From 081e35004680ac78f2a7e6a5ea58877242c0fe14 Mon Sep 17 00:00:00 2001
From: Antonin Amand <antonin.amand@gmail.com>
Date: Tue, 22 Oct 2024 11:41:09 +0200
Subject: [PATCH] Fix arrow-json writer empty

The Writer output with the ArrayJson formatter was empty
when no record were in the output (no write or empty batch write).
It should output an empty JSON array `[]` instead.
---
 arrow-json/src/writer/mod.rs | 42 ++++++++++++++++++++++++++++++++++--
 1 file changed, 40 insertions(+), 2 deletions(-)

diff --git a/arrow-json/src/writer/mod.rs b/arrow-json/src/writer/mod.rs
index df18a9ad5fe..77e17d89f66 100644
--- a/arrow-json/src/writer/mod.rs
+++ b/arrow-json/src/writer/mod.rs
@@ -367,10 +367,15 @@ where
     /// all record batches have been produced. (e.g. producing the final `']'` if writing
     /// arrays.
     pub fn finish(&mut self) -> Result<(), ArrowError> {
-        if self.started && !self.finished {
+        if !self.started {
+            self.format.start_stream(&mut self.writer)?;
+            self.started = true;
+        }
+        if !self.finished {
             self.format.end_stream(&mut self.writer)?;
             self.finished = true;
         }
+
         Ok(())
     }
 
@@ -1147,12 +1152,45 @@ mod tests {
     }
 
     #[test]
-    fn json_writer_empty() {
+    fn json_line_writer_empty() {
+        let mut writer = LineDelimitedWriter::new(vec![] as Vec<u8>);
+        writer.finish().unwrap();
+        assert_eq!(str::from_utf8(&writer.into_inner()).unwrap(), "");
+    }
+
+    #[test]
+    fn json_array_writer_empty() {
         let mut writer = ArrayWriter::new(vec![] as Vec<u8>);
         writer.finish().unwrap();
+        assert_eq!(str::from_utf8(&writer.into_inner()).unwrap(), "[]");
+    }
+
+    #[test]
+    fn json_line_writer_empty_batch() {
+        let mut writer = LineDelimitedWriter::new(vec![] as Vec<u8>);
+
+        let array = Int32Array::from(Vec::<i32>::new());
+        let schema = Schema::new(vec![Field::new("c", DataType::Int32, true)]);
+        let batch = RecordBatch::try_new(Arc::new(schema), vec![Arc::new(array)]).unwrap();
+
+        writer.write(&batch).unwrap();
+        writer.finish().unwrap();
         assert_eq!(str::from_utf8(&writer.into_inner()).unwrap(), "");
     }
 
+    #[test]
+    fn json_array_writer_empty_batch() {
+        let mut writer = ArrayWriter::new(vec![] as Vec<u8>);
+
+        let array = Int32Array::from(Vec::<i32>::new());
+        let schema = Schema::new(vec![Field::new("c", DataType::Int32, true)]);
+        let batch = RecordBatch::try_new(Arc::new(schema), vec![Arc::new(array)]).unwrap();
+
+        writer.write(&batch).unwrap();
+        writer.finish().unwrap();
+        assert_eq!(str::from_utf8(&writer.into_inner()).unwrap(), "[]");
+    }
+
     #[test]
     fn json_struct_array_nulls() {
         let inner = ListArray::from_iter_primitive::<Int32Type, _, _>(vec![