at-container-registry/pkg/appview/overlay.go

package appview

import (
	"io/fs"
	"sort"
)

// overlayFS layers two fs.FS instances: upper wins, lower is fallback.
// ReadDir merges entries from both layers (upper entries replace lower).
type overlayFS struct {
	upper fs.FS
	lower fs.FS
}

// newOverlayFS creates a layered filesystem. Files in upper take priority over lower.
func newOverlayFS(upper, lower fs.FS) fs.FS {
	return &overlayFS{upper: upper, lower: lower}
}

func (o *overlayFS) Open(name string) (fs.File, error) {
	f, err := o.upper.Open(name)
	if err == nil {
		return f, nil
	}
	return o.lower.Open(name)
}

// ReadFile implements fs.ReadFileFS for efficient file reads.
func (o *overlayFS) ReadFile(name string) ([]byte, error) {
	if rfs, ok := o.upper.(fs.ReadFileFS); ok {
		data, err := rfs.ReadFile(name)
		if err == nil {
			return data, nil
		}
	}
	if rfs, ok := o.lower.(fs.ReadFileFS); ok {
		return rfs.ReadFile(name)
	}
	// Fallback: open and read
	f, err := o.Open(name)
	if err != nil {
		return nil, err
	}
	defer f.Close()
	stat, err := f.Stat()
	if err != nil {
		return nil, err
	}
	buf := make([]byte, stat.Size())
	_, err = f.(interface{ Read([]byte) (int, error) }).Read(buf)
	return buf, err
}

// ReadDir implements fs.ReadDirFS with merge semantics: entries from upper
// replace entries from lower with the same name.
func (o *overlayFS) ReadDir(name string) ([]fs.DirEntry, error) {
	merged := make(map[string]fs.DirEntry)

	// Read lower first
	if rdfs, ok := o.lower.(fs.ReadDirFS); ok {
		entries, err := rdfs.ReadDir(name)
		if err == nil {
			for _, e := range entries {
				merged[e.Name()] = e
			}
		}
	}

	// Upper entries override lower
	if rdfs, ok := o.upper.(fs.ReadDirFS); ok {
		entries, err := rdfs.ReadDir(name)
		if err == nil {
			for _, e := range entries {
				merged[e.Name()] = e
			}
		}
	}

	if len(merged) == 0 {
		return nil, &fs.PathError{Op: "readdir", Path: name, Err: fs.ErrNotExist}
	}

	result := make([]fs.DirEntry, 0, len(merged))
	for _, e := range merged {
		result = append(result, e)
	}
	sort.Slice(result, func(i, j int) bool {
		return result[i].Name() < result[j].Name()
	})
	return result, nil
}