Change the Fixture shutdown code so that the server fds are still polled

(to flush out any writes) when the server is shutdown.

test/cpp/microbenchmarks/fullstack_fixtures.h

@@ -61,6 +61,15 @@ class FixtureConfiguration {
 
 class BaseFixture : public TrackCounters {};
 
+// Special tag to be used in Server shutdown. This tag is *NEVER* returned when
+// Cq->Next() API is called (This is because FinalizeResult() function in this
+// class always returns 'false'). This is intentional and makes writing shutdown
+// code easier.
+class ShutdownTag : public internal::CompletionQueueTag {
+ public:
+  bool FinalizeResult(void** tag, bool* status) { return false; }
+};
+
 class FullstackFixture : public BaseFixture {
  public:
   FullstackFixture(Service* service, const FixtureConfiguration& config,
@@ -84,7 +93,11 @@ class FullstackFixture : public BaseFixture {
   }
 
   virtual ~FullstackFixture() {
-    server_->Shutdown(gpr_inf_past(GPR_CLOCK_MONOTONIC));
+    // Dummy shutdown tag (this tag is swallowed by cq->Next() and is not
+    // returned to the user) see ShutdownTag definition for more details
+    ShutdownTag shutdown_tag;
+    grpc_server_shutdown_and_notify(server_->c_server(), cq_->cq(),
+                                    &shutdown_tag);
     cq_->Shutdown();
     void* tag;
     bool ok;
@@ -208,7 +221,11 @@ class EndpointPairFixture : public BaseFixture {
   }
 
   virtual ~EndpointPairFixture() {
-    server_->Shutdown(gpr_inf_past(GPR_CLOCK_MONOTONIC));
+    // Dummy shutdown tag (this tag is swallowed by cq->Next() and is not
+    // returned to the user) see ShutdownTag definition for more details
+    ShutdownTag shutdown_tag;
+    grpc_server_shutdown_and_notify(server_->c_server(), cq_->cq(),
+                                    &shutdown_tag);
     cq_->Shutdown();
     void* tag;
     bool ok;