[SPARK-19761][SQL]create InMemoryFileIndex with an empty rootPaths when set PARALLEL_PARTITION_DISCOVERY_THRESHOLD to zero failed

Author: windpiger

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/PartitioningAwareFileIndex.scala

@@ -300,7 +300,7 @@ object PartitioningAwareFileIndex extends Logging {
       sparkSession: SparkSession): Seq[(Path, Seq[FileStatus])] = {
 
     // Short-circuits parallel listing when serial listing is likely to be faster.
-    if (paths.size < sparkSession.sessionState.conf.parallelPartitionDiscoveryThreshold) {
+    if (paths.size <= sparkSession.sessionState.conf.parallelPartitionDiscoveryThreshold) {
       return paths.map { path =>
         (path, listLeafFiles(path, hadoopConf, filter, Some(sparkSession)))
       }

sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/FileIndexSuite.scala

@@ -27,6 +27,7 @@ import org.apache.hadoop.fs.{FileStatus, Path, RawLocalFileSystem}
 
 import org.apache.spark.metrics.source.HiveCatalogMetrics
 import org.apache.spark.sql.catalyst.util._
+import org.apache.spark.sql.internal.SQLConf
 import org.apache.spark.sql.test.SharedSQLContext
 
 class FileIndexSuite extends SharedSQLContext {
@@ -178,6 +179,12 @@ class FileIndexSuite extends SharedSQLContext {
       assert(catalog2.allFiles().nonEmpty)
     }
   }
+
+  test("InMemoryFileIndex with empty rootPaths when PARALLEL_PARTITION_DISCOVERY_THRESHOLD is 0") {
+    withSQLConf(SQLConf.PARALLEL_PARTITION_DISCOVERY_THRESHOLD.key -> "0") {
+      new InMemoryFileIndex(spark, Seq.empty, Map.empty, None)
+    }
+  }
 }
 
 class FakeParentPathFileSystem extends RawLocalFileSystem {