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acquire_handler.go
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/
acquire_handler.go
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package v2
import (
"errors"
"fmt"
"net/http"
"path"
"strconv"
"time"
etcdErr "github.com/coreos/etcd/error"
"github.com/coreos/etcd/third_party/github.com/coreos/go-etcd/etcd"
"github.com/coreos/etcd/third_party/github.com/gorilla/mux"
)
// acquireHandler attempts to acquire a lock on the given key.
// The "key" parameter specifies the resource to lock.
// The "value" parameter specifies a value to associate with the lock.
// The "ttl" parameter specifies how long the lock will persist for.
// The "timeout" parameter specifies how long the request should wait for the lock.
func (h *handler) acquireHandler(w http.ResponseWriter, req *http.Request) error {
h.client.SyncCluster()
// Setup connection watcher.
closeNotifier, _ := w.(http.CloseNotifier)
closeChan := closeNotifier.CloseNotify()
// Wrap closeChan so we can pass it to subsequent components
timeoutChan := make(chan bool)
stopChan := make(chan bool)
go func() {
select {
case <-closeChan:
// Client closed connection
stopChan <- true
case <-timeoutChan:
// Timeout expired
stopChan <- true
case <-stopChan:
}
close(stopChan)
}()
// Parse the lock "key".
vars := mux.Vars(req)
keypath := path.Join(prefix, vars["key"])
value := req.FormValue("value")
// Parse "timeout" parameter.
var timeout int
var err error
if req.FormValue("timeout") == "" {
timeout = -1
} else if timeout, err = strconv.Atoi(req.FormValue("timeout")); err != nil {
return etcdErr.NewError(etcdErr.EcodeTimeoutNaN, "Acquire", 0)
}
// Parse TTL.
ttl, err := strconv.Atoi(req.FormValue("ttl"))
if err != nil {
return etcdErr.NewError(etcdErr.EcodeTTLNaN, "Acquire", 0)
}
// Search for the node
_, index, pos := h.findExistingNode(keypath, value)
if index == 0 {
// Node doesn't exist; Create it
pos = -1 // Invalidate previous position
index, err = h.createNode(keypath, value, ttl)
if err != nil {
return err
}
}
indexpath := path.Join(keypath, strconv.Itoa(index))
// If pos != 0, we do not already have the lock
if pos != 0 {
if timeout == 0 {
// Attempt to get lock once, no waiting
err = h.get(keypath, index)
} else {
// Keep updating TTL while we wait
go h.ttlKeepAlive(keypath, value, ttl, stopChan)
// Start timeout
go h.timeoutExpire(timeout, timeoutChan, stopChan)
// wait for lock
err = h.watch(keypath, index, stopChan)
}
}
// Return on error, deleting our lock request on the way
if err != nil {
if index > 0 {
h.client.Delete(indexpath, false)
}
return err
}
// Check for connection disconnect before we write the lock index.
select {
case <-stopChan:
err = errors.New("user interrupted")
default:
}
// Update TTL one last time if lock was acquired. Otherwise delete.
if err == nil {
h.client.Update(indexpath, value, uint64(ttl))
} else {
h.client.Delete(indexpath, false)
}
// Write response.
w.Write([]byte(strconv.Itoa(index)))
return nil
}
// createNode creates a new lock node and watches it until it is acquired or acquisition fails.
func (h *handler) createNode(keypath string, value string, ttl int) (int, error) {
// Default the value to "-" if it is blank.
if len(value) == 0 {
value = "-"
}
// Create an incrementing id for the lock.
resp, err := h.client.AddChild(keypath, value, uint64(ttl))
if err != nil {
return 0, err
}
indexpath := resp.Node.Key
index, err := strconv.Atoi(path.Base(indexpath))
return index, err
}
// findExistingNode search for a node on the lock with the given value.
func (h *handler) findExistingNode(keypath string, value string) (*etcd.Node, int, int) {
if len(value) > 0 {
resp, err := h.client.Get(keypath, true, true)
if err == nil {
nodes := lockNodes{resp.Node.Nodes}
if node, pos := nodes.FindByValue(value); node != nil {
index, _ := strconv.Atoi(path.Base(node.Key))
return node, index, pos
}
}
}
return nil, 0, 0
}
// ttlKeepAlive continues to update a key's TTL until the stop channel is closed.
func (h *handler) ttlKeepAlive(k string, value string, ttl int, stopChan chan bool) {
for {
select {
case <-time.After(time.Duration(ttl/2) * time.Second):
h.client.Update(k, value, uint64(ttl))
case <-stopChan:
return
}
}
}
// timeoutExpire sets the countdown timer is a positive integer
// cancels on stopChan, sends true on timeoutChan after timer expires
func (h *handler) timeoutExpire(timeout int, timeoutChan chan bool, stopChan chan bool) {
// Set expiration timer if timeout is 1 or higher
if timeout < 1 {
timeoutChan = nil
return
}
select {
case <-stopChan:
return
case <-time.After(time.Duration(timeout) * time.Second):
timeoutChan <- true
return
}
}
func (h *handler) getLockIndex(keypath string, index int) (int, int, error) {
// Read all nodes for the lock.
resp, err := h.client.Get(keypath, true, true)
if err != nil {
return 0, 0, fmt.Errorf("lock watch lookup error: %s", err.Error())
}
nodes := lockNodes{resp.Node.Nodes}
prevIndex, modifiedIndex := nodes.PrevIndex(index)
return prevIndex, modifiedIndex, nil
}
// get tries once to get the lock; no waiting
func (h *handler) get(keypath string, index int) error {
prevIndex, _, err := h.getLockIndex(keypath, index)
if err != nil {
return err
}
if prevIndex == 0 {
// Lock acquired
return nil
}
return fmt.Errorf("failed to acquire lock")
}
// watch continuously waits for a given lock index to be acquired or until lock fails.
// Returns a boolean indicating success.
func (h *handler) watch(keypath string, index int, closeChan <-chan bool) error {
// Wrap close chan so we can pass it to Client.Watch().
stopWatchChan := make(chan bool)
stopWrapChan := make(chan bool)
go func() {
select {
case <-closeChan:
stopWatchChan <- true
case <-stopWrapChan:
stopWatchChan <- true
case <-stopWatchChan:
}
}()
defer close(stopWrapChan)
for {
prevIndex, modifiedIndex, err := h.getLockIndex(keypath, index)
// If there is no previous index then we have the lock.
if prevIndex == 0 {
return nil
}
// Wait from the last modification of the node.
waitIndex := modifiedIndex + 1
_, err = h.client.Watch(path.Join(keypath, strconv.Itoa(prevIndex)), uint64(waitIndex), false, nil, stopWatchChan)
if err == etcd.ErrWatchStoppedByUser {
return fmt.Errorf("lock watch closed")
} else if err != nil {
return fmt.Errorf("lock watch error: %s", err.Error())
}
return nil
}
}