[apache#5611] Improvement(iceberg): support schema exists interface f…

…or Iceberg REST server (apache#5616)

### What changes were proposed in this pull request?

schema exists interface for Iceberg REST server

### Why are the changes needed?

Close: apache#5611

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

Unit test

iceberg/iceberg-common/src/main/java/org/apache/gravitino/iceberg/common/ops/IcebergCatalogWrapper.java

@@ -134,6 +134,11 @@ public GetNamespaceResponse loadNamespace(Namespace namespace) {
     return CatalogHandlers.loadNamespace(asNamespaceCatalog, namespace);
   }
 
+  public boolean existNamespace(Namespace namespace) {
+    validateNamespace(Optional.of(namespace));
+    return asNamespaceCatalog.namespaceExists(namespace);
+  }
+
   public ListNamespacesResponse listNamespace(Namespace parent) {
     validateNamespace(Optional.empty());
     return CatalogHandlers.listNamespaces(asNamespaceCatalog, parent);

iceberg/iceberg-rest-server/src/main/java/org/apache/gravitino/iceberg/service/rest/IcebergNamespaceOperations.java

@@ -26,6 +26,7 @@
 import javax.ws.rs.DELETE;
 import javax.ws.rs.DefaultValue;
 import javax.ws.rs.GET;
+import javax.ws.rs.HEAD;
 import javax.ws.rs.POST;
 import javax.ws.rs.Path;
 import javax.ws.rs.PathParam;
@@ -95,6 +96,24 @@ public Response loadNamespace(
     return IcebergRestUtils.ok(getNamespaceResponse);
   }
 
+  @HEAD
+  @Path("{namespace}")
+  @Produces(MediaType.APPLICATION_JSON)
+  @Timed(name = "namespace-exists." + MetricNames.HTTP_PROCESS_DURATION, absolute = true)
+  @ResponseMetered(name = "namespace-exists", absolute = true)
+  public Response namespaceExists(
+      @PathParam("prefix") String prefix, @PathParam("namespace") String namespace) {
+    boolean exists =
+        icebergCatalogWrapperManager
+            .getOps(prefix)
+            .existNamespace(RESTUtil.decodeNamespace(namespace));
+    if (exists) {
+      return IcebergRestUtils.noContent();
+    } else {
+      return IcebergRestUtils.notExists();
+    }
+  }
+
   @DELETE
   @Path("{namespace}")
   @Produces(MediaType.APPLICATION_JSON)

iceberg/iceberg-rest-server/src/test/java/org/apache/gravitino/iceberg/service/rest/TestIcebergNamespaceOperations.java

@@ -93,6 +93,10 @@ private Response doLoadNamespace(String name) {
     return getNamespaceClientBuilder(Optional.of(name)).get();
   }
 
+  private Response doNamespaceExists(String name) {
+    return getNamespaceClientBuilder(Optional.of(name)).head();
+  }
+
   private Response doDropNamespace(String name) {
     return getNamespaceClientBuilder(Optional.of(name)).delete();
   }
@@ -121,6 +125,11 @@ private void verifyDropNamespaceFail(int status, String name) {
     Assertions.assertEquals(status, response.getStatus());
   }
 
+  private void verifyNamespaceExistsStatusCode(int status, String name) {
+    Response response = doNamespaceExists(name);
+    Assertions.assertEquals(status, response.getStatus());
+  }
+
   protected void verifyCreateNamespaceSucc(String... name) {
     Response response = doCreateNamespace(name);
     Assertions.assertEquals(Status.OK.getStatusCode(), response.getStatus());
@@ -167,6 +176,13 @@ void testLoadNamespace() {
     verifyLoadNamespaceFail(404, "load_foo2");
   }
 
+  @Test
+  void testNamespaceExists() {
+    verifyNamespaceExistsStatusCode(404, "exists_foo1");
+    verifyCreateNamespaceSucc("exists_foo1");
+    verifyNamespaceExistsStatusCode(204, "exists_foo1");
+  }
+
   @Test
   void testDropNamespace() {
     verifyCreateNamespaceSucc("drop_foo1");