os: don't let File.Readdir return an empty slice and nil error

In the case of a file being deleted while Readdir was running, it was
possible for File.Readdir to return an empty slice and a nil error,
counter to its documentation.

Fixes #16919

Change-Id: If0e42882eea52fbf5530317a1895f3829ea8e67b
Reviewed-on: https://go-review.googlesource.com/28056Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/dir_unix.go

@@ -34,6 +34,11 @@ func (f *File) readdir(n int) (fi []FileInfo, err error) {
 		}
 		fi = append(fi, fip)
 	}
+	if len(fi) == 0 && err == nil && n > 0 {
+		// Per File.Readir, the slice must be non-empty or err
+		// must be non-nil if n > 0.
+		err = io.EOF
+	}
 	return fi, err
 }
 

src/os/os_unix_test.go

@@ -7,8 +7,12 @@
 package os_test
 
 import (
+	"io"
+	"io/ioutil"
 	. "os"
+	"path/filepath"
 	"runtime"
+	"strings"
 	"syscall"
 	"testing"
 )
@@ -178,3 +182,38 @@ func TestLchown(t *testing.T) {
 		checkUidGid(t, f.Name(), int(sys.Uid), int(sys.Gid))
 	}
 }
+
+// Issue 16919: Readdir must return a non-empty slice or an error.
+func TestReaddirRemoveRace(t *testing.T) {
+	oldStat := *LstatP
+	defer func() { *LstatP = oldStat }()
+	*LstatP = func(name string) (FileInfo, error) {
+		if strings.HasSuffix(name, "some-file") {
+			// Act like it's been deleted.
+			return nil, ErrNotExist
+		}
+		return oldStat(name)
+	}
+	dir := newDir("TestReaddirRemoveRace", t)
+	defer RemoveAll(dir)
+	if err := ioutil.WriteFile(filepath.Join(dir, "some-file"), []byte("hello"), 0644); err != nil {
+		t.Fatal(err)
+	}
+	d, err := Open(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer d.Close()
+	fis, err := d.Readdir(2) // notably, greater than zero
+	if len(fis) == 0 && err == nil {
+		// This is what used to happen (Issue 16919)
+		t.Fatal("Readdir = empty slice & err == nil")
+	}
+	if len(fis) != 0 || err != io.EOF {
+		t.Errorf("Readdir = %d entries: %v; want 0, io.EOF", len(fis), err)
+		for i, fi := range fis {
+			t.Errorf("  entry[%d]: %q, %v", i, fi.Name(), fi.Mode())
+		}
+		t.FailNow()
+	}
+}