fix scratch tests

Signed-off-by: Adrian Cole <[email protected]>

Author: Adrian Cole

internal/syscallfs/syscallfs.go

@@ -143,7 +143,6 @@ type readFile interface {
 type file interface {
 	readFile
 	io.Writer
-	io.WriterAt // for pwrite
 	syncer
 }
 

internal/syscallfs/syscallfs_bench_test.go

@@ -9,7 +9,7 @@ import (
 
 func BenchmarkReaderAtOffset(b *testing.B) {
 	dirFS := os.DirFS("testdata")
-	embedFS, err := fs.Sub(embedFS, "testdata")
+	embedFS, err := fs.Sub(readerAtFS, "testdata")
 	if err != nil {
 		b.Fatal(err)
 	}

internal/syscallfs/syscallfs_test.go

@@ -261,14 +261,30 @@ func requireErrno(t *testing.T, expected syscall.Errno, actual error) {
 	require.True(t, errors.Is(actual, expected), "expected %v, but was %v", expected, actual)
 }
 
-//go:embed testdata/wazero.txt
-var embedFS embed.FS
+var (
+	//go:embed testdata/wazero.txt
+	readerAtFS   embed.FS
+	readerAtFile = "wazero.txt"
+)
 
 func TestReaderAtOffset(t *testing.T) {
-	dirFS := os.DirFS("testdata")
-	embedFS, err := fs.Sub(embedFS, "testdata")
+	embedFS, err := fs.Sub(readerAtFS, "testdata")
+	require.NoError(t, err)
+
+	d, err := embedFS.Open(readerAtFile)
 	require.NoError(t, err)
-	mapFS := fstest.MapFS{"wazero.txt": &fstest.MapFile{Data: []byte("wazero")}}
+	defer d.Close()
+
+	bytes, err := io.ReadAll(d)
+	require.NoError(t, err)
+
+	mapFS := fstest.MapFS{readerAtFile: &fstest.MapFile{Data: bytes}}
+
+	// Write a file as can't open "testdata" in scratch tests because they
+	// can't read the original filesystem.
+	tmpDir := t.TempDir()
+	require.NoError(t, os.WriteFile(path.Join(tmpDir, readerAtFile), bytes, 0o0600))
+	dirFS := os.DirFS(tmpDir)
 
 	tests := []struct {
 		name string