testserver.py listens on ephemeral ports by default.

net/test/test_server_win.cc

Index: net/test/test_server_win.cc
diff --git a/net/test/test_server_win.cc b/net/test/test_server_win.cc
index c6e10d5bbc61d2d6cac8b5e69fa3b9e2a3bfe64b..64437cd46d6a2aa8348d67c8b63f0400a8b71d7c 100644
--- a/net/test/test_server_win.cc
+++ b/net/test/test_server_win.cc
@@ -156,11 +156,20 @@ bool TestServer::WaitToStart() {
       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();
@@ -169,7 +178,12 @@ bool TestServer::WaitToStart() {
   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() {