Comment the requirements for changing grpc_poll_function and do

poll overrides in such a way as to avoid polling races

src/core/iomgr/pollset_posix.h

@@ -142,6 +142,10 @@ int grpc_pollset_has_workers(grpc_pollset *pollset);
 void grpc_remove_fd_from_all_epoll_sets(int fd);
 
 /* override to allow tests to hook poll() usage */
+/* NOTE: Any changes to grpc_poll_function must take place when the gRPC
+   is certainly not doing any polling anywhere.
+   Otherwise, there might be a race between changing the variable and actually
+   doing a polling operation */
 typedef int (*grpc_poll_function_type)(struct pollfd *, nfds_t, int);
 extern grpc_poll_function_type grpc_poll_function;
 extern grpc_wakeup_fd grpc_global_wakeup_fd;

test/cpp/end2end/async_end2end_test.cc

@@ -86,21 +86,21 @@ class PollOverride {
   grpc_poll_function_type prev_;
 };
 
-class PollingCheckRegion : public PollOverride {
+class PollingOverrider : public PollOverride {
  public:
-  explicit PollingCheckRegion(bool allow_blocking)
+  explicit PollingOverrider(bool allow_blocking)
       : PollOverride(allow_blocking ? poll : assert_non_blocking_poll) {}
 };
 #else
-class PollingCheckRegion {
+class PollingOverrider {
  public:
-  explicit PollingCheckRegion(bool allow_blocking) {}
+  explicit PollingOverrider(bool allow_blocking) {}
 };
 #endif
 
-class Verifier : public PollingCheckRegion {
+class Verifier {
  public:
-  explicit Verifier(bool spin) : PollingCheckRegion(!spin), spin_(spin) {}
+  explicit Verifier(bool spin) : spin_(spin) {}
   Verifier& Expect(int i, bool expect_ok) {
     expectations_[tag(i)] = expect_ok;
     return *this;
@@ -180,7 +180,7 @@ class Verifier : public PollingCheckRegion {
 
 class AsyncEnd2endTest : public ::testing::TestWithParam<bool> {
  protected:
-  AsyncEnd2endTest() {}
+  AsyncEnd2endTest(): poll_override_(GetParam()) {}
 
   void SetUp() GRPC_OVERRIDE {
     int port = grpc_pick_unused_port_or_die();
@@ -249,6 +249,8 @@ class AsyncEnd2endTest : public ::testing::TestWithParam<bool> {
   std::unique_ptr<Server> server_;
   grpc::testing::EchoTestService::AsyncService service_;
   std::ostringstream server_address_;
+
+  PollingOverrider poll_override_;
 };
 
 TEST_P(AsyncEnd2endTest, SimpleRpc) {