lib/file: reimplement os.OpenFile allowing rename/delete open files under Windows

Normally os.OpenFile under Windows does not allow renaming or deleting
open file handles.  This package provides equivelents for os.OpenFile,
os.Open and os.Create which do allow that.

lib/file/file_test.go

@@ -0,0 +1,154 @@
+package file
+
+import (
+	"fmt"
+	"io"
+	"io/ioutil"
+	"os"
+	"path"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// Create a test directory then tidy up
+func testDir(t *testing.T) (string, func()) {
+	dir, err := ioutil.TempDir("", "rclone-test")
+	require.NoError(t, err)
+	return dir, func() {
+		assert.NoError(t, os.RemoveAll(dir))
+	}
+}
+
+// This lists dir and checks the listing is as expected without checking the size
+func checkListingNoSize(t *testing.T, dir string, want []string) {
+	var got []string
+	nodes, err := ioutil.ReadDir(dir)
+	require.NoError(t, err)
+	for _, node := range nodes {
+		got = append(got, fmt.Sprintf("%s,%v", node.Name(), node.IsDir()))
+	}
+	assert.Equal(t, want, got)
+}
+
+// This lists dir and checks the listing is as expected
+func checkListing(t *testing.T, dir string, want []string) {
+	var got []string
+	nodes, err := ioutil.ReadDir(dir)
+	require.NoError(t, err)
+	for _, node := range nodes {
+		got = append(got, fmt.Sprintf("%s,%d,%v", node.Name(), node.Size(), node.IsDir()))
+	}
+	assert.Equal(t, want, got)
+}
+
+// Test we can rename an open file
+func TestOpenFileRename(t *testing.T) {
+	dir, tidy := testDir(t)
+	defer tidy()
+
+	filepath := path.Join(dir, "file1")
+	f, err := Create(filepath)
+	require.NoError(t, err)
+
+	_, err = f.Write([]byte("hello"))
+	assert.NoError(t, err)
+
+	checkListingNoSize(t, dir, []string{
+		"file1,false",
+	})
+
+	// Delete the file first
+	assert.NoError(t, os.Remove(filepath))
+
+	// .. then close it
+	assert.NoError(t, f.Close())
+
+	checkListing(t, dir, nil)
+}
+
+// Test we can delete an open file
+func TestOpenFileDelete(t *testing.T) {
+	dir, tidy := testDir(t)
+	defer tidy()
+
+	filepath := path.Join(dir, "file1")
+	f, err := Create(filepath)
+	require.NoError(t, err)
+
+	_, err = f.Write([]byte("hello"))
+	assert.NoError(t, err)
+
+	checkListingNoSize(t, dir, []string{
+		"file1,false",
+	})
+
+	// Rename the file while open
+	filepath2 := path.Join(dir, "file2")
+	assert.NoError(t, os.Rename(filepath, filepath2))
+
+	checkListingNoSize(t, dir, []string{
+		"file2,false",
+	})
+
+	// .. then close it
+	assert.NoError(t, f.Close())
+
+	checkListing(t, dir, []string{
+		"file2,5,false",
+	})
+}
+
+// Smoke test the Open, OpenFile and Create functions
+func TestOpenFileOperations(t *testing.T) {
+	dir, tidy := testDir(t)
+	defer tidy()
+
+	filepath := path.Join(dir, "file1")
+
+	// Create the file
+
+	f, err := Create(filepath)
+	require.NoError(t, err)
+
+	_, err = f.Write([]byte("hello"))
+	assert.NoError(t, err)
+
+	assert.NoError(t, f.Close())
+
+	checkListing(t, dir, []string{
+		"file1,5,false",
+	})
+
+	// Append onto the file
+
+	f, err = OpenFile(filepath, os.O_RDWR|os.O_APPEND, 0666)
+	require.NoError(t, err)
+
+	_, err = f.Write([]byte("HI"))
+	assert.NoError(t, err)
+
+	assert.NoError(t, f.Close())
+
+	checkListing(t, dir, []string{
+		"file1,7,false",
+	})
+
+	// Read it back in
+
+	f, err = Open(filepath)
+	require.NoError(t, err)
+	var b = make([]byte, 10)
+	n, err := f.Read(b)
+	assert.True(t, err == io.EOF || err == nil)
+	assert.Equal(t, 7, n)
+	assert.Equal(t, "helloHI", string(b[:n]))
+
+	assert.NoError(t, f.Close())
+
+	checkListing(t, dir, []string{
+		"file1,7,false",
+	})
+
+}