1 files changed, 90 insertions, 0 deletions
diff --git a/client/file_store.go b/client/file_store.go
new file mode 100644
index 0000000..520bbe7
--- /dev/null
+++ b/client/file_store.go
@@ -0,0 +1,90 @@
+package client
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+)
+
+// FileRemoteStore provides a RemoteStore interface compatible
+// implementation that can be used where the RemoteStore is backed by a
+// fs.FS. This is useful for example in air-gapped environments where there's no
+// possibility to make outbound network connections.
+// By having this be a fs.FS instead of directories allows the repository to
+// be backed by something that's not persisted to disk.
+func NewFileRemoteStore(fsys fs.FS, targetDir string) (*FileRemoteStore, error) {
+	if fsys == nil {
+		return nil, errors.New("nil fs.FS")
+	}
+	t := targetDir
+	if t == "" {
+		t = "targets"
+	}
+	// Make sure directory exists
+	d, err := fsys.Open(t)
+	if err != nil {
+		return nil, fmt.Errorf("failed to open targets directory %s: %w", t, err)
+	}
+	fi, err := d.Stat()
+	if err != nil {
+		return nil, fmt.Errorf("failed to stat targets directory %s: %w", t, err)
+	}
+	if !fi.IsDir() {
+		return nil, fmt.Errorf("targets directory not a directory %s", t)
+	}
+
+	fsysT, err := fs.Sub(fsys, t)
+	if err != nil {
+		return nil, fmt.Errorf("failed to open targets directory %s: %w", t, err)
+	}
+	return &FileRemoteStore{fsys: fsys, targetDir: fsysT}, nil
+}
+
+type FileRemoteStore struct {
+	// Meta directory fs
+	fsys fs.FS
+	// Target directory fs.
+	targetDir fs.FS
+	// In order to be able to make write operations (create, delete) we can't
+	// use fs.FS for it (it's read only), so we have to know the underlying
+	// directory that add/delete test methods can use. This is only necessary
+	// for testing purposes.
+	testDir string
+}
+
+func (f *FileRemoteStore) GetMeta(name string) (io.ReadCloser, int64, error) {
+	rc, b, err := f.get(f.fsys, name)
+	return handleErrors(name, rc, b, err)
+}
+
+func (f *FileRemoteStore) GetTarget(name string) (io.ReadCloser, int64, error) {
+	rc, b, err := f.get(f.targetDir, name)
+	return handleErrors(name, rc, b, err)
+}
+
+func (f *FileRemoteStore) get(fsys fs.FS, s string) (io.ReadCloser, int64, error) {
+	if !fs.ValidPath(s) {
+		return nil, 0, fmt.Errorf("invalid path %s", s)
+	}
+
+	b, err := fs.ReadFile(fsys, s)
+	if err != nil {
+		return nil, -1, err
+	}
+	return io.NopCloser(bytes.NewReader(b)), int64(len(b)), nil
+}
+
+// handleErrors converts NotFound errors to something that TUF knows how to
+// handle properly. For example, when looking for n+1 root files, this is a
+// signal that it will stop looking.
+func handleErrors(name string, rc io.ReadCloser, b int64, err error) (io.ReadCloser, int64, error) {
+	if err == nil {
+		return rc, b, err
+	}
+	if errors.Is(err, fs.ErrNotExist) {
+		return rc, b, ErrNotFound{name}
+	}
+	return rc, b, err
+}