Implement HTTP POST based RPC (#5840)

Added support for new RPC support using HTTP POST.  RPC's 
arguments and reply are Gob encoded and sent as HTTP 
request/response body.

This patch also removes Go RPC based implementation.

cmd/local-locker.go

@@ -0,0 +1,151 @@
+/*
+ * Minio Cloud Storage, (C) 2018 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 (
+	"fmt"
+	"sync"
+	"time"
+
+	"github.com/minio/dsync"
+)
+
+// lockRequesterInfo stores various info from the client for each lock that is requested.
+type lockRequesterInfo struct {
+	writer          bool      // Bool whether write or read lock.
+	node            string    // Network address of client claiming lock.
+	serviceEndpoint string    // RPC path of client claiming lock.
+	uid             string    // UID to uniquely identify request of client.
+	timestamp       time.Time // Timestamp set at the time of initialization.
+	timeLastCheck   time.Time // Timestamp for last check of validity of lock.
+}
+
+// isWriteLock returns whether the lock is a write or read lock.
+func isWriteLock(lri []lockRequesterInfo) bool {
+	return len(lri) == 1 && lri[0].writer
+}
+
+// localLocker implements Dsync.NetLocker
+type localLocker struct {
+	mutex           sync.Mutex
+	serviceEndpoint string
+	serverAddr      string
+	lockMap         map[string][]lockRequesterInfo
+}
+
+func (l *localLocker) ServerAddr() string {
+	return l.serverAddr
+}
+
+func (l *localLocker) ServiceEndpoint() string {
+	return l.serviceEndpoint
+}
+
+func (l *localLocker) Lock(args dsync.LockArgs) (reply bool, err error) {
+	l.mutex.Lock()
+	defer l.mutex.Unlock()
+	_, isLockTaken := l.lockMap[args.Resource]
+	if !isLockTaken { // No locks held on the given name, so claim write lock
+		l.lockMap[args.Resource] = []lockRequesterInfo{
+			{
+				writer:          true,
+				node:            args.ServerAddr,
+				serviceEndpoint: args.ServiceEndpoint,
+				uid:             args.UID,
+				timestamp:       UTCNow(),
+				timeLastCheck:   UTCNow(),
+			},
+		}
+	}
+	// return reply=true if lock was granted.
+	return !isLockTaken, nil
+}
+
+func (l *localLocker) Unlock(args dsync.LockArgs) (reply bool, err error) {
+	l.mutex.Lock()
+	defer l.mutex.Unlock()
+	var lri []lockRequesterInfo
+	if lri, reply = l.lockMap[args.Resource]; !reply {
+		// No lock is held on the given name
+		return reply, fmt.Errorf("Unlock attempted on an unlocked entity: %s", args.Resource)
+	}
+	if reply = isWriteLock(lri); !reply {
+		// Unless it is a write lock
+		return reply, fmt.Errorf("Unlock attempted on a read locked entity: %s (%d read locks active)", args.Resource, len(lri))
+	}
+	if !l.removeEntry(args.Resource, args.UID, &lri) {
+		return false, fmt.Errorf("Unlock unable to find corresponding lock for uid: %s", args.UID)
+	}
+	return true, nil
+
+}
+
+func (l *localLocker) RLock(args dsync.LockArgs) (reply bool, err error) {
+	l.mutex.Lock()
+	defer l.mutex.Unlock()
+	lrInfo := lockRequesterInfo{
+		writer:          false,
+		node:            args.ServerAddr,
+		serviceEndpoint: args.ServiceEndpoint,
+		uid:             args.UID,
+		timestamp:       UTCNow(),
+		timeLastCheck:   UTCNow(),
+	}
+	if lri, ok := l.lockMap[args.Resource]; ok {
+		if reply = !isWriteLock(lri); reply {
+			// Unless there is a write lock
+			l.lockMap[args.Resource] = append(l.lockMap[args.Resource], lrInfo)
+		}
+	} else {
+		// No locks held on the given name, so claim (first) read lock
+		l.lockMap[args.Resource] = []lockRequesterInfo{lrInfo}
+		reply = true
+	}
+	return reply, nil
+}
+
+func (l *localLocker) RUnlock(args dsync.LockArgs) (reply bool, err error) {
+	l.mutex.Lock()
+	defer l.mutex.Unlock()
+	var lri []lockRequesterInfo
+	if lri, reply = l.lockMap[args.Resource]; !reply {
+		// No lock is held on the given name
+		return reply, fmt.Errorf("RUnlock attempted on an unlocked entity: %s", args.Resource)
+	}
+	if reply = !isWriteLock(lri); !reply {
+		// A write-lock is held, cannot release a read lock
+		return reply, fmt.Errorf("RUnlock attempted on a write locked entity: %s", args.Resource)
+	}
+	if !l.removeEntry(args.Resource, args.UID, &lri) {
+		return false, fmt.Errorf("RUnlock unable to find corresponding read lock for uid: %s", args.UID)
+	}
+	return reply, nil
+}
+
+func (l *localLocker) ForceUnlock(args dsync.LockArgs) (reply bool, err error) {
+	l.mutex.Lock()
+	defer l.mutex.Unlock()
+	if len(args.UID) != 0 {
+		return false, fmt.Errorf("ForceUnlock called with non-empty UID: %s", args.UID)
+	}
+	if _, ok := l.lockMap[args.Resource]; ok {
+		// Only clear lock when it is taken
+		// Remove the lock (irrespective of write or read lock)
+		delete(l.lockMap, args.Resource)
+	}
+	return true, nil
+}