minio/pkg/dsync/drwmutex.go

/*
 * 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 dsync

import (
	"context"
	"errors"
	"math/rand"
	"os"
	"sync"
	"time"

	"github.com/minio/minio/pkg/console"
	"github.com/minio/minio/pkg/retry"
)

// Indicator if logging is enabled.
var dsyncLog bool

func init() {
	// Check for MINIO_DSYNC_TRACE env variable, if set logging will be enabled for failed REST operations.
	dsyncLog = os.Getenv("MINIO_DSYNC_TRACE") == "1"
	rand.Seed(time.Now().UnixNano())
}

func log(format string, data ...interface{}) {
	if dsyncLog {
		console.Printf(format, data...)
	}
}

// DRWMutexAcquireTimeout - tolerance limit to wait for lock acquisition before.
const DRWMutexAcquireTimeout = 1 * time.Second // 1 second.
const drwMutexInfinite = 1<<63 - 1

// A DRWMutex is a distributed mutual exclusion lock.
type DRWMutex struct {
	Names        []string
	writeLocks   []string   // Array of nodes that granted a write lock
	readersLocks [][]string // Array of array of nodes that granted reader locks
	m            sync.Mutex // Mutex to prevent multiple simultaneous locks from this node
	clnt         *Dsync
}

// Granted - represents a structure of a granted lock.
type Granted struct {
	index   int
	lockUID string // Locked if set with UID string, unlocked if empty
}

func (g *Granted) isLocked() bool {
	return isLocked(g.lockUID)
}

func isLocked(uid string) bool {
	return len(uid) > 0
}

// NewDRWMutex - initializes a new dsync RW mutex.
func NewDRWMutex(clnt *Dsync, names ...string) *DRWMutex {
	return &DRWMutex{
		writeLocks: make([]string, len(clnt.GetLockersFn())),
		Names:      names,
		clnt:       clnt,
	}
}

// Lock holds a write lock on dm.
//
// If the lock is already in use, the calling go routine
// blocks until the mutex is available.
func (dm *DRWMutex) Lock(id, source string) {

	isReadLock := false
	dm.lockBlocking(context.Background(), id, source, isReadLock, Options{
		Timeout: drwMutexInfinite,
	})
}

// Options lock options.
type Options struct {
	Timeout   time.Duration
	Tolerance int
}

// GetLock tries to get a write lock on dm before the timeout elapses.
//
// If the lock is already in use, the calling go routine
// blocks until either the mutex becomes available and return success or
// more time has passed than the timeout value and return false.
func (dm *DRWMutex) GetLock(ctx context.Context, id, source string, opts Options) (locked bool) {

	isReadLock := false
	return dm.lockBlocking(ctx, id, source, isReadLock, opts)
}

// RLock holds a read lock on dm.
//
// If one or more read locks are already in use, it will grant another lock.
// Otherwise the calling go routine blocks until the mutex is available.
func (dm *DRWMutex) RLock(id, source string) {

	isReadLock := true
	dm.lockBlocking(context.Background(), id, source, isReadLock, Options{
		Timeout: drwMutexInfinite,
	})
}

// GetRLock tries to get a read lock on dm before the timeout elapses.
//
// If one or more read locks are already in use, it will grant another lock.
// Otherwise the calling go routine blocks until either the mutex becomes
// available and return success or more time has passed than the timeout
// value and return false.
func (dm *DRWMutex) GetRLock(ctx context.Context, id, source string, opts Options) (locked bool) {

	isReadLock := true
	return dm.lockBlocking(ctx, id, source, isReadLock, opts)
}

// lockBlocking will try to acquire either a read or a write lock
//
// The function will loop using a built-in timing randomized back-off
// algorithm until either the lock is acquired successfully or more
// time has elapsed than the timeout value.
func (dm *DRWMutex) lockBlocking(ctx context.Context, id, source string, isReadLock bool, opts Options) (locked bool) {
	restClnts := dm.clnt.GetLockersFn()

	retryCtx, cancel := context.WithTimeout(ctx, opts.Timeout)

	defer cancel()

	// Use incremental back-off algorithm for repeated attempts to acquire the lock
	for range retry.NewTimer(retryCtx) {
		// Create temp array on stack.
		locks := make([]string, len(restClnts))

