add file copy helper method to iohelp package (#1403)

go/libraries/utils/iohelp/copy.go

@@ -0,0 +1,58 @@
+// Copyright 2021 Dolthub, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package iohelp
+
+import (
+	"fmt"
+	"io"
+	"os"
+)
+
+func CopyFile(src, dest string) (size int64, err error) {
+	stat, err := os.Stat(src)
+	if err != nil {
+		return 0, err
+	}
+
+	if !stat.Mode().IsRegular() {
+		return 0, fmt.Errorf("%s is not a regular file", src)
+	}
+
+	srcf, err := os.Open(src)
+	if err != nil {
+		return 0, err
+	}
+	defer func() {
+		closeErr := srcf.Close()
+
+		if err == nil {
+			err = closeErr
+		}
+	}()
+
+	destf, err := os.Create(dest)
+	if err != nil {
+		return 0, err
+	}
+	defer func() {
+		closeErr := destf.Close()
+
+		if err == nil {
+			err = closeErr
+		}
+	}()
+
+	return io.Copy(destf, srcf)
+}