MOD error on empty reader

src/io/ndjson/read/deserialize.rs

@@ -15,6 +15,13 @@ use super::super::super::json::read::_deserialize;
 /// # Errors
 /// This function errors iff any of the rows is not a valid JSON (i.e. the format is not valid NDJSON).
 pub fn deserialize(rows: &[String], data_type: DataType) -> Result<Arc<dyn Array>, ArrowError> {
+    if rows.is_empty() {
+        return Err(ArrowError::ExternalFormat(
+            "Cannot deserialize 0 NDJSON rows because empty string is not a valid JSON value"
+                .to_string(),
+        ));
+    }
+
     // deserialize strings to `Value`s
     let rows = rows
         .iter()

src/io/ndjson/read/file.rs

@@ -104,6 +104,12 @@ pub fn infer<R: std::io::BufRead>(
     reader: &mut R,
     number_of_rows: Option<usize>,
 ) -> Result<DataType> {
+    if !reader.fill_buf().map(|b| !b.is_empty())? {
+        return Err(ArrowError::ExternalFormat(
+            "Cannot infer NDJSON types on empty reader because empty string is not a valid JSON value".to_string(),
+        ));
+    }
+
     let rows = vec!["".to_string(); 1]; // 1 <=> read row by row
     let mut reader = FileReader::new(reader, rows, number_of_rows);
 

tests/it/io/ndjson/read.rs

@@ -3,7 +3,7 @@ use std::sync::Arc;
 
 use arrow2::array::*;
 use arrow2::datatypes::{DataType, Field};
-use arrow2::error::Result;
+use arrow2::error::{ArrowError, Result};
 use arrow2::io::ndjson::read as ndjson_read;
 use arrow2::io::ndjson::read::FallibleStreamingIterator;
 
@@ -74,14 +74,15 @@ fn read_null() -> Result<()> {
 #[test]
 fn read_empty_reader() -> Result<()> {
     let ndjson = "";
-    let expected_data_type = DataType::Null;
 
-    let data_type = infer(ndjson)?;
-    assert_eq!(expected_data_type, data_type);
+    let infer_error = infer(ndjson);
+    assert!(matches!(infer_error, Err(ArrowError::ExternalFormat(_))));
 
-    let arrays = read_and_deserialize(ndjson, &data_type, 1000)?;
-    let expected: Vec<Arc<dyn Array>> = vec![];
-    assert_eq!(expected, arrays);
+    let deserialize_error = ndjson_read::deserialize(&[], DataType::Null);
+    assert!(matches!(
+        deserialize_error,
+        Err(ArrowError::ExternalFormat(_))
+    ));
     Ok(())
 }