extractZst: allow overwriting image files

There is an issue with the new pull image functionality, where
ovewriting an existing file does not work and no errors are generated.

$ rm -rf _data/images/kind_rhel8.qcow2{,.zst}
$ echo "PIZZA" >  _data/images/kind_rhel8.qcow2
$ ~/src/little-vm-helper/lvh images pull  --cache quay.io/lvh-images/kind-ci:rhel8-20240201.122048
$ cat  _data/images/kind_rhel8.qcow2
PIZZA
$ ls _data/images/kind_rhel8.qcow2{,.zst}
_data/images/kind_rhel8.qcow2  _data/images/kind_rhel8.qcow2.zst

There is a check at extractZst for the existance of the destination
file. If the file exists, we erturn os.ErrExist. This error is
propagated up to ExtractImage but is turned to a non-error by its
caller.

This patch adds support for overwriting images. We create a tempfile for
the uncompression and if everything goes well, we rename it to the
intended destination.

Signed-off-by: Kornilios Kourtis <[email protected]>

pkg/images/pull.go

@@ -165,11 +165,20 @@ func handleTarObject(ctx context.Context, tr *tar.Reader, hdr *tar.Header, conf
 }
 
 func extractZst(ctx context.Context, reader io.Reader, dstPath string) error {
-	if _, err := os.Stat(dstPath); err == nil {
-		return os.ErrExist
+	dstDir, dstName := filepath.Split(dstPath)
+
+	var tmpPath string
+	if tmpf, err := os.CreateTemp(dstDir, fmt.Sprintf("%s-*", dstName)); err != nil {
+		return err
+	} else {
+		tmpPath = tmpf.Name()
+		tmpf.Close()
+		// NB: remove temp file in case something goes wrong
+		defer os.Remove(tmpPath)
 	}
 
-	cmd := exec.CommandContext(ctx, "zstd", "-d", "-", "-o", dstPath)
+	// NB: we need to force with -f, because the destination file exists (we created it)
+	cmd := exec.CommandContext(ctx, "zstd", "-d", "-", "-o", tmpPath, "-f")
 	cmd.Stdin = reader
 
 	if _, err := cmd.Output(); err != nil {
@@ -180,5 +189,5 @@ func extractZst(ctx context.Context, reader io.Reader, dstPath string) error {
 		return fmt.Errorf("failed during zst decompression to %s: %w", dstPath, err)
 	}
 
-	return nil
+	return os.Rename(tmpPath, dstPath)
 }