fn: add synchronous write file

The fn.WriteFile writes data like os.WriteFile but in sync mode.
Also adds a behaviour flag that enables removal of file on error.

Co-authored-by: Maurice Poirrier Chuden <mauricepoirrier@gmail.com>
Co-authored-by: Greg Weber <1183+gregwebs@users.noreply.github.com>

fn/io.go

@@ -0,0 +1,45 @@
+package fn
+
+import (
+	"os"
+)
+
+// WriteFile synchronously writes data to the named file.
+// If the file does not exist, WriteFile creates it with permissions perm
+// (before umask); otherwise WriteFile truncates it before writing, without
+// changing permissions.
+// By opening the file with O_SYNC, it ensures the data is written to disk.
+// If an error occurs, it does not remove the file.
+func WriteFile(name string, data []byte, perm os.FileMode) error {
+	f, err := os.OpenFile(
+		name, os.O_WRONLY|os.O_CREATE|os.O_TRUNC|os.O_SYNC, perm,
+	)
+	if err != nil {
+		return err
+	}
+
+	_, err = f.Write(data)
+
+	// Prioritize the error on Write but make sure to call Close regardless
+	// to avoid leaking a file handle.
+	if err1 := f.Close(); err1 != nil && err == nil {
+		err = err1
+	}
+
+	return err
+}
+
+// WriteFileRemove synchronously writes data to the named file.
+// If the file does not exist, WriteFileRemove creates it with permissions perm
+// (before umask); otherwise WriteFileRemove truncates it before writing,
+// without changing permissions.
+// By opening the file with O_SYNC, it ensures the data is written to disk.
+// If an error occurs, it removes the file.
+func WriteFileRemove(name string, data []byte, perm os.FileMode) error {
+	err := WriteFile(name, data, perm)
+	if err != nil {
+		_ = os.Remove(name)
+	}
+
+	return err
+}

fn/io_test.go

@@ -0,0 +1,96 @@
+package fn
+
+import (
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+var (
+	data1 = "The network is robust in its unstructured simplicity."
+	data2 = "Nodes work all at once with little coordination."
+)
+
+// TestWriteFile uses same scenario of ioutil asserting the content created and
+// stored on new file with the original one.
+func TestWriteFile(t *testing.T) {
+	t.Parallel()
+
+	f, deferred := ensureTempfile(t)
+	filename := f.Name()
+	defer deferred()
+
+	// Write file.
+	err := WriteFile(filename, []byte(data2), 0644)
+	require.NoError(t, err, "couldn't write file")
+	ensureFileContents(t, filename, data2)
+
+	// Overwrite file.
+	err = WriteFile(filename, []byte(data1), 0644)
+	require.NoError(t, err, "couldn't overwrite file")
+	ensureFileContents(t, filename, data1)
+
+	// Change file permission to read-only.
+	err = os.Chmod(filename, 0444)
+	require.NoError(t, err, "couldn't change file to read-only")
+
+	// Write must fail and keep the file.
+	err = WriteFile(filename, []byte(data2), 0644)
+	require.Error(t, err, "shouldn't write a read-only file")
+
+	_, err = os.Stat(filename)
+	require.NoError(t, err, "shouldn't remove file on error")
+	ensureFileContents(t, filename, data1)
+}
+
+func TestWriteFileRemove(t *testing.T) {
+	t.Parallel()
+
+	f, deferred := ensureTempfile(t)
+	filename := f.Name()
+	defer deferred()
+
+	// Write file.
+	err := WriteFileRemove(filename, []byte(data2), 0644)
+	require.NoError(t, err, "couldn't write file")
+	ensureFileContents(t, filename, data2)
+
+	// Overwrite file.
+	err = WriteFileRemove(filename, []byte(data1), 0644)
+	require.NoError(t, err, "couldn't overwrite file")
+	ensureFileContents(t, filename, data1)
+
+	// Change file permission to read-only.
+	err = os.Chmod(filename, 0444)
+	require.NoError(t, err, "couldn't change file to read-only")
+
+	// Write must fail and remove the file.
+	err = WriteFileRemove(filename, []byte(data2), 0644)
+	require.Error(t, err, "shouldn't write a read-only file")
+
+	_, err = os.Stat(filename)
+	require.ErrorContains(
+		t, err, "no such file or directory",
+		"should remove file on error",
+	)
+}
+
+func ensureTempfile(t *testing.T) (*os.File, func()) {
+	t.Helper()
+	f, err := os.CreateTemp("", "fn-io-test-TestWriteFile")
+	require.NoError(t, err, "couldn't create temporary file")
+
+	return f, func() {
+		f.Close()
+		os.Remove(f.Name())
+	}
+}
+
+func ensureFileContents(t *testing.T, filename string, data string) {
+	t.Helper()
+	contents, err := os.ReadFile(filename)
+	require.NoError(t, err, "couldn't read file")
+
+	require.Equal(t, data, string(contents))
+}