os: support deleting read-only files in RemoveAll on older Windows versions

The Windows implementation of RemoveAll supports deleting read-only
files only on file systems that supports POSIX semantics and on
newer Windows versions (Windows 10 RS5 and latter).

For all the other cases, the read-only bit was not clearer before
deleting read-only files, so they fail to delete.

Note that this case was supported prior to CL 75922, which landed on
Go 1.25.

Fixes #75922

Change-Id: Id6e6477f42e1952d08318ca3e4ab7c1648969f66
Reviewed-on: https://go-review.googlesource.com/c/go/+/713480
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Damien Neil <dneil@google.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/src/os/path_windows_test.go b/src/os/path_windows_test.go
index 3fa02e2a65..eea2b58ee0 100644
--- a/src/os/path_windows_test.go
+++ b/src/os/path_windows_test.go
@@ -236,6 +236,23 @@ func TestRemoveAllLongPathRelative(t *testing.T) {
 	}
 }
 
+func TestRemoveAllFallback(t *testing.T) {
+	windows.TestDeleteatFallback = true
+	t.Cleanup(func() { windows.TestDeleteatFallback = false })
+
+	dir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(dir, "file1"), []byte{}, 0700); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(dir, "file2"), []byte{}, 0400); err != nil { // read-only file
+		t.Fatal(err)
+	}
+
+	if err := os.RemoveAll(dir); err != nil {
+		t.Fatal(err)
+	}
+}
+
 func testLongPathAbs(t *testing.T, target string) {
 	t.Helper()
 	testWalkFn := func(path string, info os.FileInfo, err error) error {