os: add TestReadNonDir, fix problem it found on darwin

Author: dkegel-fastly

src/os/dir_darwin.go

@@ -26,6 +26,19 @@ func (d *dirInfo) close() {
 
 func (f *File) readdir(n int, mode readdirMode) (names []string, dirents []DirEntry, infos []FileInfo, err error) {
 	if f.dirinfo == nil {
+		// Begin workaround
+		// Without this test, darwinOpenDir would happily open non-directories, causing trouble in at least Glob().
+		// Upstream doesn't have to check f.Stat; darwin's
+		// fdopendir seems to take care of that when called
+		// normally, perhaps we are bypassing that?
+		sfi, err := f.Stat()
+		if err != nil {
+			return nil, nil, nil, &PathError{Op: "stat", Path: f.name, Err: err}
+		}
+		if !sfi.IsDir() {
+			return nil, nil, nil, &PathError{Op: "readdir", Path: f.name, Err: syscall.ENOTDIR}
+		}
+		// End workaround
 		dir, call, errno := darwinOpenDir(syscallFd(f.handle.(unixFileHandle)))
 		if errno != nil {
 			return nil, nil, nil, &PathError{Op: call, Path: f.name, Err: errno}

src/os/dir_test.go

@@ -169,3 +169,13 @@ func TestReadDir(t *testing.T) {
 		t.Fatalf("ReadDir %s: testdata directory not found", dirname)
 	}
 }
+
+// TestReadNonDir is a tinygo regression test to verify that opening a non-directory returns an error.
+// See note re f.Stat() in dir_darwin.go.
+func TestReadNonDir(t *testing.T) {
+	dirname := "read_test.go"
+	_, err := ReadDir(dirname)
+	if err == nil {
+		t.Fatalf("ReadDir %s: error on non-dir expected, none found", dirname)
+	}
+}