layer tree: be more resilient

Make operating on the layer tree more resilient towards parallel image
removals by checking for unknown-image errors.  This partially fixes
below linked BZ.

At the time of writing, I do not have a reliable easy (and short
running) reproducer.  Hence, I suggest to merge this change without
tests and rely on either Podman CI (which doesn't have a reliable test
yet either) and the QE team (will be informed).

BZ: bugzilla.redhat.com/show_bug.cgi?id=2216700
Signed-off-by: Valentin Rothberg <vrothberg@redhat.com>

common/libimage/layer_tree.go

@@ -2,6 +2,7 @@ package libimage
 
 import (
 	"context"
+	"errors"
 
 	"github.com/containers/storage"
 	ociv1 "github.com/opencontainers/image-spec/specs-go/v1"
@@ -31,6 +32,9 @@ func (t *layerTree) node(layerID string) *layerNode {
 }
 
 // toOCI returns an OCI image for the specified image.
+//
+// WARNING: callers are responsible for handling cases where the target image
+// has been removed and need to check for `storage.ErrImageUnknown`.
 func (t *layerTree) toOCI(ctx context.Context, i *Image) (*ociv1.Image, error) {
 	var err error
 	oci, exists := t.ociCache[i.ID()]
@@ -155,6 +159,9 @@ func (t *layerTree) children(ctx context.Context, parent *Image, all bool) ([]*I
 	parentID := parent.ID()
 	parentOCI, err := t.toOCI(ctx, parent)
 	if err != nil {
+		if errors.Is(err, storage.ErrImageUnknown) {
+			return nil, nil
+		}
 		return nil, err
 	}
 
@@ -165,6 +172,9 @@ func (t *layerTree) children(ctx context.Context, parent *Image, all bool) ([]*I
 		}
 		childOCI, err := t.toOCI(ctx, child)
 		if err != nil {
+			if errors.Is(err, storage.ErrImageUnknown) {
+				return false, nil
+			}
 			return false, err
 		}
 		// History check.
@@ -255,6 +265,9 @@ func (t *layerTree) parent(ctx context.Context, child *Image) (*Image, error) {
 	childID := child.ID()
 	childOCI, err := t.toOCI(ctx, child)
 	if err != nil {
+		if errors.Is(err, storage.ErrImageUnknown) {
+			return nil, nil
+		}
 		return nil, err
 	}
 
@@ -268,6 +281,9 @@ func (t *layerTree) parent(ctx context.Context, child *Image) (*Image, error) {
 			}
 			emptyOCI, err := t.toOCI(ctx, empty)
 			if err != nil {
+				if errors.Is(err, storage.ErrImageUnknown) {
+					return nil, nil
+				}
 				return nil, err
 			}
 			// History check.
@@ -300,6 +316,9 @@ func (t *layerTree) parent(ctx context.Context, child *Image) (*Image, error) {
 		}
 		parentOCI, err := t.toOCI(ctx, parent)
 		if err != nil {
+			if errors.Is(err, storage.ErrImageUnknown) {
+				return nil, nil
+			}
 			return nil, err
 		}
 		// History check.