Implement io.WriterTo interface on Tx.

This commit moves the functionality in Tx.Copy() to Tx.WriteTo(). This
allows Tx to be used as an io.WriterTo which makes it easier to mock.

The Tx.Copy() function still exists but it's simply a wrapper around
Tx.WriteTo().

README.md

@@ -328,7 +328,7 @@ 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
@@ -343,7 +343,8 @@ 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.