[SPARK-3138][SQL] sqlContext.parquetFile should be able to take a single file as parameter

```if (!fs.getFileStatus(path).isDir) throw Exception``` make no sense after this commit #1370

be careful if someone is working on SPARK-2551, make sure the new change passes test case ```test("Read a parquet file instead of a directory")```

Author: chutium <teng.qiu@gmail.com>

Closes #2044 from chutium/parquet-singlefile and squashes the following commits:

4ae477f [chutium] [SPARK-3138][SQL] sqlContext.parquetFile should be able to take a single file as parameter

2 files changed, 26 insertions, 8 deletions

diff --git a/sql/core/src/main/scala/org/apache/spark/sql/parquet/ParquetTypes.scala b/sql/core/src/main/scala/org/apache/spark/sql/parquet/ParquetTypes.scala
index 1a52377651..2941b97935 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/parquet/ParquetTypes.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/parquet/ParquetTypes.scala
@@ -394,17 +394,14 @@ private[parquet] object ParquetTypesConverter extends Logging {
       throw new IllegalArgumentException(s"Incorrectly formatted Parquet metadata path $origPath")
     }
     val path = origPath.makeQualified(fs)
-    if (!fs.getFileStatus(path).isDir) {
-      throw new IllegalArgumentException(
-        s"Expected $path for be a directory with Parquet files/metadata")
-    }
-    ParquetRelation.enableLogForwarding()
 
     val children = fs.listStatus(path).filterNot { status =>
       val name = status.getPath.getName
       (name(0) == '.' || name(0) == '_') && name != ParquetFileWriter.PARQUET_METADATA_FILE
     }
 
+    ParquetRelation.enableLogForwarding()
+
     // NOTE (lian): Parquet "_metadata" file can be very slow if the file consists of lots of row
     // groups. Since Parquet schema is replicated among all row groups, we only need to touch a
     // single row group to read schema related metadata. Notice that we are making assumptions that
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/parquet/ParquetQuerySuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/parquet/ParquetQuerySuite.scala
index 4219cc0800..42923b6a28 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/parquet/ParquetQuerySuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/parquet/ParquetQuerySuite.scala
@@ -35,7 +35,6 @@ import org.apache.spark.sql.test.TestSQLContext
 import org.apache.spark.sql.test.TestSQLContext._
 import org.apache.spark.util.Utils
 
-
 case class TestRDDEntry(key: Int, value: String)
 
 case class NullReflectData(
@@ -420,8 +419,30 @@ class ParquetQuerySuite extends QueryTest with FunSuiteLike with BeforeAndAfterA
     val rdd_copy = sql("SELECT * FROM tmpx").collect()
     val rdd_orig = rdd.collect()
     for(i <- 0 to 99) {
-      assert(rdd_copy(i).apply(0) === rdd_orig(i).key,  s"key error in line $i")
-      assert(rdd_copy(i).apply(1) === rdd_orig(i).value, s"value in line $i")
+      assert(rdd_copy(i).apply(0) === rdd_orig(i).key,   s"key error in line $i")
+      assert(rdd_copy(i).apply(1) === rdd_orig(i).value, s"value error in line $i")
+    }
+    Utils.deleteRecursively(file)
+  }
+
+  test("Read a parquet file instead of a directory") {
+    val file = getTempFilePath("parquet")
+    val path = file.toString
+    val fsPath = new Path(path)
+    val fs: FileSystem = fsPath.getFileSystem(TestSQLContext.sparkContext.hadoopConfiguration)
+    val rdd = TestSQLContext.sparkContext.parallelize((1 to 100))
+      .map(i => TestRDDEntry(i, s"val_$i"))
+    rdd.coalesce(1).saveAsParquetFile(path)
+
+    val children = fs.listStatus(fsPath).filter(_.getPath.getName.endsWith(".parquet"))
+    assert(children.length > 0)
+    val readFile = parquetFile(path + "/" + children(0).getPath.getName)
+    readFile.registerTempTable("tmpx")
+    val rdd_copy = sql("SELECT * FROM tmpx").collect()
+    val rdd_orig = rdd.collect()
+    for(i <- 0 to 99) {
+      assert(rdd_copy(i).apply(0) === rdd_orig(i).key,   s"key error in line $i")
+      assert(rdd_copy(i).apply(1) === rdd_orig(i).value, s"value error in line $i")
     }
     Utils.deleteRecursively(file)
   }