fs: Re-implement object layer to remember the fd (#3509)

This patch re-writes FS backend to support shared backend sharing locks for safe concurrent access across multiple servers.

pkg/lock/lock_nix.go

@@ -0,0 +1,75 @@
+// +build !windows,!plan9,!solaris
+
+/*
+ * Minio Cloud Storage, (C) 2016 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 lock
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+)
+
+// LockedOpenFile - initializes a new lock and protects
+// the file from concurrent access across mount points.
+// This implementation doesn't support all the open
+// flags and shouldn't be considered as replacement
+// for os.OpenFile().
+func LockedOpenFile(path string, flag int, perm os.FileMode) (*LockedFile, error) {
+	var lockType int
+	switch flag {
+	case syscall.O_RDONLY:
+		lockType = syscall.LOCK_SH
+	case syscall.O_WRONLY:
+		fallthrough
+	case syscall.O_RDWR:
+		fallthrough
+	case syscall.O_WRONLY | syscall.O_CREAT:
+		fallthrough
+	case syscall.O_RDWR | syscall.O_CREAT:
+		lockType = syscall.LOCK_EX
+	default:
+		return nil, fmt.Errorf("Unsupported flag (%d)", flag)
+	}
+
+	f, err := os.OpenFile(path, flag|syscall.O_SYNC, perm)
+	if err != nil {
+		return nil, err
+	}
+
+	if err = syscall.Flock(int(f.Fd()), lockType); err != nil {
+		f.Close()
+		return nil, err
+	}
+
+	st, err := os.Stat(path)
+	if err != nil {
+		f.Close()
+		return nil, err
+	}
+
+	if st.IsDir() {
+		f.Close()
+		return nil, &os.PathError{
+			Op:   "open",
+			Path: path,
+			Err:  syscall.EISDIR,
+		}
+	}
+
+	return &LockedFile{File: f, size: st.Size()}, nil
+}