ociarchive: Add new ImageStructureNotFound

This is for containers/skopeo#2114
which is in turn a dependency of coreos/rpm-ostree#4598

Basically I want to map ENOENT to a clear error, because the build
tooling wants to treat "target image is not present" differently
from "DNS lookup failed" or "we got EPERM".

There's a bit of code motion here because we need to move
the `os.Open()` call before creating a temporary directory.

Signed-off-by: Colin Walters <[email protected]>

oci/archive/oci_src.go

@@ -28,6 +28,18 @@ func (e ImageNotFoundError) Error() string {
 	return fmt.Sprintf("no descriptor found for reference %q", e.ref.image)
 }
 
+// ImageStructureNotFoundError occurs when the archive file does not exist.
+type ImageStructureNotFoundError struct {
+	// ref is the image reference
+	ref ociArchiveReference
+	// path is the file path that was not present
+	path string
+}
+
+func (e ImageStructureNotFoundError) Error() string {
+	return fmt.Sprintf("no archive file present for reference %q", e.ref.image)
+}
+
 type ociArchiveImageSource struct {
 	impl.Compat
 

oci/archive/oci_transport.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io/fs"
 	"os"
 	"strings"
 
@@ -171,18 +172,24 @@ func createOCIRef(sys *types.SystemContext, image string) (tempDirOCIRef, error)
 
 // creates the temporary directory and copies the tarred content to it
 func createUntarTempDir(sys *types.SystemContext, ref ociArchiveReference) (tempDirOCIRef, error) {
+	src := ref.resolvedFile
+	arch, err := os.Open(src)
+	if err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			return tempDirOCIRef{}, ImageStructureNotFoundError{ref: ref, path: src}
+		} else {
+			return tempDirOCIRef{}, err
+		}
+	}
+	defer arch.Close()
+
 	tempDirRef, err := createOCIRef(sys, ref.image)
 	if err != nil {
 		return tempDirOCIRef{}, fmt.Errorf("creating oci reference: %w", err)
 	}
-	src := ref.resolvedFile
 	dst := tempDirRef.tempDirectory
+
 	// TODO: This can take quite some time, and should ideally be cancellable using a context.Context.
-	arch, err := os.Open(src)
-	if err != nil {
-		return tempDirOCIRef{}, err
-	}
-	defer arch.Close()
 	if err := archive.NewDefaultArchiver().Untar(arch, dst, &archive.TarOptions{NoLchown: true}); err != nil {
 		if err := tempDirRef.deleteTempDir(); err != nil {
 			return tempDirOCIRef{}, fmt.Errorf("deleting temp directory %q: %w", tempDirRef.tempDirectory, err)