Better support of empty directories (#5890)

Better support of HEAD and listing of zero sized objects with trailing
slash (a.k.a empty directory). For that, isLeafDir function is added
to indicate if the specified object is an empty directory or not. Each
backend (xl, fs) has the responsibility to store that information.
Currently, in both of XL & FS, an empty directory is represented by
an empty directory in the backend.

isLeafDir() checks if the given path is an empty directory or not,
since dir listing is costly if the latter contains too many objects,
readDirN() is added in this PR to list only N number of entries.
In isLeadDir(), we will only list one entry to check if a directory
is empty or not.

cmd/xl-v1-common.go

@@ -76,6 +76,32 @@ func (xl xlObjects) isObject(bucket, prefix string) (ok bool) {
 	return false
 }
 
+// isObjectDir returns if the specified path represents an empty directory.
+func (xl xlObjects) isObjectDir(bucket, prefix string) (ok bool) {
+	for _, disk := range xl.getLoadBalancedDisks() {
+		if disk == nil {
+			continue
+		}
+		// Check if 'prefix' is an object on this 'disk', else continue the check the next disk
+		ctnts, err := disk.ListDir(bucket, prefix, 1)
+		if err == nil {
+			if len(ctnts) == 0 {
+				return true
+			}
+			return false
+		}
+		// Ignore for file not found,  disk not found or faulty disk.
+		if IsErrIgnored(err, xlTreeWalkIgnoredErrs...) {
+			continue
+		}
+		reqInfo := &logger.ReqInfo{BucketName: bucket}
+		reqInfo.AppendTags("prefix", prefix)
+		ctx := logger.SetReqInfo(context.Background(), reqInfo)
+		logger.LogIf(ctx, err)
+	} // Exhausted all disks - return false.
+	return false
+}
+
 // Calculate the space occupied by an object in a single disk
 func (xl xlObjects) sizeOnDisk(fileSize int64, blockSize int64, dataBlocks int) int64 {
 	numBlocks := fileSize / blockSize