feat: add tuf resolver test

pkg/tuf/mock.go

@@ -1,5 +1,67 @@
 package tuf
 
+import (
+	"io"
+	"os"
+	"path/filepath"
+
+	"github.com/docker/attest/internal/util"
+)
+
+type MockTufClient struct {
+	srcPath string
+	dstPath string
+}
+
+func NewMockTufClient(srcPath string, dstPath string) *MockTufClient {
+	if srcPath == "" {
+		panic("srcPath must be set")
+	}
+	if dstPath == "" {
+		panic("dstPath must be set")
+	}
+	return &MockTufClient{
+		srcPath: srcPath,
+		dstPath: dstPath,
+	}
+}
+
+func (dc *MockTufClient) DownloadTarget(target string, filePath string) (file *TargetFile, err error) {
+	targetPath := filepath.Join(dc.srcPath, target)
+	src, err := os.Open(targetPath)
+	if err != nil {
+		return nil, err
+	}
+	defer src.Close()
+
+	var dstFilePath string
+	if filePath == "" {
+		dstFilePath = filepath.Join(dc.dstPath, filepath.FromSlash(target))
+	} else {
+		dstFilePath = filePath
+	}
+
+	err = os.MkdirAll(filepath.Dir(dstFilePath), os.ModePerm)
+	if err != nil {
+		return nil, err
+	}
+	dst, err := os.Create(dstFilePath)
+	if err != nil {
+		return nil, err
+	}
+	defer dst.Close()
+
+	// reading from tee will read from src and write to dst at the same time
+	tee := io.TeeReader(src, dst)
+
+	b, err := io.ReadAll(tee)
+	if err != nil {
+		return nil, err
+	}
+
+	return &TargetFile{ActualFilePath: dstFilePath, TargetURI: targetPath, Data: b, Digest: util.SHA256Hex(b)}, nil
+}
+
 type MockVersionChecker struct {
 	err error
 }