Fix up tests.

common/fs/fs.go

@@ -2,10 +2,51 @@ package fs
 
 import (
 	"io"
+	"io/ioutil"
 	"os"
+	"syscall"
 )
 
-// Open is a mockable version of os.Open
-var Open = func(path string) (io.ReadWriteCloser, error) {
+// T is the filesystem interface type.
+type T interface {
+	ReadDir(string) ([]os.FileInfo, error)
+	ReadFile(string) ([]byte, error)
+	Lstat(string, *syscall.Stat_t) error
+	Stat(string, *syscall.Stat_t) error
+	Open(string) (io.ReadWriteCloser, error)
+}
+
+type realFS struct{}
+
+// FS is the way you should access the filesystem.
+var FS T = realFS{}
+
+// Mock is used to switch out the filesystem for a mock.
+func Mock(fs T) {
+	FS = fs
+}
+
+// Restore puts back the real filesystem.
+func Restore() {
+	FS = realFS{}
+}
+
+func (realFS) ReadDir(path string) ([]os.FileInfo, error) {
+	return ioutil.ReadDir(path)
+}
+
+func (realFS) ReadFile(path string) ([]byte, error) {
+	return ioutil.ReadFile(path)
+}
+
+func (realFS) Lstat(path string, stat *syscall.Stat_t) error {
+	return syscall.Lstat(path, stat)
+}
+
+func (realFS) Stat(path string, stat *syscall.Stat_t) error {
+	return syscall.Stat(path, stat)
+}
+
+func (realFS) Open(path string) (io.ReadWriteCloser, error) {
 	return os.Open(path)
 }