Merge pull request #330 from benbjohnson/tx-writer-to

Implement io.WriterTo interface on Tx.

README.md

@@ -370,7 +370,7 @@ func (*Bucket) DeleteBucket(key []byte) error
 
 ### Database backups
 
-Bolt is a single file so it's easy to backup. You can use the `Tx.Copy()`
+Bolt is a single file so it's easy to backup. You can use the `Tx.WriteTo()`
 function to write a consistent view of the database to a writer. If you call
 this from a read-only transaction, it will perform a hot backup and not block
 your other database reads and writes. It will also use `O_DIRECT` when available
@@ -385,7 +385,8 @@ func BackupHandleFunc(w http.ResponseWriter, req *http.Request) {
 		w.Header().Set("Content-Type", "application/octet-stream")
 		w.Header().Set("Content-Disposition", `attachment; filename="my.db"`)
 		w.Header().Set("Content-Length", strconv.Itoa(int(tx.Size())))
-		return tx.Copy(w)
+		_, err := tx.WriteTo(w)
+		return err
 	})
 	if err != nil {
 		http.Error(w, err.Error(), http.StatusInternalServerError)

tx.go

@@ -252,37 +252,42 @@ func (tx *Tx) close() {
 }
 
 // Copy writes the entire database to a writer.
-// A reader transaction is maintained during the copy so it is safe to continue
+// This function exists for backwards compatibility. Use WriteTo() in
-// using the database while a copy is in progress.
-// Copy will write exactly tx.Size() bytes into the writer.
 func (tx *Tx) Copy(w io.Writer) error {
-	var f *os.File
+	_, err := tx.WriteTo(w)
-	var err error
+	return err
+}
 
+// WriteTo writes the entire database to a writer.
+// If err == nil then exactly tx.Size() bytes will be written into the writer.
+func (tx *Tx) WriteTo(w io.Writer) (n int64, err error) {
 	// Attempt to open reader directly.
+	var f *os.File
 	if f, err = os.OpenFile(tx.db.path, os.O_RDONLY|odirect, 0); err != nil {
 		// Fallback to a regular open if that doesn't work.
 		if f, err = os.OpenFile(tx.db.path, os.O_RDONLY, 0); err != nil {
-			return err
+			return 0, err
 		}
 	}
 
 	// Copy the meta pages.
 	tx.db.metalock.Lock()
-	_, err = io.CopyN(w, f, int64(tx.db.pageSize*2))
+	n, err = io.CopyN(w, f, int64(tx.db.pageSize*2))
 	tx.db.metalock.Unlock()
 	if err != nil {
 		_ = f.Close()
-		return fmt.Errorf("meta copy: %s", err)
+		return n, fmt.Errorf("meta copy: %s", err)
 	}
 
 	// Copy data pages.
-	if _, err := io.CopyN(w, f, tx.Size()-int64(tx.db.pageSize*2)); err != nil {
+	wn, err := io.CopyN(w, f, tx.Size()-int64(tx.db.pageSize*2))
+	n += wn
+	if err != nil {
 		_ = f.Close()
-		return err
+		return n, err
 	}
 
-	return f.Close()
+	return n, f.Close()
 }
 
 // CopyFile copies the entire database to file at the given path.