alts: Metadata server address modification to account for default port

Fixing the utilization of the GCE Metadata host server address
environment variable to account for the case where the user does not
specify a port (defaults to port 8080)

b/451639946

Author: srkethireddy

alts/src/main/java/io/grpc/alts/HandshakerServiceChannel.java

@@ -16,7 +16,6 @@
 
 package io.grpc.alts;
 
-import com.google.common.base.MoreObjects;
 import io.grpc.CallOptions;
 import io.grpc.Channel;
 import io.grpc.ClientCall;
@@ -38,13 +37,29 @@
  * application will have at most one connection to the handshaker service.
  */
 final class HandshakerServiceChannel {
+  // Port 8080 is necessary for ALTS handshake.
+  private static final int ALTS_PORT = 8080;
+  private static final String DEFAULT_TARGET = "metadata.google.internal.:8080";
 
   static final Resource<Channel> SHARED_HANDSHAKER_CHANNEL =
-      new ChannelResource(
-          MoreObjects.firstNonNull(
-              System.getenv("GCE_METADATA_HOST"), "metadata.google.internal.:8080"));
-
-
+      new ChannelResource(getHandshakerTarget(System.getenv("GCE_METADATA_HOST")));
+  
+  /**
+   * Returns handshaker target. When GCE_METADATA_HOST is provided, it might contain port which we
+   * will discard and use ALTS_PORT instead.
+   */
+  static String getHandshakerTarget(String envValue) {
+    if (envValue == null || envValue.isEmpty()) {
+      return DEFAULT_TARGET;
+    }
+    String host = envValue;
+    int portIndex = host.lastIndexOf(':');
+    if (portIndex != -1) {
+      host = host.substring(0, portIndex); // Discard port if specified
+    }
+    return host + ":" + ALTS_PORT; // Utilize ALTS port in all cases
+  }
+  
   /** Returns a resource of handshaker service channel for testing only. */
   static Resource<Channel> getHandshakerChannelForTesting(String handshakerAddress) {
     return new ChannelResource(handshakerAddress);

alts/src/test/java/io/grpc/alts/HandshakerServiceChannelTest.java

@@ -67,6 +67,24 @@ public void sharedChannel_authority() {
     }
   }
 
+  @Test
+  public void getHandshakerTarget_nullEnvVar() {
+    assertThat(HandshakerServiceChannel.getHandshakerTarget(null))
+        .isEqualTo("metadata.google.internal.:8080");
+  }
+
+  @Test
+  public void getHandshakerTarget_envVarWithPort() {
+    assertThat(HandshakerServiceChannel.getHandshakerTarget("169.254.169.254:80"))
+        .isEqualTo("169.254.169.254:8080");
+  }
+
+  @Test
+  public void getHandshakerTarget_envVarWithHostOnly() {
+    assertThat(HandshakerServiceChannel.getHandshakerTarget("169.254.169.254"))
+        .isEqualTo("169.254.169.254:8080");
+  }
+  
   @Test
   public void resource_works() {
     Channel channel = resource.create();