open-consul/api/lock.go

386 lines
10 KiB
Go

package api
import (
"fmt"
"sync"
"time"
)
const (
// DefaultLockSessionName is the Session Name we assign if none is provided
DefaultLockSessionName = "Consul API Lock"
// DefaultLockSessionTTL is the default session TTL if no Session is provided
// when creating a new Lock. This is used because we do not have another
// other check to depend upon.
DefaultLockSessionTTL = "15s"
// DefaultLockWaitTime is how long we block for at a time to check if lock
// acquisition is possible. This affects the minimum time it takes to cancel
// a Lock acquisition.
DefaultLockWaitTime = 15 * time.Second
// DefaultLockRetryTime is how long we wait after a failed lock acquisition
// before attempting to do the lock again. This is so that once a lock-delay
// is in effect, we do not hot loop retrying the acquisition.
DefaultLockRetryTime = 5 * time.Second
// DefaultMonitorRetryTime is how long we wait after a failed monitor check
// of a lock (500 response code). This allows the monitor to ride out brief
// periods of unavailability, subject to the MonitorRetries setting in the
// lock options which is by default set to 0, disabling this feature. This
// affects locks and semaphores.
DefaultMonitorRetryTime = 2 * time.Second
// LockFlagValue is a magic flag we set to indicate a key
// is being used for a lock. It is used to detect a potential
// conflict with a semaphore.
LockFlagValue = 0x2ddccbc058a50c18
)
var (
// ErrLockHeld is returned if we attempt to double lock
ErrLockHeld = fmt.Errorf("Lock already held")
// ErrLockNotHeld is returned if we attempt to unlock a lock
// that we do not hold.
ErrLockNotHeld = fmt.Errorf("Lock not held")
// ErrLockInUse is returned if we attempt to destroy a lock
// that is in use.
ErrLockInUse = fmt.Errorf("Lock in use")
// ErrLockConflict is returned if the flags on a key
// used for a lock do not match expectation
ErrLockConflict = fmt.Errorf("Existing key does not match lock use")
)
// Lock is used to implement client-side leader election. It is follows the
// algorithm as described here: https://www.consul.io/docs/guides/leader-election.html.
type Lock struct {
c *Client
opts *LockOptions
isHeld bool
sessionRenew chan struct{}
lockSession string
l sync.Mutex
}
// LockOptions is used to parameterize the Lock behavior.
type LockOptions struct {
Key string // Must be set and have write permissions
Value []byte // Optional, value to associate with the lock
Session string // Optional, created if not specified
SessionOpts *SessionEntry // Optional, options to use when creating a session
SessionName string // Optional, defaults to DefaultLockSessionName (ignored if SessionOpts is given)
SessionTTL string // Optional, defaults to DefaultLockSessionTTL (ignored if SessionOpts is given)
MonitorRetries int // Optional, defaults to 0 which means no retries
MonitorRetryTime time.Duration // Optional, defaults to DefaultMonitorRetryTime
LockWaitTime time.Duration // Optional, defaults to DefaultLockWaitTime
LockTryOnce bool // Optional, defaults to false which means try forever
}
// LockKey returns a handle to a lock struct which can be used
// to acquire and release the mutex. The key used must have
// write permissions.
func (c *Client) LockKey(key string) (*Lock, error) {
opts := &LockOptions{
Key: key,
}
return c.LockOpts(opts)
}
// LockOpts returns a handle to a lock struct which can be used
// to acquire and release the mutex. The key used must have
// write permissions.
func (c *Client) LockOpts(opts *LockOptions) (*Lock, error) {
if opts.Key == "" {
return nil, fmt.Errorf("missing key")
}
if opts.SessionName == "" {
opts.SessionName = DefaultLockSessionName
}
if opts.SessionTTL == "" {
opts.SessionTTL = DefaultLockSessionTTL
} else {
if _, err := time.ParseDuration(opts.SessionTTL); err != nil {
return nil, fmt.Errorf("invalid SessionTTL: %v", err)
}
}
if opts.MonitorRetryTime == 0 {
opts.MonitorRetryTime = DefaultMonitorRetryTime
}
if opts.LockWaitTime == 0 {
opts.LockWaitTime = DefaultLockWaitTime
}
l := &Lock{
c: c,
opts: opts,
}
return l, nil
}
// Lock attempts to acquire the lock and blocks while doing so.
// Providing a non-nil stopCh can be used to abort the lock attempt.
// Returns a channel that is closed if our lock is lost or an error.
// This channel could be closed at any time due to session invalidation,
// communication errors, operator intervention, etc. It is NOT safe to
// assume that the lock is held until Unlock() unless the Session is specifically
// created without any associated health checks. By default Consul sessions
// prefer liveness over safety and an application must be able to handle
// the lock being lost.
func (l *Lock) Lock(stopCh <-chan struct{}) (<-chan struct{}, error) {
// Hold the lock as we try to acquire
l.l.Lock()
defer l.l.Unlock()
// Check if we already hold the lock
if l.isHeld {
return nil, ErrLockHeld
}
// Check if we need to create a session first
l.lockSession = l.opts.Session
if l.lockSession == "" {
s, err := l.createSession()
if err != nil {
return nil, fmt.Errorf("failed to create session: %v", err)
}
l.sessionRenew = make(chan struct{})
l.lockSession = s
session := l.c.Session()
go session.RenewPeriodic(l.opts.SessionTTL, s, nil, l.sessionRenew)
// If we fail to acquire the lock, cleanup the session
defer func() {
if !l.isHeld {
close(l.sessionRenew)
l.sessionRenew = nil
}
}()
}
// Setup the query options
kv := l.c.KV()
qOpts := &QueryOptions{
WaitTime: l.opts.LockWaitTime,
}
start := time.Now()
attempts := 0
WAIT:
// Check if we should quit
select {
case <-stopCh:
return nil, nil
default:
}
// Handle the one-shot mode.