		// Try to acquire the lock.
		locked = lock(retryCtx, dm.clnt, &locks, id, source, isReadLock, opts.Tolerance, dm.Names...)
		if !locked {
			continue
		}

		dm.m.Lock()

		// If success, copy array to object
		if isReadLock {
			// Append new array of strings at the end
			dm.readersLocks = append(dm.readersLocks, make([]string, len(restClnts)))
			// and copy stack array into last spot
			copy(dm.readersLocks[len(dm.readersLocks)-1], locks[:])
		} else {
			copy(dm.writeLocks, locks[:])
		}

		dm.m.Unlock()
		return locked
	}

	// Failed to acquire the lock on this attempt, incrementally wait
	// for a longer back-off time and try again afterwards.
	return locked
}

// lock tries to acquire the distributed lock, returning true or false.
func lock(ctx context.Context, ds *Dsync, locks *[]string, id, source string, isReadLock bool, tolerance int, lockNames ...string) bool {

	restClnts := ds.GetLockersFn()

	// Tolerance is not set, defaults to half of the locker clients.
	if tolerance == 0 {
		tolerance = len(restClnts) / 2
	}

	// Quorum is effectively = total clients subtracted with tolerance limit
	quorum := len(restClnts) - tolerance
	if !isReadLock {
		// In situations for write locks, as a special case
		// to avoid split brains we make sure to acquire
		// quorum + 1 when tolerance is exactly half of the
		// total locker clients.
		if quorum == tolerance {
			quorum++
		}
	}

	tolerance = len(restClnts) - quorum

	// Create buffered channel of size equal to total number of nodes.
	ch := make(chan Granted, len(restClnts))
	defer close(ch)

	var wg sync.WaitGroup
	for index, c := range restClnts {

		wg.Add(1)
		// broadcast lock request to all nodes
		go func(index int, isReadLock bool, c NetLocker) {
			defer wg.Done()

			g := Granted{index: index}
			if c == nil {
				log("dsync: nil locker")
				ch <- g
				return
			}

			args := LockArgs{
				UID:       id,
				Resources: lockNames,
				Source:    source,
			}

			var locked bool
			var err error
			if isReadLock {
				if locked, err = c.RLock(ctx, args); err != nil {
					log("dsync: Unable to call RLock failed with %s for %#v at %s\n", err, args, c)
				}
			} else {
				if locked, err = c.Lock(ctx, args); err != nil {
					log("dsync: Unable to call Lock failed with %s for %#v at %s\n", err, args, c)
				}
			}

			if locked {
				g.lockUID = args.UID
			}

			ch <- g

		}(index, isReadLock, c)
	}

	quorumMet := false

	wg.Add(1)
	go func(isReadLock bool) {

		// Wait until we have either
		//
		// a) received all lock responses
		// b) received too many 'non-'locks for quorum to be still possible
		// c) timedout
		//
		i, locksFailed := 0, 0
		done := false
		timeout := time.After(DRWMutexAcquireTimeout)

		for ; i < len(restClnts); i++ { // Loop until we acquired all locks

			select {
			case grant := <-ch:
				if grant.isLocked() {
					// Mark that this node has acquired the lock
					(*locks)[grant.index] = grant.lockUID
				} else {
					locksFailed++
					if locksFailed > tolerance {
						// We know that we are not going to get the lock anymore,
						// so exit out and release any locks that did get acquired
						done = true
						// Increment the number of grants received from the buffered channel.
						i++
						releaseAll(ds, locks, isReadLock, restClnts, lockNames...)
					}
				}
			case <-timeout:
				done = true
				// timeout happened, maybe one of the nodes is slow, count
				// number of locks to check whether we have quorum or not
				if !checkQuorumMet(locks, quorum) {
					log("Quorum not met after timeout\n")
					releaseAll(ds, locks, isReadLock, restClnts, lockNames...)
				} else {
					log("Quorum met after timeout\n")
				}
			}

			if done {
				break
			}
		}

		// Count locks in order to determine whether we have quorum or not
		quorumMet = checkQuorumMet(locks, quorum)

