grpclb: Be strict about only a single path segment in target URI

Guard this behavior change behind the RFC 3986 parser flag.

Author: jdcormie

grpclb/src/main/java/io/grpc/grpclb/SecretGrpclbNameResolverProvider.java

@@ -64,7 +64,7 @@ public NameResolver newNameResolver(Uri targetUri, final NameResolver.Args args)
       if (SCHEME.equals(targetUri.getScheme())) {
         List<String> pathSegments = targetUri.getPathSegments();
         Preconditions.checkArgument(
-            !pathSegments.isEmpty(),
+            pathSegments.size() == 1,
             "expected 1 path segment in target %s but found %s",
             targetUri,
             pathSegments);

grpclb/src/test/java/io/grpc/grpclb/SecretGrpclbNameResolverProviderTest.java

@@ -17,6 +17,8 @@
 package io.grpc.grpclb;
 
 import static com.google.common.truth.Truth.assertThat;
+import static com.google.common.truth.TruthJUnit.assume;
+import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.fail;
 import static org.mockito.Mockito.mock;
 
@@ -103,6 +105,22 @@ public void validDnsNameWithPort() throws Exception {
     testValidUri("dns:/foo.googleapis.com:456");
   }
 
+  @Test
+  public void newNameResolver_rejectsExtraPathSegments() {
+    assume().that(enableRfc3986UrisParam).isTrue();
+    IllegalArgumentException iae =
+        assertThrows(
+            IllegalArgumentException.class,
+            () -> newNameResolver("dns:///localhost:443/extras", args));
+    assertThat(iae).hasMessageThat().contains("expected 1 path segment in target");
+  }
+
+  @Test
+  public void newNameResolver_toleratesExtraPathSegments() {
+    assume().that(enableRfc3986UrisParam).isFalse();
+    newNameResolver("dns:///localhost:443/extras", args);
+  }
+
   private void testInvalidUri(String uri) {
     try {
       newNameResolver(uri, args);