[runtime/utils/buffer/write] skip sync if unnecessary

Author: andresilva

runtime/src/utils/buffer/mod.rs

@@ -1491,6 +1491,13 @@ mod tests {
             let blob = RangeSyncBlob::new();
             let writer = Write::from_pooler(&context, blob.clone(), 0, NZUsize!(8));
 
+            // A fresh writer has no buffered bytes or prior plain blob mutation, so sync is a no-op.
+            writer.sync().await.unwrap();
+            let (durable, full_syncs, range_syncs) = blob.snapshot();
+            assert!(durable.is_empty());
+            assert_eq!(full_syncs, 0);
+            assert_eq!(range_syncs, 0);
+
             // The write remains entirely buffered, so sync can make just this range durable.
             writer.write_at(0, b"abc").await.unwrap();
             writer.sync().await.unwrap();
@@ -1500,6 +1507,13 @@ mod tests {
             assert_eq!(durable.as_slice(), b"abc");
             assert_eq!(full_syncs, 0);
             assert_eq!(range_syncs, 1);
+
+            // The prior sync used write_at_sync, so there is still no pending full-sync barrier.
+            writer.sync().await.unwrap();
+            let (durable, full_syncs, range_syncs) = blob.snapshot();
+            assert_eq!(durable.as_slice(), b"abc");
+            assert_eq!(full_syncs, 0);
+            assert_eq!(range_syncs, 1);
         });
     }
 
@@ -1521,6 +1535,13 @@ mod tests {
             assert_eq!(full_syncs, 1);
             assert_eq!(range_syncs, 0);
 
+            // With no new writes, sync has no work left.
+            writer.sync().await.unwrap();
+            let (durable, full_syncs, range_syncs) = blob.snapshot();
+            assert_eq!(durable.as_slice(), b"abcdefg");
+            assert_eq!(full_syncs, 1);
+            assert_eq!(range_syncs, 0);
+
             // After the full sync, the next buffer-only write can use range sync again.
             writer.write_at(7, b"h").await.unwrap();
             writer.sync().await.unwrap();

runtime/src/utils/buffer/write.rs

@@ -240,6 +240,11 @@ impl<B: Blob> Write<B> {
             state.needs_sync = false;
             return Ok(());
         }
+
+        if !state.needs_sync {
+            return Ok(());
+        }
+
         self.blob.sync().await?;
         state.needs_sync = false;
         Ok(())