lock.go

package command

import (
	"flag"
	"fmt"
	"os"
	"path"
	"strings"
	"sync"
	"syscall"
	"time"

	"github.com/hashicorp/consul/api"
	"github.com/hashicorp/consul/command/agent"
	"github.com/mitchellh/cli"
)

const (
	// lockKillGracePeriod is how long we allow a child between
	// a SIGTERM and a SIGKILL. This is to let the child cleanup
	// any necessary state. We have to balance this with the risk
	// of a split-brain where multiple children may be acting as if
	// they hold a lock. This value is currently based on the default
	// lock-delay value of 15 seconds. This only affects locks and not
	// semaphores.
	lockKillGracePeriod = 5 * time.Second
)

// LockCommand is a Command implementation that is used to setup
// a "lock" which manages lock acquisition and invokes a sub-process
type LockCommand struct {
	ShutdownCh <-chan struct{}
	Ui         cli.Ui

	child     *os.Process
	childLock sync.Mutex
	verbose   bool
}

func (c *LockCommand) Help() string {
	helpText := `
Usage: consul lock [options] prefix child...

  Acquires a lock or semaphore at a given path, and invokes a child
  process when successful. The child process can assume the lock is
  held while it executes. If the lock is lost or communication is
  disrupted the child process will be sent a SIGTERM signal and given
  time to gracefully exit. After the grace period expires the process
  will be hard terminated.
  For Consul agents on Windows, the child process is always hard
  terminated with a SIGKILL, since Windows has no POSIX compatible
  notion for SIGTERM.

  When -n=1, only a single lock holder or leader exists providing
  mutual exclusion. Setting a higher value switches to a semaphore
  allowing multiple holders to coordinate.

  The prefix provided must have write privileges.

Options:

  -http-addr=127.0.0.1:8500  HTTP address of the Consul agent.
  -n=1                       Maximum number of allowed lock holders. If this
                             value is one, it operates as a lock, otherwise
                             a semaphore is used.
  -name=""                   Optional name to associate with lock session.
  -token=""                  ACL token to use. Defaults to that of agent.
  -pass-stdin                Pass stdin to child process.
  -verbose                   Enables verbose output
`
	return strings.TrimSpace(helpText)
}

func (c *LockCommand) Run(args []string) int {
	var childDone chan struct{}
	var name, token string
	var limit int
	var passStdin bool
	cmdFlags := flag.NewFlagSet("watch", flag.ContinueOnError)
	cmdFlags.Usage = func() { c.Ui.Output(c.Help()) }
	cmdFlags.IntVar(&limit, "n", 1, "")
	cmdFlags.StringVar(&name, "name", "", "")
	cmdFlags.StringVar(&token, "token", "", "")
	cmdFlags.BoolVar(&passStdin, "pass-stdin", false, "")
	cmdFlags.BoolVar(&c.verbose, "verbose", false, "")
	httpAddr := HTTPAddrFlag(cmdFlags)
	if err := cmdFlags.Parse(args); err != nil {
		return 1
	}

	// Check the limit
	if limit <= 0 {
		c.Ui.Error(fmt.Sprintf("Lock holder limit must be positive"))
		return 1
	}

	// Verify the prefix and child are provided
	extra := cmdFlags.Args()
	if len(extra) < 2 {
		c.Ui.Error("Key prefix and child command must be specified")
		c.Ui.Error("")
		c.Ui.Error(c.Help())
		return 1
	}
	prefix := extra[0]
	prefix = strings.TrimPrefix(prefix, "/")
	script := strings.Join(extra[1:], " ")

	// Calculate a session name if none provided
	if name == "" {
		name = fmt.Sprintf("Consul lock for '%s' at '%s'", script, prefix)
	}

