libimage: load: improve error messages

When loading a path we have to "guess" the underlying format and hence
attempt loading supported formats in a specific order.  When all
attempts have failed make sure that all loading errors are reported up,
in addition to debug logs, such that users can parse them for useful
information.

Fixes: github.com/containers/podman/issues/11730
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

libimage/load.go

@@ -2,7 +2,7 @@ package libimage
 
 import (
 	"context"
-	"errors"
+	"fmt"
 	"os"
 	"time"
 
@@ -28,66 +28,69 @@ func (r *Runtime) Load(ctx context.Context, path string, options *LoadOptions) (
 		defer r.writeEvent(&Event{ID: "", Name: path, Time: time.Now(), Type: EventTypeImageLoad})
 	}
 
-	var (
-		loadedImages []string
-		loadError    error
-	)
-
 	if options == nil {
 		options = &LoadOptions{}
 	}
 
-	for _, f := range []func() ([]string, error){
+	var loadErrors []error
+
+	for _, f := range []func() ([]string, string, error){
 		// OCI
-		func() ([]string, error) {
+		func() ([]string, string, error) {
 			logrus.Debugf("-> Attempting to load %q as an OCI directory", path)
 			ref, err := ociTransport.NewReference(path, "")
 			if err != nil {
-				return nil, err
+				return nil, ociTransport.Transport.Name(), err
 			}
-			return r.copyFromDefault(ctx, ref, &options.CopyOptions)
+			images, err := r.copyFromDefault(ctx, ref, &options.CopyOptions)
+			return images, ociTransport.Transport.Name(), err
 		},
 
 		// OCI-ARCHIVE
-		func() ([]string, error) {
+		func() ([]string, string, error) {
 			logrus.Debugf("-> Attempting to load %q as an OCI archive", path)
 			ref, err := ociArchiveTransport.NewReference(path, "")
 			if err != nil {
-				return nil, err
+				return nil, ociArchiveTransport.Transport.Name(), err
 			}
-			return r.copyFromDefault(ctx, ref, &options.CopyOptions)
+			images, err := r.copyFromDefault(ctx, ref, &options.CopyOptions)
+			return images, ociArchiveTransport.Transport.Name(), err
 		},
 
 		// DIR
-		func() ([]string, error) {
+		func() ([]string, string, error) {
 			logrus.Debugf("-> Attempting to load %q as a Docker dir", path)
 			ref, err := dirTransport.NewReference(path)
 			if err != nil {
-				return nil, err
+				return nil, dirTransport.Transport.Name(), err
 			}
-			return r.copyFromDefault(ctx, ref, &options.CopyOptions)
+			images, err := r.copyFromDefault(ctx, ref, &options.CopyOptions)
+			return images, dirTransport.Transport.Name(), err
 		},
 
 		// DOCKER-ARCHIVE
-		func() ([]string, error) {
+		func() ([]string, string, error) {
 			logrus.Debugf("-> Attempting to load %q as a Docker archive", path)
 			ref, err := dockerArchiveTransport.ParseReference(path)
 			if err != nil {
-				return nil, err
+				return nil, dockerArchiveTransport.Transport.Name(), err
 			}
-			return r.loadMultiImageDockerArchive(ctx, ref, &options.CopyOptions)
-		},
-
-		// Give a decent error message if nothing above worked.
-		func() ([]string, error) {
-			return nil, errors.New("payload does not match any of the supported image formats (oci, oci-archive, dir, docker-archive)")
+			images, err := r.loadMultiImageDockerArchive(ctx, ref, &options.CopyOptions)
+			return images, dockerArchiveTransport.Transport.Name(), err
 		},
 	} {
-		loadedImages, loadError = f()
-		if loadError == nil {
-			return loadedImages, loadError
+		loadedImages, transportName, err := f()
+		if err == nil {
+			return loadedImages, nil
 		}
-		logrus.Debugf("Error loading %s: %v", path, loadError)
+		logrus.Debugf("Error loading %s (%s): %v", path, transportName, err)
+		loadErrors = append(loadErrors, fmt.Errorf("%s: %v", transportName, err))
+	}
+
+	// Give a decent error message if nothing above worked.
+	loadError := fmt.Errorf("payload does not match any of the supported image formats:")
+	for _, err := range loadErrors {
+		loadError = fmt.Errorf("%w\n * %v", loadError, err)
 	}
 
 	return nil, loadError