s2a: Restore static token state mutated in tests

This is the easy-to-fix state, but GetAuthenticationMechanisms can save
these temporary states used in tests, so more fixes will be necessary.

Author: ejona86

s2a/src/main/java/io/grpc/s2a/internal/handshaker/tokenmanager/SingleTokenFetcher.java

@@ -41,6 +41,11 @@ public static void setAccessToken(String token) {
     accessToken = token;
   }
 
+  @VisibleForTesting
+  public static String getAccessToken() {
+    return accessToken;
+  }
+
   private SingleTokenFetcher(String token) {
     this.token = token;
   }
@@ -54,4 +59,4 @@ public String getDefaultToken() {
   public String getToken(S2AIdentity identity) {
     return token;
   }
-}
+}

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

@@ -20,6 +20,7 @@
 import io.grpc.s2a.internal.handshaker.S2AIdentity;
 import io.grpc.s2a.internal.handshaker.tokenmanager.SingleTokenFetcher;
 import java.util.Optional;
+import org.junit.AfterClass;
 import org.junit.BeforeClass;
 import org.junit.Rule;
 import org.junit.Test;
@@ -31,13 +32,20 @@
 public final class GetAuthenticationMechanismsTest {
   @Rule public final Expect expect = Expect.create();
   private static final String TOKEN = "access_token";
+  private static String originalAccessToken;
 
   @BeforeClass
   public static void setUpClass() {
+    originalAccessToken = SingleTokenFetcher.getAccessToken();
     // Set the token that the client will use to authenticate to the S2A.
     SingleTokenFetcher.setAccessToken(TOKEN);
   }
 
+  @AfterClass
+  public static void tearDownClass() {
+    SingleTokenFetcher.setAccessToken(originalAccessToken);
+  }
+
   @Test
   public void getAuthMechanisms_emptyIdentity_success() {
     expect
@@ -58,4 +66,4 @@ public void getAuthMechanisms_nonEmptyIdentity_success() {
                     .setToken("access_token")
                     .build()));
   }
-}
+}

s2a/src/test/java/io/grpc/s2a/internal/handshaker/tokenmanager/SingleTokenAccessTokenManagerTest.java

@@ -20,6 +20,7 @@
 
 import io.grpc.s2a.internal.handshaker.S2AIdentity;
 import java.util.Optional;
+import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
@@ -30,11 +31,19 @@ public final class SingleTokenAccessTokenManagerTest {
   private static final S2AIdentity IDENTITY = S2AIdentity.fromSpiffeId("spiffe_id");
   private static final String TOKEN = "token";
 
+  private String originalAccessToken;
+
   @Before
   public void setUp() {
+    originalAccessToken = SingleTokenFetcher.getAccessToken();
     SingleTokenFetcher.setAccessToken(null);
   }
 
+  @After
+  public void tearDown() {
+    SingleTokenFetcher.setAccessToken(originalAccessToken);
+  }
+
   @Test
   public void getDefaultToken_success() throws Exception {
     SingleTokenFetcher.setAccessToken(TOKEN);
@@ -68,4 +77,4 @@ public void create_success() throws Exception {
   public void create_noEnvironmentVariable() throws Exception {
     assertThat(AccessTokenManager.create()).isEmpty();
   }
-}
+}