if l.opts.LockTryOnce && attempts > 0 {
elapsed := time.Now().Sub(start)
if elapsed > qOpts.WaitTime {
return nil, nil
}
qOpts.WaitTime -= elapsed
}
attempts++
// Look for an existing lock, blocking until not taken
pair, meta, err := kv.Get(l.opts.Key, qOpts)
if err != nil {
return nil, fmt.Errorf("failed to read lock: %v", err)
}
if pair != nil && pair.Flags != LockFlagValue {
return nil, ErrLockConflict
}
locked := false
if pair != nil && pair.Session == l.lockSession {
goto HELD
}
if pair != nil && pair.Session != "" {
qOpts.WaitIndex = meta.LastIndex
goto WAIT
}
// Try to acquire the lock
pair = l.lockEntry(l.lockSession)
locked, _, err = kv.Acquire(pair, nil)
if err != nil {
return nil, fmt.Errorf("failed to acquire lock: %v", err)
}
// Handle the case of not getting the lock
if !locked {
// Determine why the lock failed
qOpts.WaitIndex = 0
pair, meta, err = kv.Get(l.opts.Key, qOpts)
if pair != nil && pair.Session != "" {
//If the session is not null, this means that a wait can safely happen
//using a long poll
qOpts.WaitIndex = meta.LastIndex
goto WAIT
} else {
// If the session is empty and the lock failed to acquire, then it means
// a lock-delay is in effect and a timed wait must be used
select {
case <-time.After(DefaultLockRetryTime):
goto WAIT
case <-stopCh:
return nil, nil
}
}
}
HELD:
// Watch to ensure we maintain leadership
leaderCh := make(chan struct{})
go l.monitorLock(l.lockSession, leaderCh)
// Set that we own the lock
l.isHeld = true
// Locked! All done
return leaderCh, nil
}
// Unlock released the lock. It is an error to call this
// if the lock is not currently held.
func (l *Lock) Unlock() error {
// Hold the lock as we try to release
l.l.Lock()
defer l.l.Unlock()
// Ensure the lock is actually held
if !l.isHeld {
return ErrLockNotHeld
}
// Set that we no longer own the lock
l.isHeld = false
// Stop the session renew
if l.sessionRenew != nil {
defer func() {
close(l.sessionRenew)
l.sessionRenew = nil
}()
}
// Get the lock entry, and clear the lock session
lockEnt := l.lockEntry(l.lockSession)
l.lockSession = ""
// Release the lock explicitly
kv := l.c.KV()
_, _, err := kv.Release(lockEnt, nil)
if err != nil {
return fmt.Errorf("failed to release lock: %v", err)
}
return nil
}
// Destroy is used to cleanup the lock entry. It is not necessary
// to invoke. It will fail if the lock is in use.
func (l *Lock) Destroy() error {
// Hold the lock as we try to release
l.l.Lock()
defer l.l.Unlock()
// Check if we already hold the lock
if l.isHeld {
return ErrLockHeld
}
// Look for an existing lock
kv := l.c.KV()
pair, _, err := kv.Get(l.opts.Key, nil)
if err != nil {
return fmt.Errorf("failed to read lock: %v", err)
}
// Nothing to do if the lock does not exist
if pair == nil {
return nil
}
// Check for possible flag conflict
if pair.Flags != LockFlagValue {
return ErrLockConflict
}
// Check if it is in use
if pair.Session != "" {
return ErrLockInUse
}
// Attempt the delete
didRemove, _, err := kv.DeleteCAS(pair, nil)
if err != nil {
return fmt.Errorf("failed to remove lock: %v", err)
}
if !didRemove {
return ErrLockInUse
}
return nil
}
// createSession is used to create a new managed session
func (l *Lock) createSession() (string, error) {
session := l.c.Session()
se := l.opts.SessionOpts
if se == nil {
se = &SessionEntry{
Name: l.opts.SessionName,
TTL: l.opts.SessionTTL,
}
}
id, _, err := session.Create(se, nil)
if err != nil {
return "", err
}
return id, nil
}
// lockEntry returns a formatted KVPair for the lock
func (l *Lock) lockEntry(session string) *KVPair {
return &KVPair{
Key: l.opts.Key,
Value: l.opts.Value,
Session: session,
Flags: LockFlagValue,
}
}
// monitorLock is a long running routine to monitor a lock ownership
// It closes the stopCh if we lose our leadership.
func (l *Lock) monitorLock(session string, stopCh chan struct{}) {
defer close(stopCh)
kv := l.c.KV()
opts := &QueryOptions{RequireConsistent: true}
WAIT:
retries := l.opts.MonitorRetries
RETRY:
pair, meta, err := kv.Get(l.opts.Key, opts)
if err != nil {
// If configured we can try to ride out a brief Consul unavailability
// by doing retries. Note that we have to attempt the retry in a non-
// blocking fashion so that we have a clean place to reset the retry
// counter if service is restored.
if retries > 0 && IsRetryableError(err) {
time.Sleep(l.opts.MonitorRetryTime)
retries--
opts.WaitIndex = 0
goto RETRY
}
return
}
if pair != nil && pair.Session == session {
opts.WaitIndex = meta.LastIndex
goto WAIT
}
}