNUSED static void
+set_thread_name (int (*set_name) (pthread_t, const char *, void *),
+				  const char *name)
+{
+  set_name (pthread_self (), "%s", const_cast<char *> (name));
+}
+
+ATTRIBUTE_UNUSED static void
+set_thread_name (int (*set_name) (pthread_t, const char *), const char *name)
+{
+  set_name (pthread_self (), name);
+}
+
+/* The macOS man page says that pthread_setname_np returns "void", but
+   the headers actually declare it returning "int".  */
+ATTRIBUTE_UNUSED static void
+set_thread_name (int (*set_name) (const char *), const char *name)
+{
+  set_name (name);
+}
+
+#endif	/* USE_PTHREAD_SETNAME_NP */
+
+namespace gdb
+{
+
+/* The thread pool detach()s its threads, so that the threads will not
+   prevent the process from exiting.  However, it was discovered that
+   if any detached threads were still waiting on a condition variable,
+   then the condition variable's destructor would wait for the threads
+   to exit -- defeating the purpose.
+
+   Allocating the thread pool on the heap and simply "leaking" it
+   avoids this problem.
+*/
+thread_pool *thread_pool::g_thread_pool = new thread_pool ();
+
+thread_pool::~thread_pool ()
+{
+  /* Because this is a singleton, we don't need to clean up.  The
+     threads are detached so that they won't prevent process exit.
+     And, cleaning up here would be actively harmful in at least one
+     case -- see the comment by the definition of g_thread_pool.  */
+}
+
+void
+thread_pool::set_thread_count (size_t num_threads)
+{
+  std::lock_guard<std::mutex> guard (m_tasks_mutex);
+
+  /* If the new size is larger, start some new threads.  */
+  if (m_thread_count < num_threads)
+    {
+      /* Ensure that signals used by gdb are blocked in the new
+	 threads.  */
+      block_signals blocker;
+      for (size_t i = m_thread_count; i < num_threads; ++i)
+	{
+	  std::thread thread (&thread_pool::thread_function, this);
+	  thread.detach ();
+	}
+    }
+  /* If the new size is smaller, terminate some existing threads.  */
+  if (num_threads < m_thread_count)
+    {
+      for (size_t i = num_threads; i < m_thread_count; ++i)
+	m_tasks.emplace ();
+      m_tasks_cv.notify_all ();
+    }
+
+  m_thread_count = num_threads;
+}
+
+std::future<void>
+thread_pool::post_task (std::function<void ()> func)
+{
+  std::packaged_task<void ()> t (func);
+  std::future<void> f = t.get_future ();
+
+  if (m_thread_count == 0)
+    {
+      /* Just execute it now.  */
+      t ();
+    }
+  else
+    {
+      std::lock_guard<std::mutex> guard (m_tasks_mutex);
+      m_tasks.emplace (std::move (t));
+      m_tasks_cv.notify_one ();
+    }
+  return f;
+}
+
+void
+thread_pool::thread_function ()
+{
+#ifdef USE_PTHREAD_SETNAME_NP
+  /* This must be done here, because on macOS one can only set the
+     name of the current thread.  */
+  set_thread_name (pthread_setname_np, "gdb worker");
+#endif
+
+  /* Ensure that SIGSEGV is delivered to an alternate signal
+     stack.  */
+  gdb::alternate_signal_stack signal_stack;
+
+  while (true)
+    {
+      optional<task> t;
+
+      {
+	/* We want to hold the lock while examining the task list, but
+	   not while invoking the task function.  */
+	std::unique_lock<std::mutex> guard (m_tasks_mutex);
+	while (m_tasks.empty ())
+	  m_tasks_cv.wait (guard);
+	t = std::move (m_tasks.front());
+	m_tasks.pop ();
+      }
+
+      if (!t.has_value ())
+	break;
+      (*t) ();
+    }
+}
+
+}
+
+#endif /* CXX_STD_THREAD */