Optimize R2 flush part implementation

drivers/r2/driver.go

@@ -1236,14 +1236,28 @@ func (w *writer) flushPart() error {
 		// nothing to write
 		return nil
 	}
-	if len(w.pendingPart) < int(w.driver.ChunkSize) {
-		// closing with a small pending part
-		// combine ready and pending to avoid writing a small part
-		w.readyPart = append(w.readyPart, w.pendingPart...)
-		w.pendingPart = nil
+
+	// Check if the ready part is less than the chunk size
+	if len(w.readyPart) < int(w.driver.ChunkSize) {
+		// If there's enough in the pending part to fill the ready part up to the chunk size
+		if len(w.pendingPart) >= int(w.driver.ChunkSize)-len(w.readyPart) {
+			fillSize := int(w.driver.ChunkSize) - len(w.readyPart)
+			w.readyPart = append(w.readyPart, w.pendingPart[:fillSize]...)
+			w.pendingPart = w.pendingPart[fillSize:]
+		}
 	}
+
 	ctx := context.Background()
+	// Check if the ready part is the right size to upload
+	// ready part must be equal to chunk size,
+	// except for the last part where it can be equal or smaller than the chunk size
+	isReadyToUpload := len(w.readyPart) == int(w.driver.ChunkSize) || (len(w.readyPart) < int(w.driver.ChunkSize) && len(w.pendingPart) == 0)
+	if !isReadyToUpload {
+		// If the ready part is not yet full and it's not the last part, wait for more data
+		return nil
+	}
 
+	// Upload the part
 	partNumber := aws.Int32(int32(len(w.parts) + 1))
 	resp, err := w.driver.R2.UploadPart(ctx, &s3.UploadPartInput{
 		Bucket:     aws.String(w.driver.Bucket),
@@ -1255,12 +1269,22 @@ func (w *writer) flushPart() error {
 	if err != nil {
 		return err
 	}
+
+	// Append the uploaded part's info to the parts slice
 	w.parts = append(w.parts, types.Part{
 		ETag:       resp.ETag,
 		PartNumber: partNumber,
 		Size:       aws.Int64(int64(len(w.readyPart))),
 	})
+
+	// Prepare for the next part
 	w.readyPart = w.pendingPart
 	w.pendingPart = nil
-	return w.flushPart()
+
+	// If there's more to upload, continue
+	if len(w.readyPart) > 0 {
+		return w.flushPart()
+	}
+
+	return nil
 }