os: improve Readdir test coverage, fix Readdir(0) on EOF

Adds tests for Readdir and Readdirnames with different n
values.  No good way to inject faults during full reads,
though.

Also fixes bug report from fshahriar:
Readdir(0) wasn't behaving like Readdir(-1).

R=rsc, fshahriar
CC=golang-dev
https://golang.org/cl/4529092

src/pkg/os/dir_unix.go

@@ -32,7 +32,7 @@ func (f *File) Readdirnames(n int) (names []string, err Error) {
 		f.dirinfo.buf = make([]byte, blockSize)
 	}
 	d := f.dirinfo
-	wantAll := n < 0
+	wantAll := n <= 0
 
 	size := n
 	if size < 0 {

src/pkg/os/os_test.go

@@ -286,6 +286,71 @@ func TestReaddirnamesOneAtATime(t *testing.T) {
 	}
 }
 
+func TestReaddirNValues(t *testing.T) {
+	if testing.Short() {
+		t.Logf("test.short; skipping")
+		return
+	}
+	dir, err := ioutil.TempDir("", "")
+	if err != nil {
+		t.Fatalf("TempDir: %v", err)
+	}
+	defer RemoveAll(dir)
+	for i := 1; i <= 20; i++ {
+		f, err := Create(filepath.Join(dir, fmt.Sprintf("%d", i)))
+		if err != nil {
+			t.Fatalf("Create: %v", err)
+		}
+		f.Write([]byte(strings.Repeat("X", i)))
+		f.Close()
+	}
+
+	var d *File
+	openDir := func() {
+		var err Error
+		d, err = Open(dir)
+		if err != nil {
+			t.Fatalf("Open directory: %v", err)
+		}
+	}
+
+	readDirExpect := func(n, want int, wantErr Error) {
+		fi, err := d.Readdir(n)
+		if err != wantErr {
+			t.Fatalf("Readdir of %d got error %v, want %v", n, err, wantErr)
+		}
+		if g, e := len(fi), want; g != e {
+			t.Errorf("Readdir of %d got %d files, want %d", n, g, e)
+		}
+	}
+
+	readDirNamesExpect := func(n, want int, wantErr Error) {
+		fi, err := d.Readdirnames(n)
+		if err != wantErr {
+			t.Fatalf("Readdirnames of %d got error %v, want %v", n, err, wantErr)
+		}
+		if g, e := len(fi), want; g != e {
+			t.Errorf("Readdirnames of %d got %d files, want %d", n, g, e)
+		}
+	}
+
+	for _, fn := range []func(int, int, Error){readDirExpect, readDirNamesExpect} {
+		// Test the -1 case
+		openDir()
+		fn(-1, 20, nil)
+		fn(-2, 0, nil)
+		fn(0, 0, nil)
+		d.Close()
+
+		// Test the bounded case
+		openDir()
+		fn(19, 19, nil)
+		fn(18, 1, nil)
+		fn(17, 0, EOF)
+		d.Close()
+	}
+}
+
 func TestHardLink(t *testing.T) {
 	// Hardlinks are not supported under windows.
 	if syscall.OS == "windows" {