util/lineread: add little package to read lines from files/Readers

util/lineread/lineread.go

@@ -0,0 +1,33 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package lineread reads lines from files. It's not fancy, but it got repetitive.
+package lineread
+
+import (
+	"bufio"
+	"io"
+	"os"
+)
+
+// File opens name and calls fn for each line. It returns an error if the Open failed
+// or once fn returns an error.
+func File(name string, fn func(line []byte) error) error {
+	f, err := os.Open(name)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+	return Reader(f, fn)
+}
+
+func Reader(r io.Reader, fn func(line []byte) error) error {
+	bs := bufio.NewScanner(r)
+	for bs.Scan() {
+		if err := fn(bs.Bytes()); err != nil {
+			return err
+		}
+	}
+	return bs.Err()
+}