feat: add billfs billy to io/fs adaptor

pkg/billyfs/billy.go

@@ -0,0 +1,89 @@
+package billyfs
+
+import (
+	"io/fs"
+
+	"github.com/go-git/go-billy/v5"
+)
+
+// BillyFS is the minimum interface from the billy package required
+// to implement an fs.FS implementation.
+type BillyFS interface {
+	billy.Basic
+	billy.Dir
+}
+
+// FS is an implementation of fs.FS which wraps a billy filesystem abtraction.
+type FS struct {
+	fs BillyFS
+}
+
+func New(fs BillyFS) *FS {
+	return &FS{fs}
+}
+
+// ReadDir reads the named directory
+// and returns a list of directory entries sorted by filename.
+func (f *FS) ReadDir(name string) ([]fs.DirEntry, error) {
+	infos, err := f.fs.ReadDir(name)
+	if err != nil {
+		return nil, err
+	}
+
+	entries := make([]fs.DirEntry, 0, len(infos))
+	for _, info := range infos {
+		entries = append(entries, fs.FileInfoToDirEntry(info))
+	}
+
+	return entries, nil
+}
+
+// Open opens the named file.
+// When Open returns an error, it should be of type *PathError
+// with the Op field set to "open", the Path field set to name,
+// and the Err field describing the problem.
+//
+// Open should reject attempts to open names that do not satisfy
+// ValidPath(name), returning a *PathError with Err set to
+// ErrInvalid or ErrNotExist.
+func (f *FS) Open(name string) (fs.File, error) {
+	fi, err := f.fs.Open(name)
+	if err != nil {
+		return nil, err
+	}
+
+	return &File{fi, name, f}, nil
+}
+
+// File is an implementation of fs.File which wraps a billy.File instance.
+type File struct {
+	billy.File
+
+	name string
+	fs   *FS
+}
+
+// ReadDir reads the contents of the directory and returns
+// a slice of up to n DirEntry values in directory order.
+// Subsequent calls on the same file will yield further DirEntry values.
+//
+// If n > 0, ReadDir returns at most n DirEntry structures.
+// In this case, if ReadDir returns an empty slice, it will return
+// a non-nil error explaining why.
+// At the end of a directory, the error is io.EOF.
+// (ReadDir must return io.EOF itself, not an error wrapping io.EOF.)
+//
+// If n <= 0, ReadDir returns all the DirEntry values from the directory
+// in a single slice. In this case, if ReadDir succeeds (reads all the way
+// to the end of the directory), it returns the slice and a nil error.
+// If it encounters an error before the end of the directory,
+// ReadDir returns the DirEntry list read until that point and a non-nil error.
+func (f *File) ReadDir(n int) ([]fs.DirEntry, error) {
+	return f.fs.ReadDir(f.name)
+}
+
+// Stat returns a FileInfo describing the file.
+// If there is an error, it should be of type *PathError.
+func (f *File) Stat() (fs.FileInfo, error) {
+	return f.fs.fs.Stat(f.name)
+}