[SPARK-25660][SQL] Fix for the backward slash as CSV fields delimiter

## What changes were proposed in this pull request?

The PR addresses the exception raised on accessing chars out of delimiter string. In particular, the backward slash `\` as the CSV fields delimiter causes the following exception on reading `abc\1`:
```Scala
String index out of range: 1
java.lang.StringIndexOutOfBoundsException: String index out of range: 1
	at java.lang.String.charAt(String.java:658)
```
because `str.charAt(1)` tries to access a char out of `str` in `CSVUtils.toChar`

## How was this patch tested?

Added tests for empty string and string containing the backward slash to `CSVUtilsSuite`. Besides of that I added an end-to-end test to check how the backward slash is handled in reading CSV string with it.

Closes apache#22654 from MaxGekk/csv-slash-delim.

Authored-by: Maxim Gekk <[email protected]>
Signed-off-by: gatorsmile <[email protected]>

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

@@ -139,23 +139,25 @@ object CSVUtils {
    */
   @throws[IllegalArgumentException]
   def toChar(str: String): Char = {
-    if (str.charAt(0) == '\\') {
-      str.charAt(1)
-      match {
-        case 't' => '\t'
-        case 'r' => '\r'
-        case 'b' => '\b'
-        case 'f' => '\f'
-        case '\"' => '\"' // In case user changes quote char and uses \" as delimiter in options
-        case '\'' => '\''
-        case 'u' if str == """\u0000""" => '\u0000'
-        case _ =>
-          throw new IllegalArgumentException(s"Unsupported special character for delimiter: $str")
-      }
-    } else if (str.length == 1) {
-      str.charAt(0)
-    } else {
-      throw new IllegalArgumentException(s"Delimiter cannot be more than one character: $str")
+    (str: Seq[Char]) match {
+      case Seq() => throw new IllegalArgumentException("Delimiter cannot be empty string")
+      case Seq('\\') => throw new IllegalArgumentException("Single backslash is prohibited." +
+        " It has special meaning as beginning of an escape sequence." +
+        " To get the backslash character, pass a string with two backslashes as the delimiter.")
+      case Seq(c) => c
+      case Seq('\\', 't') => '\t'
+      case Seq('\\', 'r') => '\r'
+      case Seq('\\', 'b') => '\b'
+      case Seq('\\', 'f') => '\f'
+      // In case user changes quote char and uses \" as delimiter in options
+      case Seq('\\', '\"') => '\"'
+      case Seq('\\', '\'') => '\''
+      case Seq('\\', '\\') => '\\'
+      case _ if str == """\u0000""" => '\u0000'
+      case Seq('\\', _) =>
+        throw new IllegalArgumentException(s"Unsupported special character for delimiter: $str")
+      case _ =>
+        throw new IllegalArgumentException(s"Delimiter cannot be more than one character: $str")
     }
   }
 

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

@@ -1826,4 +1826,14 @@ class CSVSuite extends QueryTest with SharedSQLContext with SQLTestUtils with Te
     val df = spark.read.option("enforceSchema", false).csv(input)
     checkAnswer(df, Row("1", "2"))
   }
+
+  test("using the backward slash as the delimiter") {
+    val input = Seq("""abc\1""").toDS()
+    val delimiter = """\\"""
+    checkAnswer(spark.read.option("delimiter", delimiter).csv(input), Row("abc", "1"))
+    checkAnswer(spark.read.option("inferSchema", true).option("delimiter", delimiter).csv(input),
+      Row("abc", 1))
+    val schema = new StructType().add("a", StringType).add("b", IntegerType)
+    checkAnswer(spark.read.schema(schema).option("delimiter", delimiter).csv(input), Row("abc", 1))
+  }
 }

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

@@ -28,6 +28,7 @@ class CSVUtilsSuite extends SparkFunSuite {
     assert(CSVUtils.toChar("""\"""") === '\"')
     assert(CSVUtils.toChar("""\'""") === '\'')
     assert(CSVUtils.toChar("""\u0000""") === '\u0000')
+    assert(CSVUtils.toChar("""\\""") === '\\')
   }
 
   test("Does not accept delimiter larger than one character") {
@@ -44,4 +45,17 @@ class CSVUtilsSuite extends SparkFunSuite {
     assert(exception.getMessage.contains("Unsupported special character for delimiter"))
   }
 
+  test("string with one backward slash is prohibited") {
+    val exception = intercept[IllegalArgumentException]{
+      CSVUtils.toChar("""\""")
+    }
+    assert(exception.getMessage.contains("Single backslash is prohibited"))
+  }
+
+  test("output proper error message for empty string") {
+    val exception = intercept[IllegalArgumentException]{
+      CSVUtils.toChar("")
+    }
+    assert(exception.getMessage.contains("Delimiter cannot be empty string"))
+  }
 }