Add interface for async handling of sstable writer flushing and closi…

…ng (#978)

forest/sstable/sstable.go

@@ -44,3 +44,31 @@ type Writer interface {
 	// Close flushes all entries to the disk and returns the WriteResult.
 	Close() (*WriteResult, error)
 }
+
+// BatchWriterCloser collects sstable writers and handles the asynchronous
+// flushing and closing of the writers.
+// Example usage:
+// func batch(manager Manager, bwc BatchWriterCloser) {
+//	w1, _ := manager.GetWriter()
+//	_ = w1.WriteEntry(rocks.EntryRecord{Path: "foo1", Entry: &rocks.Entry{Address: "bar1"}})
+//	_ = w1.WriteEntry(rocks.EntryRecord{Path: "foo2", Entry: &rocks.Entry{Address: "bar2"}})
+//	_ = bwc.CloseWriterAsync(w1)
+
+//	w2, _ := manager.GetWriter()
+//	_ = w2.WriteEntry(rocks.EntryRecord{Path: "goo1", Entry: &rocks.Entry{Address: "baz1"}})
+//	_ = bwc.CloseWriterAsync(w2)
+
+//	// blocks until all writers finished or any writer failed
+//	res, err := bwc.Wait()
+//	// handle err, results, etc..
+// }
+type BatchWriterCloser interface {
+	// CloseWriterAsync adds Writer instance for the BatchWriterCloser to handle.
+	// Any writes executed to the writer after this call are not guaranteed to succeed.
+	// If Wait() has already been called, returns an error.
+	CloseWriterAsync(Writer) error
+
+	// Wait returns when all Writers finished.
+	// Any failure to close a single Writer will return with a nil results slice and an error.
+	Wait() ([]WriteResult, error)
+}