		// Signal that we have the quorum
		wg.Done()

		// Wait for the other responses and immediately release the locks
		// (do not add them to the locks array because the DRWMutex could
		//  already has been unlocked again by the original calling thread)
		for ; i < len(restClnts); i++ {
			grantToBeReleased := <-ch
			if grantToBeReleased.isLocked() {
				// release lock
				sendRelease(ds, restClnts[grantToBeReleased.index],
					grantToBeReleased.lockUID, isReadLock, lockNames...)
			}
		}
	}(isReadLock)

	wg.Wait()

	return quorumMet
}

// checkQuorumMet determines whether we have acquired the required quorum of underlying locks or not
func checkQuorumMet(locks *[]string, quorum int) bool {
	count := 0
	for _, uid := range *locks {
		if isLocked(uid) {
			count++
		}
	}

	return count >= quorum
}

// releaseAll releases all locks that are marked as locked
func releaseAll(ds *Dsync, locks *[]string, isReadLock bool, restClnts []NetLocker, lockNames ...string) {
	for lock := range restClnts {
		if isLocked((*locks)[lock]) {
			sendRelease(ds, restClnts[lock], (*locks)[lock], isReadLock, lockNames...)
			(*locks)[lock] = ""
		}
	}
}

// Unlock unlocks the write lock.
//
// It is a run-time error if dm is not locked on entry to Unlock.
func (dm *DRWMutex) Unlock() {

	restClnts := dm.clnt.GetLockersFn()
	// create temp array on stack
	locks := make([]string, len(restClnts))

	{
		dm.m.Lock()
		defer dm.m.Unlock()

		// Check if minimally a single bool is set in the writeLocks array
		lockFound := false
		for _, uid := range dm.writeLocks {
			if isLocked(uid) {
				lockFound = true
				break
			}
		}
		if !lockFound {
			panic("Trying to Unlock() while no Lock() is active")
		}

		// Copy write locks to stack array
		copy(locks, dm.writeLocks[:])
		// Clear write locks array
		dm.writeLocks = make([]string, len(restClnts))
	}

	isReadLock := false
	unlock(dm.clnt, locks, isReadLock, restClnts, dm.Names...)
}

// RUnlock releases a read lock held on dm.
//
// It is a run-time error if dm is not locked on entry to RUnlock.
func (dm *DRWMutex) RUnlock() {

	// create temp array on stack
	restClnts := dm.clnt.GetLockersFn()

	locks := make([]string, len(restClnts))
	{
		dm.m.Lock()
		defer dm.m.Unlock()
		if len(dm.readersLocks) == 0 {
			panic("Trying to RUnlock() while no RLock() is active")
		}
		// Copy out first element to release it first (FIFO)
		copy(locks, dm.readersLocks[0][:])
		// Drop first element from array
		dm.readersLocks = dm.readersLocks[1:]
	}

	isReadLock := true
	unlock(dm.clnt, locks, isReadLock, restClnts, dm.Names...)
}

func unlock(ds *Dsync, locks []string, isReadLock bool, restClnts []NetLocker, names ...string) {

	// We don't need to synchronously wait until we have released all the locks (or the quorum)
	// (a subsequent lock will retry automatically in case it would fail to get quorum)

	for index, c := range restClnts {

		if isLocked(locks[index]) {
			// broadcast lock release to all nodes that granted the lock
			sendRelease(ds, c, locks[index], isReadLock, names...)
		}
	}
}

// sendRelease sends a release message to a node that previously granted a lock
func sendRelease(ds *Dsync, c NetLocker, uid string, isReadLock bool, names ...string) {
	if c == nil {
		log("Unable to call RUnlock failed with %s\n", errors.New("netLocker is offline"))
		return
	}

	args := LockArgs{
		UID:       uid,
		Resources: names,
	}
	if isReadLock {
		if _, err := c.RUnlock(args); err != nil {
			log("dsync: Unable to call RUnlock failed with %s for %#v at %s\n", err, args, c)
		}
	} else {
		if _, err := c.Unlock(args); err != nil {
			log("dsync: Unable to call Unlock failed with %s for %#v at %s\n", err, args, c)
		}
	}
}