fix panic when reusing a was-empty batch (#267)

* add a test case

Signed-off-by: tabokie <xy.tao@outlook.com>

* fix the bug

Signed-off-by: tabokie <xy.tao@outlook.com>

* update changelog

Signed-off-by: tabokie <xy.tao@outlook.com>

* Address comment

Signed-off-by: Xinye Tao <xy.tao@outlook.com>

Signed-off-by: tabokie <xy.tao@outlook.com>
Signed-off-by: Xinye Tao <xy.tao@outlook.com>

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 * Unconditionally tolerate `fallocate` failures as a fix to its portability issue. Errors other than `EOPNOTSUPP` will still emit a warning.
 * Avoid leaving fractured write after failure by reseeking the file writer. Panic if the reseek fails as well.
+* Fix panic when an empty batch is written to engine and then reused.
 
 ### New Features
 

src/engine.rs

@@ -136,8 +136,12 @@ where
     /// bytes. If `sync` is true, the write will be followed by a call to
     /// `fdatasync` on the log file.
     pub fn write(&self, log_batch: &mut LogBatch, mut sync: bool) -> Result<usize> {
+        if log_batch.is_empty() {
+            return Ok(0);
+        }
         let start = Instant::now();
         let len = log_batch.finish_populate(self.cfg.batch_compression_threshold.0 as usize)?;
+        debug_assert!(len > 0);
         let block_handle = {
             let mut writer = Writer::new(log_batch, sync);
             // Snapshot and clear the current perf context temporarily, so the write group
@@ -151,16 +155,7 @@ where
                     writer.entered_time = Some(now);
                     sync |= writer.sync;
                     let log_batch = writer.mut_payload();
-                    let res = if !log_batch.is_empty() {
-                        self.pipe_log.append(LogQueue::Append, log_batch)
-                    } else {
-                        // TODO(tabokie): use Option<FileBlockHandle> instead.
-                        Ok(FileBlockHandle {
-                            id: FileId::new(LogQueue::Append, 0),
-                            offset: 0,
-                            len: 0,
-                        })
-                    };
+                    let res = self.pipe_log.append(LogQueue::Append, log_batch);
                     writer.set_output(res);
                 }
                 perf_context!(log_write_duration).observe_since(now);
@@ -185,20 +180,17 @@ where
             set_perf_context(perf_context);
             writer.finish()?
         };
-
         let mut now = Instant::now();
-        if len > 0 {
-            log_batch.finish_write(block_handle);
-            self.memtables.apply_append_writes(log_batch.drain());
-            for listener in &self.listeners {
-                listener.post_apply_memtables(block_handle.id);
-            }
-            let end = Instant::now();
-            let apply_duration = end.saturating_duration_since(now);
-            ENGINE_WRITE_APPLY_DURATION_HISTOGRAM.observe(apply_duration.as_secs_f64());
-            perf_context!(apply_duration).observe(apply_duration);
-            now = end;
-        }
+        log_batch.finish_write(block_handle);
+        self.memtables.apply_append_writes(log_batch.drain());
+        for listener in &self.listeners {
+            listener.post_apply_memtables(block_handle.id);
+        }
+        let end = Instant::now();
+        let apply_duration = end.saturating_duration_since(now);
+        ENGINE_WRITE_APPLY_DURATION_HISTOGRAM.observe(apply_duration.as_secs_f64());
+        perf_context!(apply_duration).observe(apply_duration);
+        now = end;
         ENGINE_WRITE_DURATION_HISTOGRAM.observe(now.saturating_duration_since(start).as_secs_f64());
         ENGINE_WRITE_SIZE_HISTOGRAM.observe(len as f64);
         Ok(len)
@@ -1393,6 +1385,34 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_empty_batch() {
+        let dir = tempfile::Builder::new()
+            .prefix("test_empty_batch")
+            .tempdir()
+            .unwrap();
+        let cfg = Config {
+            dir: dir.path().to_str().unwrap().to_owned(),
+            ..Default::default()
+        };
+        let engine =
+            RaftLogEngine::open_with_file_system(cfg, Arc::new(ObfuscatedFileSystem::default()))
+                .unwrap();
+        let data = vec![b'x'; 16];
+        let cases = [[false, false], [false, true], [true, true]];
+        for (i, writes) in cases.iter().enumerate() {
+            let rid = i as u64;
+            let mut batch = LogBatch::default();
+            for &has_data in writes {
+                if has_data {
+                    batch.put(rid, b"key".to_vec(), data.clone());
+                }
+                engine.write(&mut batch, true).unwrap();
+                assert!(batch.is_empty());
+            }
+        }
+    }
+
     #[test]
     fn test_dirty_recovery() {
         let dir = tempfile::Builder::new()