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.go

@@ -0,0 +1,102 @@
+/*
+ * 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 - implements filesystem locking wrappers around an
+// open file descriptor.
+package lock
+
+import (
+	"os"
+	"sync"
+)
+
+// RLockedFile represents a read locked file, implements a special
+// closer which only closes the associated *os.File when the ref count.
+// has reached zero, i.e when all the readers have given up their locks.
+type RLockedFile struct {
+	*LockedFile
+	mutex sync.Mutex
+	refs  int // Holds read lock refs.
+}
+
+// IsClosed - Check if the rlocked file is already closed.
+func (r *RLockedFile) IsClosed() bool {
+	r.mutex.Lock()
+	defer r.mutex.Unlock()
+	return r.refs == 0
+}
+
+// IncLockRef - is used by called to indicate lock refs.
+func (r *RLockedFile) IncLockRef() {
+	r.mutex.Lock()
+	r.refs++
+	r.mutex.Unlock()
+}
+
+// Close - this closer implements a special closer
+// closes the underlying fd only when the refs
+// reach zero.
+func (r *RLockedFile) Close() (err error) {
+	r.mutex.Lock()
+	defer r.mutex.Unlock()
+
+	if r.refs == 0 {
+		return os.ErrInvalid
+	}
+
+	r.refs--
+	if r.refs == 0 {
+		err = r.File.Close()
+	}
+
+	return err
+}
+
+// Provides a new initialized read locked struct from *os.File
+func newRLockedFile(lkFile *LockedFile) (*RLockedFile, error) {
+	if lkFile == nil {
+		return nil, os.ErrInvalid
+	}
+
+	return &RLockedFile{
+		LockedFile: lkFile,
+		refs:       1,
+	}, nil
+}
+
+// RLockedOpenFile - returns a wrapped read locked file, if the file
+// doesn't exist at path returns an error.
+func RLockedOpenFile(path string) (*RLockedFile, error) {
+	lkFile, err := LockedOpenFile(path, os.O_RDONLY, 0666)
+	if err != nil {
+		return nil, err
+	}
+
+	return newRLockedFile(lkFile)
+
+}
+
+// LockedFile represents a locked file, implements a helper
+// method Size(), represents the size of the underlying object.
+type LockedFile struct {
+	*os.File
+	size int64
+}
+
+// Size - size of the underlying locked file.
+func (l *LockedFile) Size() int64 {
+	return l.size
+}