os: yet more Readdir tests and fix earlier regression

R=golang-dev, fshahriar
CC=golang-dev
https://golang.org/cl/4548068

src/pkg/os/dir_unix.go

@@ -32,11 +32,11 @@ func (f *File) Readdirnames(n int) (names []string, err Error) {
 		f.dirinfo.buf = make([]byte, blockSize)
 	}
 	d := f.dirinfo
-	wantAll := n <= 0
 
 	size := n
-	if size < 0 {
+	if size <= 0 {
 		size = 100
+		n = -1
 	}
 
 	names = make([]string, 0, size) // Empty with room to grow.
@@ -60,7 +60,7 @@ func (f *File) Readdirnames(n int) (names []string, err Error) {
 		d.bufp += nb
 		n -= nc
 	}
-	if !wantAll && len(names) == 0 {
+	if n >= 0 && len(names) == 0 {
 		return names, EOF
 	}
 	return names, nil

src/pkg/os/os_test.go

@@ -296,7 +296,7 @@ func TestReaddirNValues(t *testing.T) {
 		t.Fatalf("TempDir: %v", err)
 	}
 	defer RemoveAll(dir)
-	for i := 1; i <= 20; i++ {
+	for i := 1; i <= 105; i++ {
 		f, err := Create(filepath.Join(dir, fmt.Sprintf("%d", i)))
 		if err != nil {
 			t.Fatalf("Create: %v", err)
@@ -335,18 +335,25 @@ func TestReaddirNValues(t *testing.T) {
 	}
 
 	for _, fn := range []func(int, int, Error){readDirExpect, readDirNamesExpect} {
-		// Test the -1 case
+		// Test the slurp case
 		openDir()
-		fn(-1, 20, nil)
+		fn(0, 105, nil)
+		fn(0, 0, nil)
+		d.Close()
+
+		// Slurp with -1 instead
+		openDir()
+		fn(-1, 105, 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)
+		fn(1, 1, nil)
+		fn(2, 2, nil)
+		fn(105, 102, nil) // and tests buffer >100 case
+		fn(3, 0, EOF)
 		d.Close()
 	}
 }