[SPARK-16459][SQL] Prevent dropping current database

## What changes were proposed in this pull request?

This PR prevents dropping the current database to avoid errors like the followings.

```scala
scala> sql("create database delete_db")
scala> sql("use delete_db")
scala> sql("drop database delete_db")
scala> sql("create table t as select 1")
org.apache.spark.sql.catalyst.analysis.NoSuchDatabaseException: Database `delete_db` not found;
```

## How was this patch tested?

Pass the Jenkins tests including an updated testcase.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #14115 from dongjoon-hyun/SPARK-16459.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalog.scala

@@ -34,6 +34,10 @@ import org.apache.spark.sql.catalyst.expressions.{Expression, ExpressionInfo}
 import org.apache.spark.sql.catalyst.plans.logical.{LogicalPlan, SubqueryAlias}
 import org.apache.spark.sql.catalyst.util.StringUtils
 
+object SessionCatalog {
+  val DEFAULT_DATABASE = "default"
+}
+
 /**
  * An internal catalog that is used by a Spark Session. This internal catalog serves as a
  * proxy to the underlying metastore (e.g. Hive Metastore) and it also manages temporary
@@ -47,6 +51,7 @@ class SessionCatalog(
     functionRegistry: FunctionRegistry,
     conf: CatalystConf,
     hadoopConf: Configuration) extends Logging {
+  import SessionCatalog._
   import CatalogTypes.TablePartitionSpec
 
   // For testing only.
@@ -77,7 +82,7 @@ class SessionCatalog(
   // the corresponding item in the current database.
   @GuardedBy("this")
   protected var currentDb = {
-    val defaultName = "default"
+    val defaultName = DEFAULT_DATABASE
     val defaultDbDefinition =
       CatalogDatabase(defaultName, "default database", conf.warehousePath, Map())
     // Initialize default database if it doesn't already exist
@@ -146,8 +151,10 @@ class SessionCatalog(
 
   def dropDatabase(db: String, ignoreIfNotExists: Boolean, cascade: Boolean): Unit = {
     val dbName = formatDatabaseName(db)
-    if (dbName == "default") {
+    if (dbName == DEFAULT_DATABASE) {
       throw new AnalysisException(s"Can not drop default database")
+    } else if (dbName == getCurrentDatabase) {
+      throw new AnalysisException(s"Can not drop current database `${dbName}`")
     }
     externalCatalog.dropDatabase(dbName, ignoreIfNotExists, cascade)
   }
@@ -878,14 +885,14 @@ class SessionCatalog(
    * This is mainly used for tests.
    */
   private[sql] def reset(): Unit = synchronized {
-    val default = "default"
-    listDatabases().filter(_ != default).foreach { db =>
+    setCurrentDatabase(DEFAULT_DATABASE)
+    listDatabases().filter(_ != DEFAULT_DATABASE).foreach { db =>
       dropDatabase(db, ignoreIfNotExists = false, cascade = true)
     }
-    listTables(default).foreach { table =>
+    listTables(DEFAULT_DATABASE).foreach { table =>
       dropTable(table, ignoreIfNotExists = false)
     }
-    listFunctions(default).map(_._1).foreach { func =>
+    listFunctions(DEFAULT_DATABASE).map(_._1).foreach { func =>
       if (func.database.isDefined) {
         dropFunction(func, ignoreIfNotExists = false)
       } else {
@@ -902,7 +909,6 @@ class SessionCatalog(
       require(functionBuilder.isDefined, s"built-in function '$f' is missing function builder")
       functionRegistry.registerFunction(f, expressionInfo.get, functionBuilder.get)
     }
-    setCurrentDatabase(default)
   }
 
 }

sql/core/src/test/scala/org/apache/spark/sql/execution/command/DDLSuite.scala

@@ -1270,6 +1270,15 @@ class DDLSuite extends QueryTest with SharedSQLContext with BeforeAndAfterEach {
       "WITH SERDEPROPERTIES ('spark.sql.sources.me'='anything')")
   }
 
+  test("drop current database") {
+    sql("CREATE DATABASE temp")
+    sql("USE temp")
+    val m = intercept[AnalysisException] {
+      sql("DROP DATABASE temp")
+    }.getMessage
+    assert(m.contains("Can not drop current database `temp`"))
+  }
+
   test("drop default database") {
     Seq("true", "false").foreach { caseSensitive =>
       withSQLConf(SQLConf.CASE_SENSITIVE.key -> caseSensitive) {

sql/hive/src/test/scala/org/apache/spark/sql/hive/HiveContextCompatibilitySuite.scala

@@ -93,6 +93,7 @@ class HiveContextCompatibilitySuite extends SparkFunSuite with BeforeAndAfterEac
     hc.sql("DROP TABLE mee_table")
     val tables2 = hc.sql("SHOW TABLES IN mee_db").collect().map(_.getString(0))
     assert(tables2.isEmpty)
+    hc.sql("USE default")
     hc.sql("DROP DATABASE mee_db CASCADE")
     val databases3 = hc.sql("SHOW DATABASES").collect().map(_.getString(0))
     assert(databases3.toSeq == Seq("default"))

sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveDDLSuite.scala

@@ -472,6 +472,7 @@ class HiveDDLSuite
       sql(s"DROP TABLE $tabName")
 
       assert(tmpDir.listFiles.isEmpty)
+      sql("USE default")
       sql(s"DROP DATABASE $dbName")
       assert(!fs.exists(new Path(tmpDir.toString)))
     }
@@ -526,6 +527,7 @@ class HiveDDLSuite
           assert(!tableDirectoryExists(TableIdentifier(tabName), Option(expectedDBLocation)))
         }
 
+        sql(s"USE default")
         val sqlDropDatabase = s"DROP DATABASE $dbName ${if (cascade) "CASCADE" else "RESTRICT"}"
         if (tableExists && !cascade) {
           val message = intercept[AnalysisException] {