fs: Add interface and FS implementations

This adds two implementations of the new `FS` interface: One for the local
file system (`Local`) and one for a single file read from an
`io.Reader` (`Reader`).

internal/fs/fs_track.go

@@ -0,0 +1,54 @@
+package fs
+
+import (
+	"fmt"
+	"os"
+	"runtime"
+	"runtime/debug"
+)
+
+// Track is a wrapper around another file system which installs finalizers
+// for open files which call panic() when they are not closed when the garbage
+// collector releases them. This can be used to find resource leaks via open
+// files.
+type Track struct {
+	FS
+}
+
+// Open wraps the Open method of the underlying file system.
+func (fs Track) Open(name string) (File, error) {
+	f, err := fs.FS.Open(fixpath(name))
+	if err != nil {
+		return nil, err
+	}
+
+	return newTrackFile(debug.Stack(), name, f), nil
+}
+
+// OpenFile wraps the OpenFile method of the underlying file system.
+func (fs Track) OpenFile(name string, flag int, perm os.FileMode) (File, error) {
+	f, err := fs.FS.OpenFile(fixpath(name), flag, perm)
+	if err != nil {
+		return nil, err
+	}
+
+	return newTrackFile(debug.Stack(), name, f), nil
+}
+
+type trackFile struct {
+	File
+}
+
+func newTrackFile(stack []byte, filename string, file File) *trackFile {
+	f := &trackFile{file}
+	runtime.SetFinalizer(f, func(f *trackFile) {
+		fmt.Fprintf(os.Stderr, "file %s not closed\n\nStacktrack:\n%s\n", filename, stack)
+		panic("file " + filename + " not closed")
+	})
+	return f
+}
+
+func (f *trackFile) Close() error {
+	runtime.SetFinalizer(f, nil)
+	return f.File.Close()
+}