[PATCH] Fix build on macOS

[Tom Tromey <tromey@adacore.com>]

PR build/25268 points out that the build fails on macOS, because on
macOS the "pthread_setname_np" function takes a single argument.

This patch fixes the problem, by introducing a new template adapter
function that handles both styles of pthread_setname_np.  This is a
technique I learned from Alexandre Oliva, and avoids the need for a
complicated autoconf check.

This change also meant moving the pthread_setname_np call to the
thread function, because macOS only permits setting the name of the
current thread.  This means that there can be a brief window when gdb
will see the wrong name; but I think this is a minor concern.

Tested by rebuilding on x86-64 Fedora 30, and on macOS High Sierra.
On Linux I also debugged gdb to ensure that the thread names are still
set correctly.

gdb/ChangeLog
2019-12-10  Tom Tromey  <tromey@adacore.com>

	PR build/25268:
	* gdbsupport/thread-pool.c (set_thread_name): New template
	function.
	(thread_pool::set_thread_count): Don't call pthread_setname_np.
	(thread_pool::thread_function): Call set_thread_name.

Change-Id: Id7bf28d99ca27a893a9fc87ebb90b15a9c2a9cb4
---
 gdb/ChangeLog                |  8 ++++++++
 gdb/gdbsupport/thread-pool.c | 31 +++++++++++++++++++++++++++----
 2 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/gdb/gdbsupport/thread-pool.c b/gdb/gdbsupport/thread-pool.c
index d19ae02e3ef..8e24570ee96 100644
--- a/gdb/gdbsupport/thread-pool.c
+++ b/gdb/gdbsupport/thread-pool.c
@@ -36,8 +36,28 @@
 #endif
 
 #ifdef USE_PTHREAD_SETNAME_NP
+
 #include <pthread.h>
-#endif
+
+/* Handle platform discrepancies in pthread_setname_np: macOS uses a
+   single-argument form, while Linux uses a two-argument form.  This
+   wrapper template handles the difference.  */
+
+template<typename R, typename A1, typename A2>
+void
+set_thread_name (R (*set_name) (A1, A2), const char *name)
+{
+  set_name (pthread_self (), name);
+}
+
+template<typename R, typename A1>
+void
+set_thread_name (R (*set_name) (A1), const char *name)
+{
+  set_name (name);
+}
+
+#endif	/* USE_PTHREAD_SETNAME_NP */
 
 namespace gdb
 {
@@ -75,9 +95,6 @@ thread_pool::set_thread_count (size_t num_threads)
       for (size_t i = m_thread_count; i < num_threads; ++i)
 	{
 	  std::thread thread (&thread_pool::thread_function, this);
-#ifdef USE_PTHREAD_SETNAME_NP
-	  pthread_setname_np (thread.native_handle (), "gdb worker");
-#endif
 	  thread.detach ();
 	}
     }
@@ -115,6 +132,12 @@ thread_pool::post_task (std::function<void ()> func)
 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;
-- 
2.21.0