posix: test isDirEmpty, change error conditional (#4743)

This commit adds a new test for isDirEmpty (for code coverage) and
changes around the error conditional. Previously, there was a `return
nil` statement that would only be triggered under a race condition and
would trip up our test coverage for no real reason. With this new error
conditional, there's no awkward 'else'-esque condition, which means test
coverage will not change between runs for no reason in this specific
test. It's also a cleaner read.

cmd/posix_test.go

@@ -73,6 +73,42 @@ func TestPosixGetDiskInfo(t *testing.T) {
 	}
 }
 
+func TestPosixIsDirEmpty(t *testing.T) {
+	tmp, err := ioutil.TempDir(globalTestTmpDir, "minio-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer removeAll(tmp)
+
+	// Should give false on non-existent directory.
+	dir1 := slashpath.Join(tmp, "non-existent-directory")
+	if isDirEmpty(dir1) != false {
+		t.Error("expected false for non-existent directory, got true")
+	}
+
+	// Should give false for not-a-directory.
+	dir2 := slashpath.Join(tmp, "file")
+	err = ioutil.WriteFile(dir2, []byte("hello"), 0777)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if isDirEmpty(dir2) != false {
+		t.Error("expected false for a file, got true")
+	}
+
+	// Should give true for a real empty directory.
+	dir3 := slashpath.Join(tmp, "empty")
+	err = os.Mkdir(dir3, 0777)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if isDirEmpty(dir3) != true {
+		t.Error("expected true for empty dir, got false")
+	}
+}
+
 // TestPosixReadAll - TestPosixs the functionality implemented by posix ReadAll storage API.
 func TestPosixReadAll(t *testing.T) {
 	// create posix test setup