base/file: move loopbackfs to github.com repo (grailbio/grail!14197)

Approved-by: Jaran Charumilind <jcharumilind@grailbio.com>

GitLab URL: https://gitlab.com/grailbio/grail/-/merge_requests/14197

fbshipit-source-id: 447558d

file/loopbackfs/loopbackfs.go

@@ -0,0 +1,122 @@
+package loopbackfs
+
+import (
+	"context"
+	"io/ioutil"
+	"os"
+	"path"
+	"time"
+
+	"github.com/grailbio/base/file/fsnode"
+	"github.com/grailbio/base/ioctx/fsctx"
+	"github.com/grailbio/base/ioctx/spliceio"
+)
+
+// New returns an fsnode.T representing a path on the local filesystem.
+// TODO: Replace this with a generic io/fs.FS wrapper around os.DirFS, after upgrading Go.
+func New(name string, path string) (fsnode.T, error) {
+	info, err := os.Stat(path)
+	if err != nil {
+		return nil, err
+	}
+	node := newT(name, info, path)
+	if node == nil {
+		return nil, os.ErrInvalid
+	}
+	return node, nil
+}
+
+func newT(name string, info os.FileInfo, path string) fsnode.T {
+	switch info.Mode() & os.ModeType {
+	case os.ModeDir:
+		// Temporary hack: record the original path so we can peek at it.
+		// TODO: Eventually, adapt our libraries to operate on FS's directly.
+		// TODO: Consider preserving executable bits. But, copying permissions without also checking
+		// owner UID/GID may not make sense.
+		info := fsnode.NewDirInfo(name).WithModTime(info.ModTime()).WithSys(path).
+			WithCacheableFor(time.Hour)
+		return parent{dir: path, FileInfo: info}
+	case 0:
+		info := fsnode.NewRegInfo(name).WithModTime(info.ModTime()).WithSys(path).
+			WithCacheableFor(time.Hour)
+		return leaf{path, info}
+	}
+	return nil
+}
+
+type parent struct {
+	fsnode.ParentReadOnly
+	dir string
+	fsnode.FileInfo
+}
+
+var _ fsnode.Parent = parent{}
+
+func (p parent) FSNodeT() {}
+
+func (p parent) Child(_ context.Context, name string) (fsnode.T, error) {
+	return New(name, path.Join(p.dir, name))
+}
+
+type iterator struct {
+	dir      string
+	fetched  bool
+	delegate fsnode.Iterator
+}
+
+func (p parent) Children() fsnode.Iterator { return &iterator{dir: p.dir} }
+
+func (it *iterator) ensureFetched() error {
+	if it.fetched {
+		if it.delegate == nil {
+			return os.ErrClosed
+		}
+		return nil
+	}
+	entries, err := ioutil.ReadDir(it.dir)
+	if err != nil {
+		return err
+	}
+	nodes := make([]fsnode.T, 0, len(entries))
+	for _, info := range entries {
+		fullPath := path.Join(it.dir, info.Name())
+		node := newT(info.Name(), info, fullPath)
+		if node == nil {
+			continue
+		}
+		nodes = append(nodes, node)
+	}
+	it.fetched = true
+	it.delegate = fsnode.NewIterator(nodes...)
+	return nil
+}
+
+func (it *iterator) Next(ctx context.Context) (fsnode.T, error) {
+	if err := it.ensureFetched(); err != nil {
+		return nil, err
+	}
+	return it.delegate.Next(ctx)
+}
+
+func (it *iterator) Close(context.Context) error {
+	it.fetched = true
+	it.delegate = nil
+	return nil
+}
+
+type leaf struct {
+	path string
+	fsnode.FileInfo
+}
+
+var _ fsnode.Leaf = leaf{}
+
+func (l leaf) FSNodeT() {}
+
+func (l leaf) OpenFile(context.Context, int) (fsctx.File, error) {
+	file, err := os.Open(l.path)
+	if err != nil {
+		return nil, err
+	}
+	return (*spliceio.OSFile)(file), nil
+}