gdbsupport/thread-pool.cc

/* Thread pool

   Copyright (C) 2019-2026 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 "gdbsupport/thread-pool.h"

#if CXX_STD_THREAD

#include "gdbsupport/alt-stack.h"
#include "gdbsupport/block-signals.h"
#include <algorithm>
#include <system_error>

/* 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
do_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
do_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
do_set_thread_name (int (*set_name) (const char *), const char *name)
{
  set_name (name);
}

static void
set_thread_name (const char *name)
{
  do_set_thread_name (pthread_setname_np, name);
}

#elif defined (USE_WIN32API)

#include <windows.h>

typedef HRESULT WINAPI (SetThreadDescription_ftype) (HANDLE, PCWSTR);
static SetThreadDescription_ftype *dyn_SetThreadDescription;
static bool initialized;

static void
init_windows ()
{
  initialized = true;

  HMODULE hm = LoadLibrary (TEXT ("kernel32.dll"));
  if (hm)
    dyn_SetThreadDescription
      = (SetThreadDescription_ftype *) GetProcAddress (hm,
						       "SetThreadDescription");

  /* On some versions of Windows, this function is only available in
     KernelBase.dll, not kernel32.dll.  */
  if (dyn_SetThreadDescription == nullptr)
    {
      hm = LoadLibrary (TEXT ("KernelBase.dll"));
      if (hm)
	dyn_SetThreadDescription
	  = (SetThreadDescription_ftype *) GetProcAddress (hm,
							   "SetThreadDescription");
    }
}

static void
do_set_thread_name (const wchar_t *name)
{
  if (!initialized)
    init_windows ();

  if (dyn_SetThreadDescription != nullptr)
    dyn_SetThreadDescription (GetCurrentThread (), name);
}

#define set_thread_name(NAME) do_set_thread_name (L ## NAME)

#else /* USE_WIN32API */

static void
set_thread_name (const char *name)
{
}

#endif

#endif /* CXX_STD_THREAD */

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.  */
}

size_t
thread_pool::thread_count ()
{
#if CXX_STD_THREAD
  std::lock_guard<std::mutex> guard (m_tasks_mutex);
  return m_thread_count;
#else
  return 0;
#endif
}

void
thread_pool::set_thread_count (size_t num_threads)
{
#if CXX_STD_THREAD
  std::lock_guard<std::mutex> guard (m_tasks_mutex);
  m_sized_at_least_once = true;

  /* 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)
	{
	  try
	    {
	      std::thread thread (&thread_pool::thread_function, this);
	      thread.detach ();
	    }
	  catch (const std::system_error &)
	    {
	      /* libstdc++ may not implement std::thread, and will
		 throw an exception on use.  It seems fine to ignore
		 this, and any other sort of startup failure here.  */
	      num_threads = i;
	      break;
	    }
	}
    }
  /* 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;
#else
  /* No threads available, simply ignore the request.  */
#endif /* CXX_STD_THREAD */
}

#if CXX_STD_THREAD

void
thread_pool::do_post_task (std::packaged_task<void ()> &&func)
{
  /* This is a bit strange but we don't want to hold the lock when
     calling FUNC in the case where it must be called immediately.
     Although that seems pathological, there are some weird cases (a
     moribund worker thread or FUNC itself submitting a task) and it
     seemed best to be safe.  */
  bool call_immediately = false;

  {
    std::lock_guard<std::mutex> guard (m_tasks_mutex);

    /* This assert is here to check that no tasks are posted to the
       pool between its initialization and sizing.  */
    gdb_assert (m_sized_at_least_once);

    if (m_thread_count != 0)
      {
	m_tasks.emplace (std::move (func));
	m_tasks_cv.notify_one ();
      }
    else
      call_immediately = true;
  }

  if (call_immediately)
    {
      /* Just execute it now.  */
      func ();
    }
}

void
thread_pool::thread_function ()
{
  /* This must be done here, because on macOS one can only set the
     name of the current thread.  */
  set_thread_name ("gdb worker");

  /* Ensure that SIGSEGV is delivered to an alternate signal
     stack.  */
  gdb::alternate_signal_stack signal_stack;

  while (true)
    {
      std::optional<task_t> 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 */

} /* namespace gdb */