AVRO-3881: [rust] Writer should write user metadata even if the body …

…is empty (#2545)

* writer should write user metadata even if the body is empty

* AVRO-3881: Give a better name to a new test case

---------

Co-authored-by: Martin Grigorov <[email protected]>

lang/rust/avro/src/writer.rs

@@ -323,6 +323,7 @@ impl<'a, W: Write> Writer<'a, W> {
     /// **NOTE** This function forces the written data to be flushed (an implicit
     /// call to [`flush`](struct.Writer.html#method.flush) is performed).
     pub fn into_inner(mut self) -> AvroResult<W> {
+        self.maybe_write_header()?;
         self.flush()?;
         Ok(self.writer)
     }
@@ -635,6 +636,7 @@ mod tests {
         schema::{DecimalSchema, FixedSchema, Name},
         types::Record,
         util::zig_i64,
+        Reader,
     };
     use pretty_assertions::assert_eq;
     use serde::{Deserialize, Serialize};
@@ -1139,6 +1141,22 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn test_avro_3881_metadata_empty_body() -> TestResult {
+        let schema = Schema::parse_str(SCHEMA)?;
+        let mut writer = Writer::new(&schema, Vec::new());
+        writer.add_user_metadata("a".to_string(), "b")?;
+        let result = writer.into_inner()?;
+
+        let reader = Reader::with_schema(&schema, &result[..])?;
+        let mut expected = HashMap::new();
+        expected.insert("a".to_string(), vec![b'b']);
+        assert_eq!(reader.user_metadata(), &expected);
+        assert_eq!(reader.into_iter().count(), 0);
+
+        Ok(())
+    }
+
     #[test]
     fn test_avro_3405_writer_add_metadata_failure() -> TestResult {
         let schema = Schema::parse_str(SCHEMA)?;