Bugfix: avoid partial config read/writes.

The registry is written on AFTER the file. This means that failing to
write to the registry will cause the file to be updated but not the
registry. This is leaves the config in a bad state.

To solve it, we create a backup of the file, and restore it when the
registry write fails.

The same happens when reading.

windows-agent/internal/config/config_marshal.go

@@ -19,11 +19,18 @@ type marshalHelper struct {
 func (c *Config) load() (err error) {
 	defer decorate.OnError(&err, "could not load data for Config")
 
+	// Backup the config in case the registry fails.
+	// This avoids partial loads.
+	l := c.landscape
+	s := c.subscription
+
 	if err := c.loadFile(); err != nil {
 		return fmt.Errorf("could not load config from the chache file: %v", err)
 	}
 
 	if err := c.loadRegistry(); err != nil {
+		c.landscape = l
+		c.subscription = s
 		return fmt.Errorf("could not load config from the registry: %v", err)
 	}
 
@@ -97,16 +104,44 @@ func readFromRegistry(r Registry, key uintptr, field string) (string, error) {
 func (c *Config) dump() (err error) {
 	defer decorate.OnError(&err, "could not store Config data")
 
-	var errs error
-	if err := c.dumpRegistry(); err != nil {
-		errs = errors.Join(errs, err)
+	// Backup the file in case the registry write fails.
+	// This avoids partial writes
+	restore, err := makeBackup(c.cachePath)
+	if err != nil {
+		return err
 	}
 
 	if err := c.dumpFile(); err != nil {
-		errs = errors.Join(errs, err)
+		return err
+	}
+
+	if err := c.dumpRegistry(); err != nil {
+		return errors.Join(err, restore())
+	}
+
+	return nil
+}
+
+func makeBackup(originalPath string) (func() error, error) {
+	backupPath := originalPath + ".backup"
+
+	// filterFsError is a helper to avoid having to do the errors.Is dance every time.
+	filterFsError := func(err error) error {
+		if errors.Is(err, fs.ErrNotExist) {
+			return nil
+		}
+		return err
+	}
+
+	err := os.Rename(originalPath, backupPath)
+	if filterFsError(err) != nil {
+		return nil, fmt.Errorf("could not create backup: %v", err)
 	}
 
-	return errs
+	return func() error {
+		err := os.Rename(backupPath, originalPath)
+		return filterFsError(err)
+	}, nil
 }
 
 func (c *Config) dumpRegistry() error {