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.

cmd/namespace-lock.go

@@ -28,6 +28,14 @@ import (
 // Global name space lock.
 var globalNSMutex *nsLockMap
 
+// RWLocker - locker interface extends sync.Locker
+// to introduce RLock, RUnlock.
+type RWLocker interface {
+	sync.Locker
+	RLock()
+	RUnlock()
+}
+
 // Initialize distributed locking only in case of distributed setup.
 // Returns if the setup is distributed or not on success.
 func initDsyncNodes(eps []*url.URL) error {
@@ -68,13 +76,6 @@ func initNSLock(isDistXL bool) {
 	globalNSMutex.debugLockMap = make(map[nsParam]*debugLockInfoPerVolumePath)
 }
 
-// RWLocker - interface that any read-write locking library should implement.
-type RWLocker interface {
-	sync.Locker
-	RLock()
-	RUnlock()
-}
-
 // nsParam - carries name space resource.
 type nsParam struct {
 	volume string
@@ -94,8 +95,7 @@ type nsLockMap struct {
 	counters     *lockStat
 	debugLockMap map[nsParam]*debugLockInfoPerVolumePath // Info for instrumentation on locks.
 
-	// Indicates whether the locking service is part
-	// of a distributed setup or not.
+	// Indicates if namespace is part of a distributed setup.
 	isDistXL     bool
 	lockMap      map[nsParam]*nsLock
 	lockMapMutex sync.Mutex
@@ -256,14 +256,14 @@ func (n *nsLockMap) ForceUnlock(volume, path string) {
 
 // lockInstance - frontend/top-level interface for namespace locks.
 type lockInstance struct {
-	n                   *nsLockMap
+	ns                  *nsLockMap
 	volume, path, opsID string
 }
 
 // NewNSLock - returns a lock instance for a given volume and
 // path. The returned lockInstance object encapsulates the nsLockMap,
 // volume, path and operation ID.
-func (n *nsLockMap) NewNSLock(volume, path string) *lockInstance {
+func (n *nsLockMap) NewNSLock(volume, path string) RWLocker {
 	return &lockInstance{n, volume, path, getOpsID()}
 }
 
@@ -271,24 +271,24 @@ func (n *nsLockMap) NewNSLock(volume, path string) *lockInstance {
 func (li *lockInstance) Lock() {
 	lockSource := callerSource()
 	readLock := false
-	li.n.lock(li.volume, li.path, lockSource, li.opsID, readLock)
+	li.ns.lock(li.volume, li.path, lockSource, li.opsID, readLock)
 }
 
 // Unlock - block until write lock is released.
 func (li *lockInstance) Unlock() {
 	readLock := false
-	li.n.unlock(li.volume, li.path, li.opsID, readLock)
+	li.ns.unlock(li.volume, li.path, li.opsID, readLock)
 }
 
 // RLock - block until read lock is taken.
 func (li *lockInstance) RLock() {
 	lockSource := callerSource()
 	readLock := true
-	li.n.lock(li.volume, li.path, lockSource, li.opsID, readLock)
+	li.ns.lock(li.volume, li.path, lockSource, li.opsID, readLock)
 }
 
 // RUnlock - block until read lock is released.
 func (li *lockInstance) RUnlock() {
 	readLock := true
-	li.n.unlock(li.volume, li.path, li.opsID, readLock)
+	li.ns.unlock(li.volume, li.path, li.opsID, readLock)
 }