[SPARK-17899][SQL] add a debug mode to keep raw table properties in HiveExternalCatalog

## What changes were proposed in this pull request?

Currently `HiveExternalCatalog` will filter out the Spark SQL internal table properties, e.g. `spark.sql.sources.provider`, `spark.sql.sources.schema`, etc. This is reasonable for external users as they don't want to see these internal properties in `DESC TABLE`.

However, as a Spark developer, sometimes we do wanna see the raw table properties. This PR adds a new internal SQL conf, `spark.sql.debug`, to enable debug mode and keep these raw table properties.

This config can also be used in similar places where we wanna retain debug information in the future.

## How was this patch tested?

new test in MetastoreDataSourcesSuite

Author: Wenchen Fan <wenchen@databricks.com>

Closes #15458 from cloud-fan/debug.

sql/core/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -915,4 +915,9 @@ object StaticSQLConf {
     .internal()
     .intConf
     .createWithDefault(4000)
+
+  val DEBUG_MODE = buildConf("spark.sql.debug")
+    .internal()
+    .booleanConf
+    .createWithDefault(false)
 }

sql/core/src/test/scala/org/apache/spark/sql/internal/SQLConfSuite.scala

@@ -19,6 +19,7 @@ package org.apache.spark.sql.internal
 
 import org.apache.hadoop.fs.Path
 
+import org.apache.spark.SparkContext
 import org.apache.spark.sql._
 import org.apache.spark.sql.execution.WholeStageCodegenExec
 import org.apache.spark.sql.internal.StaticSQLConf._
@@ -254,18 +255,21 @@ class SQLConfSuite extends QueryTest with SharedSQLContext {
     }
   }
 
-  test("global SQL conf comes from SparkConf") {
-    val newSession = SparkSession.builder()
-      .config(SCHEMA_STRING_LENGTH_THRESHOLD.key, "2000")
-      .getOrCreate()
-
-    assert(newSession.conf.get(SCHEMA_STRING_LENGTH_THRESHOLD.key) == "2000")
-    checkAnswer(
-      newSession.sql(s"SET ${SCHEMA_STRING_LENGTH_THRESHOLD.key}"),
-      Row(SCHEMA_STRING_LENGTH_THRESHOLD.key, "2000"))
+  test("static SQL conf comes from SparkConf") {
+    val previousValue = sparkContext.conf.get(SCHEMA_STRING_LENGTH_THRESHOLD)
+    try {
+      sparkContext.conf.set(SCHEMA_STRING_LENGTH_THRESHOLD, 2000)
+      val newSession = new SparkSession(sparkContext)
+      assert(newSession.conf.get(SCHEMA_STRING_LENGTH_THRESHOLD) == 2000)
+      checkAnswer(
+        newSession.sql(s"SET ${SCHEMA_STRING_LENGTH_THRESHOLD.key}"),
+        Row(SCHEMA_STRING_LENGTH_THRESHOLD.key, "2000"))
+    } finally {
+      sparkContext.conf.set(SCHEMA_STRING_LENGTH_THRESHOLD, previousValue)
+    }
   }
 
-  test("cannot set/unset global SQL conf") {
+  test("cannot set/unset static SQL conf") {
     val e1 = intercept[AnalysisException](sql(s"SET ${SCHEMA_STRING_LENGTH_THRESHOLD.key}=10"))
     assert(e1.message.contains("Cannot modify the value of a static config"))
     val e2 = intercept[AnalysisException](spark.conf.unset(SCHEMA_STRING_LENGTH_THRESHOLD.key))

sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveExternalCatalog.scala

@@ -37,7 +37,7 @@ import org.apache.spark.sql.execution.command.{ColumnStatStruct, DDLUtils}
 import org.apache.spark.sql.execution.datasources.CaseInsensitiveMap
 import org.apache.spark.sql.hive.client.HiveClient
 import org.apache.spark.sql.internal.HiveSerDe
-import org.apache.spark.sql.internal.StaticSQLConf.SCHEMA_STRING_LENGTH_THRESHOLD
+import org.apache.spark.sql.internal.StaticSQLConf._
 import org.apache.spark.sql.types.{DataType, StructType}
 
 
@@ -461,13 +461,18 @@ private[spark] class HiveExternalCatalog(conf: SparkConf, hadoopConf: Configurat
         } else {
           table.storage
         }
+        val tableProps = if (conf.get(DEBUG_MODE)) {
+          table.properties
+        } else {
+          getOriginalTableProperties(table)
+        }
         table.copy(
           storage = storage,
           schema = getSchemaFromTableProperties(table),
           provider = Some(provider),
           partitionColumnNames = getPartitionColumnsFromTableProperties(table),
           bucketSpec = getBucketSpecFromTableProperties(table),
-          properties = getOriginalTableProperties(table))
+          properties = tableProps)
       } getOrElse {
         table.copy(provider = Some("hive"))
       }

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

@@ -23,6 +23,7 @@ import scala.collection.mutable.ArrayBuffer
 
 import org.apache.hadoop.fs.Path
 
+import org.apache.spark.SparkContext
 import org.apache.spark.sql._
 import org.apache.spark.sql.catalyst.TableIdentifier
 import org.apache.spark.sql.catalyst.catalog.{CatalogStorageFormat, CatalogTable, CatalogTableType}
@@ -31,7 +32,7 @@ import org.apache.spark.sql.hive.HiveExternalCatalog._
 import org.apache.spark.sql.hive.client.HiveClient
 import org.apache.spark.sql.hive.test.TestHiveSingleton
 import org.apache.spark.sql.internal.SQLConf
-import org.apache.spark.sql.internal.StaticSQLConf.SCHEMA_STRING_LENGTH_THRESHOLD
+import org.apache.spark.sql.internal.StaticSQLConf._
 import org.apache.spark.sql.test.SQLTestUtils
 import org.apache.spark.sql.types._
 import org.apache.spark.util.Utils
@@ -1324,4 +1325,18 @@ class MetastoreDataSourcesSuite extends QueryTest with SQLTestUtils with TestHiv
       hiveClient.dropTable("default", "t", ignoreIfNotExists = true, purge = true)
     }
   }
+
+  test("should keep data source entries in table properties when debug mode is on") {
+    val previousValue = sparkSession.sparkContext.conf.get(DEBUG_MODE)
+    try {
+      sparkSession.sparkContext.conf.set(DEBUG_MODE, true)
+      val newSession = sparkSession.newSession()
+      newSession.sql("CREATE TABLE abc(i int) USING json")
+      val tableMeta = newSession.sessionState.catalog.getTableMetadata(TableIdentifier("abc"))
+      assert(tableMeta.properties(DATASOURCE_SCHEMA_NUMPARTS).toInt == 1)
+      assert(tableMeta.properties(DATASOURCE_PROVIDER) == "json")
+    } finally {
+      sparkSession.sparkContext.conf.set(DEBUG_MODE, previousValue)
+    }
+  }
 }