ociarchive: Return ImageNotFound if the file isn't present

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

Basically I want to map ENOENT to `ImageNotFound`, 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.

Author: cgwalters

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) {
-	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
+		if errors.Is(err, fs.ErrNotExist) {
+			return tempDirOCIRef{}, ImageNotFoundError{ref: ref}
+		} 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)
+	}
+	dst := tempDirRef.tempDirectory
+
 	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)