add: remotefs/download support file download

remotefs/download.go

@@ -0,0 +1,53 @@
+package remotefs
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"io"
+	"os"
+)
+
+// ErrChecksumMismatch is returned when the checksum of the uploaded file does not match the local checksum.
+//var ErrChecksumMismatch = errors.New("checksum mismatch")
+
+// Download a file from the remote host.
+func Download(fs FS, src, dst string) error {
+	remote, err := fs.Open(src)
+	if err != nil {
+		return fmt.Errorf("open remote file for download: %w", err)
+	}
+	defer remote.Close()
+
+	remoteStat, err := remote.Stat()
+	if err != nil {
+		return fmt.Errorf("stat remote file for download: %w", err)
+	}
+
+	remoteSum := sha256.New()
+	localSum := sha256.New()
+
+	local, err := os.OpenFile(dst, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, remoteStat.Mode())
+	if err != nil {
+		return fmt.Errorf("open local file for download: %w", err)
+	}
+	defer local.Close()
+
+	remoteReader := io.TeeReader(remote, remoteSum)
+	if _, err := io.Copy(io.MultiWriter(local, localSum), remoteReader); err != nil {
+		_ = local.Close()
+		return fmt.Errorf("copy file from remote host: %w", err)
+	}
+	if err := local.Close(); err != nil {
+		return fmt.Errorf("close local file after download: %w", err)
+	}
+
+	ls := hex.EncodeToString(localSum.Sum(nil))
+	rs := hex.EncodeToString(remoteSum.Sum(nil))
+	fmt.Printf("local %s remote %s", ls, rs)
+	if ls != rs {
+		return ErrChecksumMismatch
+	}
+
+	return nil
+}