allow large buffer to list more entries per directory (#9785)

cmd/os-dirent_namelen_linux.go

@@ -0,0 +1,43 @@
+// +build linux,!appengine
+
+/*
+ * MinIO Cloud Storage, (C) 2019 MinIO, Inc.
+ *
+ * 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.
+ */
+
+package cmd
+
+import (
+	"bytes"
+	"fmt"
+	"syscall"
+	"unsafe"
+)
+
+func direntNamlen(dirent *syscall.Dirent) (uint64, error) {
+	const fixedHdr = uint16(unsafe.Offsetof(syscall.Dirent{}.Name))
+	nameBuf := (*[unsafe.Sizeof(dirent.Name)]byte)(unsafe.Pointer(&dirent.Name[0]))
+	const nameBufLen = uint16(len(nameBuf))
+	limit := dirent.Reclen - fixedHdr
+	if limit > nameBufLen {
+		limit = nameBufLen
+	}
+	// Avoid bugs in long file names
+	// https://github.com/golang/tools/commit/5f9a5413737ba4b4f692214aebee582b47c8be74
+	nameLen := bytes.IndexByte(nameBuf[:limit], 0)
+	if nameLen < 0 {
+		return 0, fmt.Errorf("failed to find terminating 0 byte in dirent")
+	}
+	return uint64(nameLen), nil
+}