testserver.py listens on ephemeral ports by default.

net/test/test_server_win.cc

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/test/test_server.h"
 
 #include <windows.h>
 #include <wincrypt.h>
 
 #include "base/base_paths.h"
@@ skipping 138 matching lines @@
   base::Thread thread("test_server_watcher");
   if (!thread.Start())
     return false;
 
   // Prepare a timeout in case the server fails to start.
   bool unblocked = false;
   thread.message_loop()->PostDelayedTask(FROM_HERE,
       NewRunnableFunction(UnblockPipe, child_write_fd_.Get(), &unblocked),
       TestTimeouts::action_max_timeout_ms());
 
-  char buf[8];
-  DWORD bytes_read;
-  BOOL result = ReadFile(child_read_fd_.Get(), buf, sizeof(buf), &bytes_read,
-                         NULL);
-
+  // Try to read two bytes from the pipe indicating the ephemeral port number.
+  uint16 port;
+  uint8* buffer = reinterpret_cast<uint8*>(&port);
+  DWORD bytes_read = 0;
+  DWORD bytes_max = sizeof(port);
+  while (bytes_read < bytes_max) {
+    DWORD num_bytes;
+    if (!ReadFile(child_read_fd_, buffer + bytes_read, bytes_max - bytes_read,
+                  &num_bytes, NULL))
+      break;
+    if (num_bytes <= 0)
+      break;
+    bytes_read += num_bytes;
+  }
   thread.Stop();
   child_read_fd_.Close();
   child_write_fd_.Close();
 
   // If we hit the timeout, fail.
   if (unblocked)
     return false;
 
-  return result && bytes_read > 0;
+  // If not enough bytes were read, fail.
+  if (bytes_read < bytes_max)
+    return false;
+
+  host_port_pair_.set_port(port);
+  return true;
 }
 
 bool TestServer::CheckCATrusted() {
   HCERTSTORE cert_store = CertOpenSystemStore(NULL, L"ROOT");
   if (!cert_store) {
     LOG(ERROR) << " could not open trusted root CA store";
     return false;
   }
   PCCERT_CONTEXT cert =
       CertFindCertificateInStore(cert_store,
@@ skipping 11 matching lines @@
                   "certificate to your trusted roots for this test to work. "
                   "For more info visit:\n"
                   "http://dev.chromium.org/developers/testing\n";
     return false;
   }
 
   return true;
 }
 
 }  // namespace net