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/posix-list-dir-nix.go

@@ -112,6 +112,12 @@ var readDirBufPool = sync.Pool{
 
 // Return all the entries at the directory dirPath.
 func readDir(dirPath string) (entries []string, err error) {
+	return readDirN(dirPath, -1)
+}
+
+// Return count entries at the directory dirPath and all entries
+// if count is set to -1
+func readDirN(dirPath string, count int) (entries []string, err error) {
 	bufp := readDirBufPool.Get().(*[]byte)
 	buf := *bufp
 	defer readDirBufPool.Put(bufp)
@@ -135,7 +141,11 @@ func readDir(dirPath string) (entries []string, err error) {
 	defer d.Close()
 
 	fd := int(d.Fd())
-	for {
+
+	remaining := count
+	done := false
+
+	for !done {
 		nbuf, err := syscall.ReadDirent(fd, buf)
 		if err != nil {
 			return nil, err
@@ -147,6 +157,13 @@ func readDir(dirPath string) (entries []string, err error) {
 		if tmpEntries, err = parseDirents(dirPath, buf[:nbuf]); err != nil {
 			return nil, err
 		}
+		if count > 0 {
+			if remaining <= len(tmpEntries) {
+				tmpEntries = tmpEntries[:remaining]
+				done = true
+			}
+			remaining -= len(tmpEntries)
+		}
 		entries = append(entries, tmpEntries...)
 	}
 	return