Support creating empty directories. (#5049)

Every so often we get requirements for creating
directories/prefixes and we end up rejecting
such requirements. This PR implements this and
allows empty directories without any new file
addition to backend.

Existing lower APIs themselves are leveraged to provide
this behavior. Only FS backend supports this for
the time being as desired.

cmd/fs-v1-metadata.go

@@ -91,6 +91,11 @@ func (m fsMetaV1) ToObjectInfo(bucket, object string, fi os.FileInfo) ObjectInfo
 		}
 	}
 
+	if hasSuffix(object, slashSeparator) {
+		m.Meta["etag"] = emptyETag // For directories etag is d41d8cd98f00b204e9800998ecf8427e
+		m.Meta["content-type"] = "application/octet-stream"
+	}
+
 	objInfo := ObjectInfo{
 		Bucket: bucket,
 		Name:   object,
@@ -101,7 +106,11 @@ func (m fsMetaV1) ToObjectInfo(bucket, object string, fi os.FileInfo) ObjectInfo
 	if fi != nil {
 		objInfo.ModTime = fi.ModTime()
 		objInfo.Size = fi.Size()
-		objInfo.IsDir = fi.IsDir()
+		if fi.IsDir() {
+			// Directory is always 0 bytes in S3 API, treat it as such.
+			objInfo.Size = 0
+			objInfo.IsDir = fi.IsDir()
+		}
 	}
 
 	// Extract etag from metadata.