Added robust file Rename function for moving files between filesystem…

…s (cf. uhppoted/uhppoted-httpd#20)

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 ### Added 
 1. `ActivateKeypads` API function for REST and MQTT `activate-keypads` command.
+2. Robust file rename that falls back to copying the file to be renamed if the 
+   OS relink failed.
 
 ### Updated
 1. Added _card.format_ to the configuration to facilitate support for card formats other

TODO.md

@@ -1,7 +1,8 @@
 # TODO
 
 - [x] Add card formats to configuration (cf. https://github.com/uhppoted/uhppote-cli/issues/12)
-- [x] Implement `activate-keypads` (cf. https://github.com/uhppoted/uhppoted/issues/35)
+- [x] Implement `activate-keypads` (cf. https://github.com/uhppoted/uhppoted/issues/35)\
+- [ ] Implement robust Rename (cf. https://github.com/uhppoted/uhppoted-httpd/issues/20)
 
 ## TODO
 

os/file.go

@@ -0,0 +1,72 @@
+package os
+
+import (
+	"io"
+	sys "os"
+	"path/filepath"
+	"strings"
+)
+
+/**
+ * Replacement implementation for os.Rename that copies the file if os.Rename
+ * fails with an error.
+ *
+ * Use os.Rename to 'move' a file between filesystems on different filesystems fails
+ * with an 'invalid cross-device link' error. The replacement implementation first
+ * attempts a 'rename' and if that fails, creates a temporary file adjacent to the
+ * destination file, copies the source to the temporary file and then does an
+ * os.Rename(...) on the temporary file to overwrite the destination file.
+ *
+ * The temporary file and original file are deleted.
+ *
+ * NB: this is very much an application specific implementation - errors deleting the
+ *     temporary or original file are discarded on the basis that this operation is
+ *     typically used to copy create an updated 'working file' from a temporary file
+ *     and the application can and will use the updated working file even if the original
+ *     file is not deleted.
+ *
+ * Ref. https://github.com/uhppoted/uhppoted-httpd/issues/20
+ *
+ * (interim implementation pending resolution of https://github.com/golang/go/issues/41487)
+ */
+func Rename(oldpath, newpath string) error {
+	if err := sys.Rename(oldpath, newpath); err == nil {
+		return nil
+	}
+
+	dir := filepath.Dir(newpath)
+	ext := filepath.Ext(newpath)
+	base := filepath.Base(newpath)
+	tmpfile := strings.TrimSuffix(base, ext)
+
+	tmp, err := sys.CreateTemp(dir, tmpfile+"*")
+	if err != nil {
+		return err
+	} else {
+		defer sys.Remove(tmp.Name())
+	}
+
+	src, err := sys.Open(oldpath)
+	if err != nil {
+		return err
+	} else {
+		defer src.Close()
+	}
+
+	if _, err := io.Copy(tmp, src); err != nil {
+		return err
+	} else if err := tmp.Sync(); err != nil {
+		return err
+	}
+
+	src.Close()
+	tmp.Close()
+
+	if err := sys.Rename(tmp.Name(), newpath); err != nil {
+		return err
+	}
+
+	sys.Remove(oldpath)
+
+	return nil
+}