Report file location and offset when CSV schema mismatch (apache#13185)

datafusion/core/src/datasource/file_format/csv.rs

@@ -325,7 +325,13 @@ impl FileFormat for CsvFormat {
             let stream = self.read_to_delimited_chunks(store, object).await;
             let (schema, records_read) = self
                 .infer_schema_from_stream(state, records_to_read, stream)
-                .await?;
+                .await
+                .map_err(|err| {
+                    DataFusionError::Context(
+                        format!("Error when processing CSV file {}", &object.location),
+                        Box::new(err),
+                    )
+                })?;
             records_to_read -= records_read;
             schemas.push(schema);
             if records_to_read == 0 {
@@ -433,11 +439,13 @@ impl CsvFormat {
         let mut total_records_read = 0;
         let mut column_names = vec![];
         let mut column_type_possibilities = vec![];
-        let mut first_chunk = true;
+        let mut record_number = -1;
 
         pin_mut!(stream);
 
         while let Some(chunk) = stream.next().await.transpose()? {
+            record_number += 1;
+            let first_chunk = record_number == 0;
             let mut format = arrow::csv::reader::Format::default()
                 .with_header(
                     first_chunk
@@ -471,14 +479,14 @@ impl CsvFormat {
                         (field.name().clone(), possibilities)
                     })
                     .unzip();
-                first_chunk = false;
             } else {
                 if fields.len() != column_type_possibilities.len() {
                     return exec_err!(
                             "Encountered unequal lengths between records on CSV file whilst inferring schema. \
-                             Expected {} records, found {} records",
+                             Expected {} fields, found {} fields at record {}",
                             column_type_possibilities.len(),
-                            fields.len()
+                            fields.len(),
+                            record_number + 1
                         );
                 }