Fix SubProcess Communicate() for macOS.

On macOS, poll(2) may return POLLNVAL for a valid file descriptor that was
closed on its remote side (by the other process) before this process calls
poll() the first time.  Treat this case as an error.

Re-enable the SubProcess tests, one of which (StdinChildExit) appeared
flaky on macOS depending on the timing of the child process exiting and
the parent process first calling poll().
Change: 139850066

tensorflow/core/BUILD

@@ -1541,6 +1541,7 @@ tf_cc_tests(
         "platform/net_test.cc",
         "platform/port_test.cc",
         "platform/profile_utils/cpu_utils_test.cc",
+        "platform/subprocess_test.cc",
     ],
     deps = [
         ":lib",
@@ -1553,21 +1554,6 @@ tf_cc_tests(
     ],
 )
 
-tf_cc_tests(
-    size = "small",
-    srcs = ["platform/subprocess_test.cc"],
-    tags = ["manual"],
-    deps = [
-        ":lib",
-        ":lib_internal",
-        ":lib_test_internal",
-        ":protos_all_cc",
-        ":test",
-        ":test_main",
-        "//third_party/eigen3",
-    ],
-)
-
 tf_cc_tests(
     size = "small",
     srcs = ["platform/env_test.cc"],

tensorflow/core/platform/posix/subprocess.cc

@@ -446,7 +446,7 @@ int SubProcess::Communicate(const string* stdin_input, string* stdout_output,
             fds[i].fd = -1;
             fd_remain--;
           }
-        } else if ((fds[i].revents & POLLERR) != 0) {
+        } else if ((fds[i].revents & (POLLERR | POLLNVAL)) != 0) {
           fds[i].fd = -1;
           fd_remain--;
         }