fix: put object on windows when parent directories dont already exists

The previous logic was not allowing put-object on windows when the parent directory did not already exist, and would
not always return the correct error if an ancestor in the path already existed as a file.

The problem is the different behavior of the os.Stat command in Windows compared to *nix in backend/posix/posix.go
in function PutObjectWithPostFunc. The os.Stat returns ENOTDIR on *nix if the parent object is a file instead of a
directory. On Windows, if the parent object does not exist at all, the return code of such os.Stat is ERROR_PATH_NOT_FOUND which is mapped to ENOTDIR. However this is inappropriate in this case. As a result, the
return code of the os.Stat is incorrectly interpreted as if the parent object is a file instead of the parent object does not
exist. Which then leads to a failed upload.

This fix validates the existing parent structure on put to make sure the correct error is returned or the put is successful.

Fixes #1702

backend/posix/parentdir_other.go

@@ -0,0 +1,25 @@
+// Copyright 2026 Versity Software
+// This file is licensed under the Apache License, Version 2.0
+// (the "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+//go:build !windows
+
+package posix
+
+import (
+	"github.com/versity/versitygw/s3err"
+)
+
+func handleParentDirError(_ string) error {
+	return s3err.GetAPIError(s3err.ErrObjectParentIsFile)
+}

backend/posix/parentdir_windows.go

@@ -0,0 +1,46 @@
+// Copyright 2026 Versity Software
+// This file is licensed under the Apache License, Version 2.0
+// (the "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+//go:build windows
+
+package posix
+
+import (
+	"os"
+	"path/filepath"
+
+	"github.com/versity/versitygw/s3err"
+)
+
+func handleParentDirError(name string) error {
+	dir := filepath.Dir(name)
+
+	// Walk up the directory hierarchy
+	for dir != "." && dir != "/" {
+		d, statErr := os.Stat(dir)
+		if statErr == nil {
+			// Path component exists
+			if !d.IsDir() {
+				// Found a file in the ancestor path
+				return s3err.GetAPIError(s3err.ErrObjectParentIsFile)
+			}
+			// Found a valid directory ancestor, parent truly doesn't exist
+			break
+		}
+		// Continue checking parent directories
+		dir = filepath.Dir(dir)
+	}
+	// Parent doesn't exist or is a directory, treat as ENOENT
+	return nil
+}

backend/posix/posix.go

@@ -3052,7 +3052,10 @@ func (p *Posix) PutObjectWithPostFunc(ctx context.Context, po s3response.PutObje
 		return s3response.PutObjectOutput{}, s3err.GetAPIError(s3err.ErrKeyTooLong)
 	}
 	if errors.Is(err, syscall.ENOTDIR) {
-		return s3response.PutObjectOutput{}, s3err.GetAPIError(s3err.ErrObjectParentIsFile)
+		parentErr := handleParentDirError(name)
+		if parentErr != nil {
+			return s3response.PutObjectOutput{}, parentErr
+		}
 	}
 	if err != nil && !errors.Is(err, fs.ErrNotExist) {
 		return s3response.PutObjectOutput{}, fmt.Errorf("stat object: %w", err)