Fix panics on reading entries by stale index.

src/engine.rs

@@ -334,9 +334,12 @@ where
         let _t = StopWatch::new(&*ENGINE_READ_ENTRY_DURATION_HISTOGRAM);
         if let Some(memtable) = self.memtables.get(region_id) {
             let mut ents_idx: Vec<EntryIndex> = Vec::with_capacity((end - begin) as usize);
-            memtable
-                .read()
-                .fetch_entries_to(begin, end, max_size, &mut ents_idx)?;
+            // Ensure that the corresponding memtable is locked with a read lock before
+            // completing the fetching of entries from the raft logs. This
+            // prevents the scenario where the index could become stale while
+            // being concurrently updated by the `rewrite` operation.
+            let immutable = memtable.read();
+            immutable.fetch_entries_to(begin, end, max_size, &mut ents_idx)?;
             for i in ents_idx.iter() {
                 vec.push(read_entry_from_file::<M, _>(self.pipe_log.as_ref(), i)?);
             }
@@ -635,9 +638,11 @@ pub(crate) mod tests {
     use crate::util::ReadableSize;
     use kvproto::raft_serverpb::RaftLocalState;
     use raft::eraftpb::Entry;
+    use rand::{thread_rng, Rng};
     use std::collections::{BTreeSet, HashSet};
     use std::fs::OpenOptions;
     use std::path::PathBuf;
+    use std::sync::atomic::{AtomicBool, Ordering};
 
     pub(crate) type RaftLogEngine<F = DefaultFileSystem> = Engine<F>;
     impl<F: FileSystem> RaftLogEngine<F> {
@@ -1929,8 +1934,6 @@ pub(crate) mod tests {
     #[cfg(feature = "nightly")]
     #[bench]
     fn bench_engine_fetch_entries(b: &mut test::Bencher) {
-        use rand::{thread_rng, Rng};
-
         let dir = tempfile::Builder::new()
             .prefix("bench_engine_fetch_entries")
             .tempdir()
@@ -2587,6 +2590,53 @@ pub(crate) mod tests {
         assert!(data.is_empty(), "data loss {:?}", data);
     }
 
+    #[test]
+    fn test_fetch_with_concurrently_rewrite() {
+        let dir = tempfile::Builder::new()
+            .prefix("test_fetch_with_concurrently_rewrite")
+            .tempdir()
+            .unwrap();
+        let cfg = Config {
+            dir: dir.path().to_str().unwrap().to_owned(),
+            target_file_size: ReadableSize(2048),
+            ..Default::default()
+        };
+        let fs = Arc::new(DeleteMonitoredFileSystem::new());
+        let engine = Arc::new(RaftLogEngine::open_with_file_system(cfg, fs.clone()).unwrap());
+        let entry_data = vec![b'x'; 128];
+        // Set up a concurrent write with purge, and fetch.
+        let mut vec: Vec<Entry> = Vec::new();
+        let fetch_engine = engine.clone();
+        let flag = Arc::new(AtomicBool::new(false));
+        let start_flag = flag.clone();
+        let th = std::thread::spawn(move || {
+            while !start_flag.load(Ordering::Acquire) {
+                std::thread::sleep(Duration::from_millis(10));
+            }
+            for _ in 0..10 {
+                let region_id = thread_rng().gen_range(1..=10);
+                // Should not return file seqno out of range error.
+                let _ = fetch_engine
+                    .fetch_entries_to::<Entry>(region_id, 1, 101, None, &mut vec)
+                    .map_err(|e| {
+                        assert!(!format!("{e}").contains("file seqno out of"));
+                    });
+                vec.clear();
+            }
+        });
+        for i in 0..10 {
+            for rid in 1..=10 {
+                engine.append(rid, 1 + i * 10, 1 + i * 10 + 10, Some(&entry_data));
+            }
+            flag.store(true, Ordering::Release);
+            for rid in 1..=10 {
+                engine.clean(rid);
+            }
+            engine.purge_expired_files().unwrap();
+        }
+        th.join().unwrap();
+    }
+
     #[test]
     fn test_internal_key_filter() {
         let dir = tempfile::Builder::new()