fix(Writer): io.ErrShortWrite caused by even byte

Issues: #4

go.mod

@@ -0,0 +1,5 @@
+module github.com/myml/ar
+
+go 1.14
+
+require github.com/google/martian v2.1.0+incompatible // indirect

go.sum

@@ -0,0 +1,2 @@
+github.com/google/martian v2.1.0+incompatible h1:/CP5g8u/VJHijgedC/Legn3BAbAaWPgecwXBIDzw5no=
+github.com/google/martian v2.1.0+incompatible/go.mod h1:9I4somxYTbIHy5NJKHRl3wXiIaQGbYVAs8BPL6v8lEs=

writer.go

@@ -1,4 +1,4 @@
-/* 
+/*
 Copyright (c) 2013 Blake Smith <[email protected]>
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
@@ -28,7 +28,7 @@ import (
 )
 
 var (
-	ErrWriteTooLong    = errors.New("ar: write too long")
+	ErrWriteTooLong = errors.New("ar: write too long")
 )
 
 // Writer provides sequential writing of an ar archive.
@@ -45,7 +45,7 @@ var (
 // }
 // io.Copy(archive, data)
 type Writer struct {
-	w io.Writer
+	w  io.Writer
 	nb int64 // number of unwritten bytes for the current file entry
 }
 
@@ -92,7 +92,7 @@ func (aw *Writer) Write(b []byte) (n int, err error) {
 
 	if len(b)%2 == 1 { // data size must be aligned to an even byte
 		n2, _ := aw.w.Write([]byte{'\n'})
-		return n+n2, err
+		return n + n2, err
 	}
 
 	return
@@ -122,3 +122,18 @@ func (aw *Writer) WriteHeader(hdr *Header) error {
 
 	return err
 }
+
+var _ io.ReaderFrom = &Writer{}
+
+// ReadFrom prevent returns io.ErrShortWrite when using io.Copy. This is to make io.Copy() to work correctly.
+func (aw *Writer) ReadFrom(r io.Reader) (written int64, err error) {
+	written, err = io.Copy(aw.w, r)
+	if err != nil {
+		return written, err
+	}
+	if written%2 == 1 { // data size must be aligned to an even byte
+		n2, err := aw.w.Write([]byte{'\n'})
+		return written + int64(n2), err
+	}
+	return written, err
+}

writer_test.go

@@ -1,4 +1,4 @@
-/* 
+/*
 Copyright (c) 2013 Blake Smith <[email protected]>
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
@@ -23,8 +23,10 @@ package ar
 
 import (
 	"bytes"
+	"io"
 	"io/ioutil"
 	"os"
+	"strings"
 	"testing"
 	"time"
 )
@@ -90,3 +92,39 @@ func TestWriteTooLong(t *testing.T) {
 		t.Errorf("Error should have been: %s", ErrWriteTooLong)
 	}
 }
+
+// Use odd byte check error
+func TestShortWrite(t *testing.T) {
+	body := strings.NewReader("Hello world!\n")
+
+	hdr := new(Header)
+	hdr.Size = body.Size()
+
+	var buf bytes.Buffer
+	writer := NewWriter(&buf)
+	writer.WriteHeader(hdr)
+	// Hide io.WriteTo interface and io.ReadFrom interface
+	_, err := io.Copy(io.MultiWriter(writer), io.MultiReader(body))
+	if err != io.ErrShortWrite {
+		t.Errorf("Error should have been: %s", io.ErrShortWrite)
+	}
+}
+
+func TestWriteCopy(t *testing.T) {
+	body := strings.NewReader("Hello world!\n")
+
+	hdr := new(Header)
+	hdr.Size = body.Size()
+
+	var buf bytes.Buffer
+	writer := NewWriter(&buf)
+	writer.WriteHeader(hdr)
+	// Only hide io.WriteTo interface
+	wn, err := io.Copy(writer, io.MultiReader(body))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if wn != hdr.Size+1 {
+		t.Errorf("Expected %d to equal %d", wn, hdr.Size+1)
+	}
+}