fix: auto downgrade index record option, instead of vint error

Prev: thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: IoError(Custom { kind: InvalidData, error: "Reach end of buffer while reading VInt" })', src/main.rs:46:14
Now: Automatic downgrade to next available level

Author: PSeitz

src/core/inverted_index_reader.rs

@@ -135,6 +135,8 @@ impl InvertedIndexReader {
         term_info: &TermInfo,
         option: IndexRecordOption,
     ) -> io::Result<SegmentPostings> {
+        let option = option.downgrade(self.record_option);
+
         let block_postings = self.read_block_postings_from_terminfo(term_info, option)?;
         let position_reader = {
             if option.has_positions() {

src/query/term_query/term_query.rs

@@ -109,6 +109,7 @@ impl TermQuery {
         } else {
             IndexRecordOption::Basic
         };
+
         Ok(TermWeight::new(
             self.term.clone(),
             index_record_option,

src/schema/index_record_option.rs

@@ -1,5 +1,7 @@
 use serde::{Deserialize, Serialize};
 
+use crate::TantivyError;
+
 /// `IndexRecordOption` describes an amount information associated
 /// with a given indexed field.
 ///
@@ -49,4 +51,17 @@ impl IndexRecordOption {
             IndexRecordOption::WithFreqsAndPositions => true,
         }
     }
+
+    /// Downgrades to the next level if provided `IndexRecordOption` is unavailable.
+    pub fn downgrade(&self, other: IndexRecordOption) -> IndexRecordOption {
+        use IndexRecordOption::*;
+
+        match (other, self) {
+            (WithFreqsAndPositions, WithFreqsAndPositions) => WithFreqsAndPositions,
+            (WithFreqs, WithFreqs) => WithFreqs,
+            (WithFreqsAndPositions, WithFreqs) => WithFreqs,
+            (WithFreqs, WithFreqsAndPositions) => WithFreqs,
+            _ => Basic,
+        }
+    }
 }