diff --git a/tools/run_tests/port_server.py b/tools/run_tests/port_server.py
index 4e473af4113..0593d7dc974 100755
--- a/tools/run_tests/port_server.py
+++ b/tools/run_tests/port_server.py
@@ -42,7 +42,7 @@ import time
 # increment this number whenever making a change to ensure that
 # the changes are picked up by running CI servers
 # note that all changes must be backwards compatible
-_MY_VERSION = 4
+_MY_VERSION = 5
 
 
 if len(sys.argv) == 2 and sys.argv[1] == 'dump_version':
@@ -124,9 +124,12 @@ class Handler(BaseHTTPServer.BaseHTTPRequestHandler):
       self.send_header('Content-Type', 'text/plain')
       self.end_headers()
       p = int(self.path[6:])
-      del in_use[p]
-      pool.append(p)
-      self.log_message('drop port %d' % p)
+      if p in in_use:
+        del in_use[p]
+        pool.append(p)
+        self.log_message('drop known port %d' % p)
+      else:
+        self.log_message('drop unknown port %d' % p)
     elif self.path == '/version_number':
       # fetch a version string and the current process pid
       self.send_response(200)
diff --git a/tools/run_tests/run_tests.py b/tools/run_tests/run_tests.py
index 71377590221..2934b5f70f8 100755
--- a/tools/run_tests/run_tests.py
+++ b/tools/run_tests/run_tests.py
@@ -709,9 +709,17 @@ def _start_port_server(port_server_port):
     fd, logfile = tempfile.mkstemp()
     os.close(fd)
     print 'starting port_server, with log file %s' % logfile
-    port_server = subprocess.Popen(
-        [sys.executable, 'tools/run_tests/port_server.py', '-p', '%d' % port_server_port, '-l', logfile],
+    args = [sys.executable, 'tools/run_tests/port_server.py', '-p', '%d' % port_server_port, '-l', logfile]
+    if platform.system() == 'Windows':
+      port_server = subprocess.Popen(
+        args,
+        creationflags = 0x00000008, # detached process
         close_fds=True)
+    else:
+      port_server = subprocess.Popen(
+          args,
+          preexec_fn=os.setsid,
+          close_fds=True)
     time.sleep(1)
     # ensure port server is up
     waits = 0