io/ioutil: return better error when TempDir called with non-extant dir

Fixes #14196

Change-Id: Ife7950289ac6adbcfc4d0f2fce31f20bc2657858
Reviewed-on: https://go-review.googlesource.com/28772Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/io/ioutil/tempfile.go

@@ -90,6 +90,11 @@ func TempDir(dir, prefix string) (name string, err error) {
 			}
 			continue
 		}
+		if os.IsNotExist(err) {
+			if _, err := os.Stat(dir); os.IsNotExist(err) {
+				return "", err
+			}
+		}
 		if err == nil {
 			name = try
 		}

src/io/ioutil/tempfile_test.go

@@ -51,3 +51,19 @@ func TestTempDir(t *testing.T) {
 		}
 	}
 }
+
+// test that we return a nice error message if the dir argument to TempDir doesn't
+// exist (or that it's empty and os.TempDir doesn't exist)
+func TestTempDir_BadDir(t *testing.T) {
+	dir, err := TempDir("", "TestTempDir_BadDir")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	badDir := filepath.Join(dir, "not-exist")
+	_, err = TempDir(badDir, "foo")
+	if pe, ok := err.(*os.PathError); !ok || !os.IsNotExist(err) || pe.Path != badDir {
+		t.Errorf("TempDir error = %#v; want PathError for path %q satisifying os.IsNotExist", err, badDir)
+	}
+}