os: add TestReadNonDir, fix problem it found on darwin

Author: dkegel-fastly

src/os/dir_darwin.go

@@ -140,6 +140,12 @@ func darwinOpenDir(fd syscallFd) (uintptr, string, error) {
 			break
 		}
 	}
+	// Without this test, darwinOpenDir would happily open non-directories, causing trouble in at least Glob().
+	// Upstream doesn't test dir for nil here, but dtruss shows stat is being called behind the scenes.
+	// Is syscall.Fdopendir not checking for errno properly?
+	if err == nil && dir == 0 {
+		err = syscall.ENOTDIR
+	}
 	if err != nil {
 		syscall.Close(fd2)
 		return 0, "fdopendir", err

src/os/dir_test.go

@@ -169,3 +169,14 @@ 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) {
+	// Use filename of this source file, it is known to exist, and go tests run in source tree.
+	dirname := "dir_test.go"
+	_, err := ReadDir(dirname)
+	if err == nil {
+		t.Fatalf("ReadDir %s: error on non-dir expected, none found", dirname)
+	}
+}