[SPARK-30605][SQL] move defaultNamespace from SupportsNamespace to Ca…

…talogPlugin

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

Move the `defaultNamespace` method from the interface `SupportsNamespace` to `CatalogPlugin`.

### Why are the changes needed?

While I'm implementing JDBC V2, I realize that the default namespace is very an important information. Even if you don't want to implement namespace manipulation functionalities like CREATE/DROP/ALTER namespace, you still need to report the default namespace.

The default namespace is not about functionality but a matter of correctness. If you don't know the default namespace of a catalog, it's wrong to assume it's `[]`. You may get table not found exception if you do so.

I think it's more reasonable to put the `defaultNamespace` method in the base class `CatalogPlugin`. It returns `[]` by default so won't bother implementation if they really don't have namespace concept.

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

yes, but for an unreleased API.

### How was this patch tested?

existing tests

Closes #27319 from cloud-fan/ns.

Authored-by: Wenchen Fan <wenchen@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

sql/catalyst/src/main/java/org/apache/spark/sql/connector/catalog/CatalogPlugin.java

@@ -58,4 +58,18 @@ public interface CatalogPlugin {
    * called to pass the catalog's name.
    */
   String name();
+
+  /**
+   * Return a default namespace for the catalog.
+   * <p>
+   * When this catalog is set as the current catalog, the namespace returned by this method will be
+   * set as the current namespace.
+   * <p>
+   * The namespace returned by this method is not required to exist.
+   *
+   * @return a multi-part namespace
+   */
+  default String[] defaultNamespace() {
+    return new String[0];
+  }
 }

sql/catalyst/src/main/java/org/apache/spark/sql/connector/catalog/DelegatingCatalogExtension.java

@@ -51,6 +51,11 @@ public String name() {
   @Override
   public final void initialize(String name, CaseInsensitiveStringMap options) {}
 
+  @Override
+  public String[] defaultNamespace() {
+    return delegate.defaultNamespace();
+  }
+
   @Override
   public Identifier[] listTables(String[] namespace) throws NoSuchNamespaceException {
     return asTableCatalog().listTables(namespace);
@@ -99,11 +104,6 @@ public void renameTable(
     asTableCatalog().renameTable(oldIdent, newIdent);
   }
 
-  @Override
-  public String[] defaultNamespace() {
-    return asNamespaceCatalog().defaultNamespace();
-  }
-
   @Override
   public String[][] listNamespaces() throws NoSuchNamespaceException {
     return asNamespaceCatalog().listNamespaces();

sql/catalyst/src/main/java/org/apache/spark/sql/connector/catalog/SupportsNamespaces.java

@@ -69,20 +69,6 @@ public interface SupportsNamespaces extends CatalogPlugin {
    */
   List<String> RESERVED_PROPERTIES = Arrays.asList(PROP_COMMENT, PROP_LOCATION, PROP_OWNER);
 
-  /**
-   * Return a default namespace for the catalog.
-   * <p>
-   * When this catalog is set as the current catalog, the namespace returned by this method will be
-   * set as the current namespace.
-   * <p>
-   * The namespace returned by this method is not required to exist.
-   *
-   * @return a multi-part namespace
-   */
-  default String[] defaultNamespace() {
-    return new String[0];
-  }
-
   /**
    * List top-level namespaces from the catalog.
    * <p>

sql/catalyst/src/main/scala/org/apache/spark/sql/connector/catalog/CatalogManager.scala

@@ -93,19 +93,14 @@ class CatalogManager(
     }.getOrElse(defaultSessionCatalog)
   }
 
-  private def getDefaultNamespace(c: CatalogPlugin) = c match {
-    case c: SupportsNamespaces => c.defaultNamespace()
-    case _ => Array.empty[String]
-  }
-
   private var _currentNamespace: Option[Array[String]] = None
 
   def currentNamespace: Array[String] = synchronized {
     _currentNamespace.getOrElse {
       if (currentCatalog.name() == SESSION_CATALOG_NAME) {
         Array(v1SessionCatalog.getCurrentDatabase)
       } else {
-        getDefaultNamespace(currentCatalog)
+        currentCatalog.defaultNamespace()
       }
     }
   }

sql/catalyst/src/test/scala/org/apache/spark/sql/connector/catalog/CatalogManagerSuite.scala

@@ -18,7 +18,6 @@
 package org.apache.spark.sql.connector.catalog
 
 import java.net.URI
-import java.util
 
 import org.apache.spark.SparkFunSuite
 import org.apache.spark.sql.catalyst.analysis.{EmptyFunctionRegistry, FakeV2SessionCatalog, NoSuchNamespaceException}
@@ -112,31 +111,11 @@ class CatalogManagerSuite extends SparkFunSuite {
   }
 }
 
-class DummyCatalog extends SupportsNamespaces {
-  override def defaultNamespace(): Array[String] = Array("a", "b")
-
-  override def listNamespaces(): Array[Array[String]] = {
-    throw new UnsupportedOperationException
-  }
-  override def listNamespaces(namespace: Array[String]): Array[Array[String]] = {
-    throw new UnsupportedOperationException
-  }
-  override def loadNamespaceMetadata(namespace: Array[String]): util.Map[String, String] = {
-    throw new UnsupportedOperationException
-  }
-  override def createNamespace(
-      namespace: Array[String], metadata: util.Map[String, String]): Unit = {
-    throw new UnsupportedOperationException
-  }
-  override def alterNamespace(namespace: Array[String], changes: NamespaceChange*): Unit = {
-    throw new UnsupportedOperationException
-  }
-  override def dropNamespace(namespace: Array[String]): Boolean = {
-    throw new UnsupportedOperationException
-  }
+class DummyCatalog extends CatalogPlugin {
   override def initialize(name: String, options: CaseInsensitiveStringMap): Unit = {
     _name = name
   }
   private var _name: String = null
   override def name(): String = _name
+  override def defaultNamespace(): Array[String] = Array("a", "b")
 }