s2a: fix flake in FakeS2AServerTest (grpc#11673)

While here:
 * add an awaitTermination to after calling shutdown on server
 * don't use port picker

Fixes grpc#11648

Author: rmehta19

s2a/src/test/java/io/grpc/s2a/internal/handshaker/FakeS2AServerTest.java

@@ -20,13 +20,13 @@
 import static java.util.concurrent.TimeUnit.SECONDS;
 
 import com.google.common.collect.ImmutableList;
+import com.google.common.util.concurrent.SettableFuture;
 import com.google.protobuf.ByteString;
 import io.grpc.Grpc;
 import io.grpc.InsecureChannelCredentials;
 import io.grpc.ManagedChannel;
 import io.grpc.Server;
 import io.grpc.ServerBuilder;
-import io.grpc.benchmarks.Utils;
 import io.grpc.s2a.internal.handshaker.ValidatePeerCertificateChainReq.VerificationMode;
 import io.grpc.stub.StreamObserver;
 import java.io.IOException;
@@ -49,51 +49,52 @@ public final class FakeS2AServerTest {
 
   private static final ImmutableList<ByteString> FAKE_CERT_DER_CHAIN =
       ImmutableList.of(ByteString.copyFrom("fake-der-chain".getBytes(StandardCharsets.US_ASCII)));
-  private int port;
   private String serverAddress;
-  private SessionResp response = null;
   private Server fakeS2AServer;
 
   @Before
   public void setUp() throws Exception {
-    port = Utils.pickUnusedPort();
-    fakeS2AServer = ServerBuilder.forPort(port).addService(new FakeS2AServer()).build();
+    fakeS2AServer = ServerBuilder.forPort(0).addService(new FakeS2AServer()).build();
     fakeS2AServer.start();
-    serverAddress = String.format("localhost:%d", port);
+    serverAddress = String.format("localhost:%d", fakeS2AServer.getPort());
   }
 
   @After
-  public void tearDown() {
+  public void tearDown() throws Exception {
     fakeS2AServer.shutdown();
+    fakeS2AServer.awaitTermination(10, SECONDS);
   }
 
   @Test
   public void callS2AServerOnce_getTlsConfiguration_returnsValidResult()
-      throws InterruptedException, IOException {
+      throws InterruptedException, IOException, java.util.concurrent.ExecutionException {
     ExecutorService executor = Executors.newSingleThreadExecutor();
     logger.info("Client connecting to: " + serverAddress);
     ManagedChannel channel =
         Grpc.newChannelBuilder(serverAddress, InsecureChannelCredentials.create())
             .executor(executor)
             .build();
-
+    SettableFuture<SessionResp> respFuture = SettableFuture.create();
     try {
       S2AServiceGrpc.S2AServiceStub asyncStub = S2AServiceGrpc.newStub(channel);
       StreamObserver<SessionReq> requestObserver =
           asyncStub.setUpSession(
               new StreamObserver<SessionResp>() {
+                SessionResp recvResp;
                 @Override
                 public void onNext(SessionResp resp) {
-                  response = resp;
+                  recvResp = resp;
                 }
 
                 @Override
                 public void onError(Throwable t) {
-                  throw new RuntimeException(t);
+                  respFuture.setException(t);
                 }
 
                 @Override
-                public void onCompleted() {}
+                public void onCompleted() {
+                  respFuture.set(recvResp);
+                }
               });
       try {
         requestObserver.onNext(
@@ -138,36 +139,39 @@ public void onCompleted() {}
                             .addCiphersuites(
                                 Ciphersuite.CIPHERSUITE_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256)))
             .build();
-    assertThat(response).ignoringRepeatedFieldOrder().isEqualTo(expected);
+    assertThat(respFuture.get()).ignoringRepeatedFieldOrder().isEqualTo(expected);
   }
 
   @Test
   public void callS2AServerOnce_validatePeerCertifiate_returnsValidResult()
-      throws InterruptedException {
+      throws InterruptedException, java.util.concurrent.ExecutionException {
     ExecutorService executor = Executors.newSingleThreadExecutor();
     logger.info("Client connecting to: " + serverAddress);
     ManagedChannel channel =
         Grpc.newChannelBuilder(serverAddress, InsecureChannelCredentials.create())
             .executor(executor)
             .build();
-
+    SettableFuture<SessionResp> respFuture = SettableFuture.create();
     try {
       S2AServiceGrpc.S2AServiceStub asyncStub = S2AServiceGrpc.newStub(channel);
       StreamObserver<SessionReq> requestObserver =
           asyncStub.setUpSession(
               new StreamObserver<SessionResp>() {
+                private SessionResp recvResp;
                 @Override
                 public void onNext(SessionResp resp) {
-                  response = resp;
+                  recvResp = resp;
                 }
 
                 @Override
                 public void onError(Throwable t) {
-                  throw new RuntimeException(t);
+                  respFuture.setException(t);
                 }
 
                 @Override
-                public void onCompleted() {}
+                public void onCompleted() {
+                  respFuture.set(recvResp);
+                }
               });
       try {
         requestObserver.onNext(
@@ -200,7 +204,7 @@ public void onCompleted() {}
                 ValidatePeerCertificateChainResp.newBuilder()
                     .setValidationResult(ValidatePeerCertificateChainResp.ValidationResult.SUCCESS))
             .build();
-    assertThat(response).ignoringRepeatedFieldOrder().isEqualTo(expected);
+    assertThat(respFuture.get()).ignoringRepeatedFieldOrder().isEqualTo(expected);
   }
 
   @Test