add test and docstring

parquet/src/column/writer/mod.rs

@@ -902,6 +902,19 @@ impl<'a, E: ColumnValueEncoder> GenericColumnWriter<'a, E> {
             .unwrap_or_else(|| (data.to_vec(), false))
     }
 
+    /// Truncates a binary statistic to at most `truncation_length` bytes, and then increment the
+    /// final byte(s) to yield a valid upper bound. This may result in a result of less than
+    /// `truncation_length` bytes if the last byte(s) overflows.
+    ///
+    /// If truncation is not possible, returns `data`.
+    ///
+    /// The `bool` in the returned tuple indicates whether truncation occurred or not.
+    ///
+    /// UTF-8 Note:
+    /// If the column type indicates UTF-8, and `data` contains valid UTF-8, then the result will
+    /// also remain valid UTF-8 (but again may be less than `truncation_length` bytes). If `data`
+    /// does not contain valid UTF-8, then truncation will occur as if the column is non-string
+    /// binary.
     fn truncate_max_value(&self, truncation_length: Option<usize>, data: &[u8]) -> (Vec<u8>, bool) {
         truncation_length
             .filter(|l| data.len() > *l)
@@ -1500,10 +1513,13 @@ fn increment(mut data: Vec<u8>) -> Option<Vec<u8>> {
 
 #[cfg(test)]
 mod tests {
-    use crate::file::properties::DEFAULT_COLUMN_INDEX_TRUNCATE_LENGTH;
+    use crate::{
+        file::{properties::DEFAULT_COLUMN_INDEX_TRUNCATE_LENGTH, writer::SerializedFileWriter},
+        schema::parser::parse_message_type,
+    };
     use core::str;
     use rand::distributions::uniform::SampleUniform;
-    use std::sync::Arc;
+    use std::{fs::File, sync::Arc};
 
     use crate::column::{
         page::PageReader,
@@ -3297,6 +3313,56 @@ mod tests {
         assert!(r.is_none());
     }
 
+    #[test]
+    // Check fallback truncation of statistics that should be UTF-8, but aren't
+    // (see https://github.com/apache/arrow-rs/pull/6870).
+    fn test_byte_array_truncate_invalid_utf8_statistics() {
+        let message_type = "
+            message test_schema {
+                OPTIONAL BYTE_ARRAY a (UTF8);
+            }
+        ";
+        let schema = Arc::new(parse_message_type(message_type).unwrap());
+
+        // Create Vec<ByteArray> containing non-UTF8 bytes
+        let data = vec![ByteArray::from(vec![128u8; 32]); 7];
+        let def_levels = [1, 1, 1, 1, 0, 1, 0, 1, 0, 1];
+        let file: File = tempfile::tempfile().unwrap();
+        let props = Arc::new(
+            WriterProperties::builder()
+                .set_statistics_enabled(EnabledStatistics::Chunk)
+                .set_statistics_truncate_length(Some(8))
+                .build(),
+        );
+
+        let mut writer = SerializedFileWriter::new(&file, schema, props).unwrap();
+        let mut row_group_writer = writer.next_row_group().unwrap();
+
+        let mut col_writer = row_group_writer.next_column().unwrap().unwrap();
+        col_writer
+            .typed::<ByteArrayType>()
+            .write_batch(&data, Some(&def_levels), None)
+            .unwrap();
+        col_writer.close().unwrap();
+        row_group_writer.close().unwrap();
+        let file_metadata = writer.close().unwrap();
+        assert!(file_metadata.row_groups[0].columns[0].meta_data.is_some());
+        let stats = file_metadata.row_groups[0].columns[0]
+            .meta_data
+            .as_ref()
+            .unwrap()
+            .statistics
+            .as_ref()
+            .unwrap();
+        assert!(!stats.is_max_value_exact.unwrap());
+        // Truncation of invalid UTF-8 should fall back to binary truncation, so last byte should
+        // be incremented by 1.
+        assert_eq!(
+            stats.max_value,
+            Some([128, 128, 128, 128, 128, 128, 128, 129].to_vec())
+        );
+    }
+
     #[test]
     fn test_increment_max_binary_chars() {
         let r = increment(vec![0xFF, 0xFE, 0xFD, 0xFF, 0xFF]);