[NaCl SDK] nacl_io: Fix select() implementation so it doesn't assume entire fs_sets.

native_client_sdk/src/libraries/nacl_io/kernel_proxy.cc

Index: native_client_sdk/src/libraries/nacl_io/kernel_proxy.cc
diff --git a/native_client_sdk/src/libraries/nacl_io/kernel_proxy.cc b/native_client_sdk/src/libraries/nacl_io/kernel_proxy.cc
index 680eaac0978acc656fa2955fde7449b376670386..0cd8c10f491d0798b1dc9352612778596d4d0a46 100644
--- a/native_client_sdk/src/libraries/nacl_io/kernel_proxy.cc
+++ b/native_client_sdk/src/libraries/nacl_io/kernel_proxy.cc
@@ -1029,28 +1029,24 @@ int KernelProxy::sigaction(int signum, const struct sigaction* action,
 
 int KernelProxy::select(int nfds, fd_set* readfds, fd_set* writefds,
                         fd_set* exceptfds, struct timeval* timeout) {
-  fd_set ignore;
   std::vector<pollfd> pollfds;
 
-  // Simplify logic, by using an IGNORE set for any undefined set
-  FD_ZERO(&ignore);
-  if (NULL == readfds)
-    readfds = &ignore;
-  if (NULL == writefds)
-    writefds = &ignore;
-  if (NULL == exceptfds)
-    exceptfds = &ignore;
-
   for (int fd = 0; fd < nfds; fd++) {
     int events = 0;
-    if (FD_ISSET(fd, readfds))
+    if (readfds && FD_ISSET(fd, readfds)) {
       events |= POLLIN;
+      FD_CLR(fd, readfds);
+    }
 
-    if (FD_ISSET(fd, writefds))
+    if (writefds && FD_ISSET(fd, writefds)) {
       events |= POLLOUT;
+      FD_CLR(fd, writefds);
+    }
 
-    if (FD_ISSET(fd, exceptfds))
+    if (exceptfds && FD_ISSET(fd, exceptfds)) {
       events |= POLLERR | POLLHUP;
+      FD_CLR(fd, exceptfds);
+    }
 
     if (events) {
       pollfd info;
@@ -1060,10 +1056,6 @@ int KernelProxy::select(int nfds, fd_set* readfds, fd_set* writefds,
     }
   }
 
-  FD_ZERO(readfds);
-  FD_ZERO(writefds);
-  FD_ZERO(exceptfds);
-
   // NULL timeout signals wait forever.
   int ms_timeout = -1;
   if (timeout != NULL) {