[SPARK-19919][SQL] Defer throwing the exception for empty paths in CSV datasource into `DataSource`

## What changes were proposed in this pull request?

This PR proposes to defer throwing the exception within `DataSource`.

Currently, if other datasources fail to infer the schema, it returns `None` and then this is being validated in `DataSource` as below:

```
scala> spark.read.json("emptydir")
org.apache.spark.sql.AnalysisException: Unable to infer schema for JSON. It must be specified manually.;
```

```
scala> spark.read.orc("emptydir")
org.apache.spark.sql.AnalysisException: Unable to infer schema for ORC. It must be specified manually.;
```

```
scala> spark.read.parquet("emptydir")
org.apache.spark.sql.AnalysisException: Unable to infer schema for Parquet. It must be specified manually.;
```

However, CSV it checks it within the datasource implementation and throws another exception message as below:

```
scala> spark.read.csv("emptydir")
java.lang.IllegalArgumentException: requirement failed: Cannot infer schema from an empty set of files
```

We could remove this duplicated check and validate this in one place in the same way with the same message.

## How was this patch tested?

Unit test in `CSVSuite` and manual test.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #17256 from HyukjinKwon/SPARK-19919.

2 files changed, 19 insertions, 10 deletions

diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/csv/CSVDataSource.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/csv/CSVDataSource.scala
index 63af18ec5b..83bdf6fe22 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/csv/CSVDataSource.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/csv/CSVDataSource.scala
@@ -54,10 +54,21 @@ abstract class CSVDataSource extends Serializable {
   /**
    * Infers the schema from `inputPaths` files.
    */
-  def infer(
+  final def inferSchema(
       sparkSession: SparkSession,
       inputPaths: Seq[FileStatus],
-      parsedOptions: CSVOptions): Option[StructType]
+      parsedOptions: CSVOptions): Option[StructType] = {
+    if (inputPaths.nonEmpty) {
+      Some(infer(sparkSession, inputPaths, parsedOptions))
+    } else {
+      None
+    }
+  }
+
+  protected def infer(
+      sparkSession: SparkSession,
+      inputPaths: Seq[FileStatus],
+      parsedOptions: CSVOptions): StructType
 
   /**
    * Generates a header from the given row which is null-safe and duplicate-safe.
@@ -131,10 +142,10 @@ object TextInputCSVDataSource extends CSVDataSource {
   override def infer(
       sparkSession: SparkSession,
       inputPaths: Seq[FileStatus],
-      parsedOptions: CSVOptions): Option[StructType] = {
+      parsedOptions: CSVOptions): StructType = {
     val csv = createBaseDataset(sparkSession, inputPaths, parsedOptions)
     val maybeFirstLine = CSVUtils.filterCommentAndEmpty(csv, parsedOptions).take(1).headOption
-    Some(inferFromDataset(sparkSession, csv, maybeFirstLine, parsedOptions))
+    inferFromDataset(sparkSession, csv, maybeFirstLine, parsedOptions)
   }
 
   /**
@@ -203,7 +214,7 @@ object WholeFileCSVDataSource extends CSVDataSource {
   override def infer(
       sparkSession: SparkSession,
       inputPaths: Seq[FileStatus],
-      parsedOptions: CSVOptions): Option[StructType] = {
+      parsedOptions: CSVOptions): StructType = {
     val csv = createBaseRdd(sparkSession, inputPaths, parsedOptions)
     csv.flatMap { lines =>
       UnivocityParser.tokenizeStream(
@@ -222,10 +233,10 @@ object WholeFileCSVDataSource extends CSVDataSource {
             parsedOptions.headerFlag,
             new CsvParser(parsedOptions.asParserSettings))
         }
-        Some(CSVInferSchema.infer(tokenRDD, header, parsedOptions))
+        CSVInferSchema.infer(tokenRDD, header, parsedOptions)
       case None =>
         // If the first row could not be read, just return the empty schema.
-        Some(StructType(Nil))
+        StructType(Nil)
     }
   }
 
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/csv/CSVFileFormat.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/csv/CSVFileFormat.scala
index eef43c7629..a99bdfee5d 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/csv/CSVFileFormat.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/csv/CSVFileFormat.scala
@@ -51,12 +51,10 @@ class CSVFileFormat extends TextBasedFileFormat with DataSourceRegister {
       sparkSession: SparkSession,
       options: Map[String, String],
       files: Seq[FileStatus]): Option[StructType] = {
-    require(files.nonEmpty, "Cannot infer schema from an empty set of files")
-
     val parsedOptions =
       new CSVOptions(options, sparkSession.sessionState.conf.sessionLocalTimeZone)
 
-    CSVDataSource(parsedOptions).infer(sparkSession, files, parsedOptions)
+    CSVDataSource(parsedOptions).inferSchema(sparkSession, files, parsedOptions)
   }
 
   override def prepareWrite(