use background thread to wait for address changes on OSX (dotnet/core…

…fx#41768)

* use background thread to wait for address changes on OSX

* remove RemoteExecutor

* update comment


Commit migrated from dotnet/corefx@08bf1fe

src/libraries/System.Net.NetworkInformation/src/System/Net/NetworkInformation/NetworkAddressChange.OSX.cs

@@ -181,6 +181,7 @@ private static unsafe void CreateAndStartRunLoop()
                 }
             }
             s_runLoopThread = new Thread(RunLoopThreadStart);
+            s_runLoopThread.IsBackground = true;
             s_runLoopThread.Start();
             s_runLoopStartedEvent.WaitOne(); // Wait for the new thread to finish initialization.
         }

src/libraries/System.Net.NetworkInformation/tests/FunctionalTests/NetworkAvailabilityChangedTests.cs

@@ -27,6 +27,20 @@ public void NetworkAvailabilityChanged_JustRemove_Success()
             NetworkChange.NetworkAvailabilityChanged -= _availabilityHandler;
         }
 
+        [Fact]
+        [ActiveIssue(33530, TestPlatforms.FreeBSD)]
+        public void NetworkAddressChanged_Add_DoesNotBlock()
+        {
+            // Register without unregistering.
+            // This should not block process exit. If it does, this test will pass
+            // but we would fail to exit test run at the end.
+            // We cannot test this via RemoteInvoke() as that calls Environment.Exit()
+            // and forces quit even when foreground threads are running.
+            NetworkChange.NetworkAddressChanged += _addressHandler;
+            {
+            };
+        }
+
         [Fact]
         [ActiveIssue(33530, TestPlatforms.FreeBSD)]
         public void NetworkAddressChanged_AddAndRemove_NetworkAvailabilityChanged_JustRemove_Success()