Remove unnecessary usage of CircularList (#358)

Implementation of CircularList is an overkill for its stated
purpose. Remove an unnecessary usage so that the remaining
use case may be refactored in isolation.

prober/src/main/java/google/registry/monitoring/blackbox/module/WebWhoisModule.java

@@ -27,7 +27,6 @@ import google.registry.monitoring.blackbox.handler.WebWhoisMessageHandler;
 import google.registry.monitoring.blackbox.message.HttpRequestMessage;
 import google.registry.monitoring.blackbox.metric.MetricsCollector;
 import google.registry.monitoring.blackbox.token.WebWhoisToken;
-import google.registry.util.CircularList;
 import google.registry.util.Clock;
 import io.netty.bootstrap.Bootstrap;
 import io.netty.channel.Channel;
@@ -191,8 +190,8 @@ public class WebWhoisModule {
   @Singleton
   @Provides
   @WebWhoisProtocol
-  CircularList<String> provideTopLevelDomains() {
+  ImmutableList<String> provideTopLevelDomains() {
-    return new CircularList.Builder<String>().add("how", "soy", "xn--q9jyb4c").build();
+    return ImmutableList.of("how", "soy", "xn--q9jyb4c");
   }
 
   @Provides

prober/src/main/java/google/registry/monitoring/blackbox/token/WebWhoisToken.java

@@ -14,11 +14,15 @@
 
 package google.registry.monitoring.blackbox.token;
 
+import static com.google.common.base.Preconditions.checkArgument;
+
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.Iterables;
+import com.google.common.collect.Iterators;
+import com.google.common.collect.PeekingIterator;
 import google.registry.monitoring.blackbox.exception.UndeterminedStateException;
 import google.registry.monitoring.blackbox.message.OutboundMessageType;
 import google.registry.monitoring.blackbox.module.WebWhoisModule.WebWhoisProtocol;
-import google.registry.util.CircularList;
 import javax.inject.Inject;
 
 /**
@@ -33,19 +37,21 @@ public class WebWhoisToken extends Token {
   /** For each top level domain (tld), we probe "prefix.tld". */
   private static final String PREFIX = "whois.nic.";
 
-  /** {@link ImmutableList} of all top level domains to be probed. */
+  /** {@link PeekingIterator} over a cycle of all top level domains to be probed. */
-  private CircularList<String> topLevelDomainsList;
+  private final PeekingIterator<String> tldCycleIterator;
 
   @Inject
-  public WebWhoisToken(@WebWhoisProtocol CircularList<String> topLevelDomainsList) {
+  public WebWhoisToken(@WebWhoisProtocol ImmutableList<String> topLevelDomainsList) {
+    checkArgument(!topLevelDomainsList.isEmpty(), "topLevelDomainsList must not be empty.");
 
-    this.topLevelDomainsList = topLevelDomainsList;
+    this.tldCycleIterator =
+        Iterators.peekingIterator(Iterables.cycle(topLevelDomainsList).iterator());
   }
 
-  /** Moves on to next top level domain in {@code topLevelDomainsList}. */
+  /** Moves on to next top level domain in {@code tldCycleIterator}. */
   @Override
   public WebWhoisToken next() {
-    topLevelDomainsList = topLevelDomainsList.next();
+    tldCycleIterator.next();
     return this;
   }
 
@@ -62,6 +68,6 @@ public class WebWhoisToken extends Token {
    */
   @Override
   public String host() {
-    return PREFIX + topLevelDomainsList.get();
+    return PREFIX + tldCycleIterator.peek();
   }
 }

prober/src/test/java/google/registry/monitoring/blackbox/token/WebWhoisTokenTest.java

@@ -16,9 +16,9 @@ package google.registry.monitoring.blackbox.token;
 
 import static com.google.common.truth.Truth.assertThat;
 
+import com.google.common.collect.ImmutableList;
 import google.registry.monitoring.blackbox.exception.UndeterminedStateException;
 import google.registry.monitoring.blackbox.message.HttpRequestMessage;
-import google.registry.util.CircularList;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
@@ -32,8 +32,8 @@ public class WebWhoisTokenTest {
   private static final String FIRST_TLD = "first_test";
   private static final String SECOND_TLD = "second_test";
   private static final String THIRD_TLD = "third_test";
-  private final CircularList<String> testDomains =
+  private final ImmutableList<String> testDomains =
-      new CircularList.Builder<String>().add(FIRST_TLD, SECOND_TLD, THIRD_TLD).build();
+      ImmutableList.of(FIRST_TLD, SECOND_TLD, THIRD_TLD);
 
   public Token webToken = new WebWhoisToken(testDomains);
 
@@ -48,10 +48,12 @@ public class WebWhoisTokenTest {
     assertThat(secondMessage.headers().get("host")).isEqualTo(PREFIX + FIRST_TLD);
   }
 
-  /**
+  @Test
-   * As Circular Linked List has not been implemented yet, we cannot yet wrap around, so we don't
+  public void testHostOfToken() {
-   * test that in testing {@code next}.
+    assertThat(webToken.host()).isEqualTo(PREFIX + FIRST_TLD);
-   */
+    assertThat(webToken.host()).isEqualTo(PREFIX + FIRST_TLD);
+  }
+
   @Test
   public void testNextToken() {
     assertThat(webToken.host()).isEqualTo(PREFIX + FIRST_TLD);