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