fix: skip writing the file if the contents haven't changed

As the controller reconciles every /etc file present, it might be called
multiple times for the same file, even if the actual contents haven't
changed.

Rewriting the file might lead to some concurrent process seeing
incomplete file contents more often than needed.

Signed-off-by: Andrey Smirnov <andrey.smirnov@siderolabs.com>

internal/app/machined/pkg/controllers/files/etcfile.go

@@ -5,6 +5,7 @@
 package files
 
 import (
+	"bytes"
 	"context"
 	"errors"
 	"fmt"
@@ -132,7 +133,7 @@ func (ctrl *EtcFileController) Run(ctx context.Context, r controller.Runtime, lo
 
 				logger.Debug("writing file contents", zap.String("dst", dst), zap.Stringer("version", spec.Metadata().Version()))
 
-				if err = os.WriteFile(dst, spec.TypedSpec().Contents, spec.TypedSpec().Mode); err != nil {
+				if err = updateFile(dst, spec.TypedSpec().Contents, spec.TypedSpec().Mode); err != nil {
 					return fmt.Errorf("error updating %q: %w", dst, err)
 				}
 
@@ -190,3 +191,14 @@ func createBindMount(src, dst string, mode os.FileMode) (err error) {
 
 	return nil
 }
+
+// updateFile is like `os.WriteFile`, but it will only update the file if the
+// contents have changed.
+func updateFile(filename string, contents []byte, mode os.FileMode) error {
+	oldContents, err := os.ReadFile(filename)
+	if err == nil && bytes.Equal(oldContents, contents) {
+		return nil
+	}
+
+	return os.WriteFile(filename, contents, mode)
+}