fix Issue 15270 - use TLS to store Thread.getThis

- as signals are no longer send during thread startup we can use
  a TLS variable to store Thread.getThis()
- make sure to set the variable (and probably trigger lazy TLS
  allocation) before adding the Thread to the global list

src/core/thread.d

@@ -267,8 +267,8 @@ else version( Posix )
             obj.m_tlsgcdata = rt_tlsgc_init();
 
             atomicStore!(MemoryOrder.raw)(obj.m_isRunning, true);
-            Thread.setThis(obj);
-            Thread.add(obj);
+            Thread.setThis(obj); // allocates lazy TLS (see Issue 11981)
+            Thread.add(obj);     // can only receive signals from here on
             scope (exit)
             {
                 Thread.remove(obj);
@@ -1130,19 +1130,7 @@ class Thread
         // NOTE: This function may not be called until thread_init has
         //       completed.  See thread_suspendAll for more information
         //       on why this might occur.
-        version( OSX )
-        {
-            return sm_this;
-        }
-        else version( Posix )
-        {
-            auto t = cast(Thread) pthread_getspecific( sm_this );
-            return t;
-        }
-        else
-        {
-            return sm_this;
-        }
+        return sm_this;
     }
 
 
@@ -1401,22 +1389,7 @@ private:
     //
     // Local storage
     //
-    version( OSX )
-    {
-        static Thread       sm_this;
-    }
-    else version( Posix )
-    {
-        // On Posix (excluding OSX), pthread_key_t is explicitly used to
-        // store and access thread reference. This is needed
-        // to avoid TLS access in signal handlers (malloc deadlock)
-        // when using shared libraries, see issue 11981.
-        __gshared pthread_key_t sm_this;
-    }
-    else
-    {
-        static Thread       sm_this;
-    }
+    static Thread       sm_this;
 
 
     //
@@ -1475,18 +1448,7 @@ private:
     //
     static void setThis( Thread t )
     {
-        version( OSX )
-        {
-            sm_this = t;
-        }
-        else version( Posix )
-        {
-            pthread_setspecific( sm_this, cast(void*) t );
-        }
-        else
-        {
-            sm_this = t;
-        }
+        sm_this = t;
     }
 
 
@@ -1998,9 +1960,6 @@ extern (C) void thread_init()
 
         status = sem_init( &suspendCount, 0, 0 );
         assert( status == 0 );
-
-        status = pthread_key_create( &Thread.sm_this, null );
-        assert( status == 0 );
     }
     Thread.sm_main = thread_attachThis();
 }
@@ -2020,14 +1979,6 @@ extern (C) void thread_term()
         Thread.pAboutToStart = null;
     }
     Thread.termLocks();
-
-    version( OSX )
-    {
-    }
-    else version( Posix )
-    {
-        pthread_key_delete( Thread.sm_this );
-    }
 }