	// Create and test the HTTP client
	conf := api.DefaultConfig()
	conf.Address = *httpAddr
	conf.Token = token
	client, err := api.NewClient(conf)
	if err != nil {
		c.Ui.Error(fmt.Sprintf("Error connecting to Consul agent: %s", err))
		return 1
	}
	_, err = client.Agent().NodeName()
	if err != nil {
		c.Ui.Error(fmt.Sprintf("Error querying Consul agent: %s", err))
		return 1
	}

	// Setup the lock or semaphore
	var lu *LockUnlock
	if limit == 1 {
		lu, err = c.setupLock(client, prefix, name)
	} else {
		lu, err = c.setupSemaphore(client, limit, prefix, name)
	}
	if err != nil {
		c.Ui.Error(fmt.Sprintf("Lock setup failed: %s", err))
		return 1
	}

	// Attempt the acquisition
	if c.verbose {
		c.Ui.Info("Attempting lock acquisition")
	}
	lockCh, err := lu.lockFn(c.ShutdownCh)
	if lockCh == nil {
		if err == nil {
			c.Ui.Error("Shutdown triggered during lock acquisition")
		} else {
			c.Ui.Error(fmt.Sprintf("Lock acquisition failed: %s", err))
		}
		return 1
	}

	// Check if we were shutdown but managed to still acquire the lock
	select {
	case <-c.ShutdownCh:
		c.Ui.Error("Shutdown triggered during lock acquisition")
		goto RELEASE
	default:
	}

	// Start the child process
	childDone = make(chan struct{})
	go func() {
		if err := c.startChild(script, childDone, passStdin); err != nil {
			c.Ui.Error(fmt.Sprintf("%s", err))
		}
	}()

	// Monitor for shutdown, child termination, or lock loss
	select {
	case <-c.ShutdownCh:
		if c.verbose {
			c.Ui.Info("Shutdown triggered, killing child")
		}
	case <-lockCh:
		if c.verbose {
			c.Ui.Info("Lock lost, killing child")
		}
	case <-childDone:
		if c.verbose {
			c.Ui.Info("Child terminated, releasing lock")
		}
		goto RELEASE
	}

	// Prevent starting a new child.  The lock is never released
	// after this point.
	c.childLock.Lock()
	// Kill any existing child
	if err := c.killChild(childDone); err != nil {
		c.Ui.Error(fmt.Sprintf("%s", err))
	}

RELEASE:
	// Release the lock before termination
	if err := lu.unlockFn(); err != nil {
		c.Ui.Error(fmt.Sprintf("Lock release failed: %s", err))
		return 1
	}

	// Cleanup the lock if no longer in use
	if err := lu.cleanupFn(); err != nil {
		if err != lu.inUseErr {
			c.Ui.Error(fmt.Sprintf("Lock cleanup failed: %s", err))
			return 1
		} else if c.verbose {
			c.Ui.Info("Cleanup aborted, lock in use")
		}
	} else if c.verbose {
		c.Ui.Info("Cleanup succeeded")
	}
	return 0
}

// setupLock is used to setup a new Lock given the API client,
// the key prefix to operate on, and an optional session name.
func (c *LockCommand) setupLock(client *api.Client, prefix, name string) (*LockUnlock, error) {
	// Use the DefaultSemaphoreKey extension, this way if a lock and
	// semaphore are both used at the same prefix, we will get a conflict
	// which we can report to the user.
	key := path.Join(prefix, api.DefaultSemaphoreKey)
	if c.verbose {
		c.Ui.Info(fmt.Sprintf("Setting up lock at path: %s", key))
	}
	opts := api.LockOptions{
		Key:         key,
		SessionName: name,
	}
	l, err := client.LockOpts(&opts)
	if err != nil {
		return nil, err
	}
	lu := &LockUnlock{
		lockFn:    l.Lock,
		unlockFn:  l.Unlock,
		cleanupFn: l.Destroy,
		inUseErr:  api.ErrLockInUse,
	}
	return lu, nil
}

// setupSemaphore is used to setup a new Semaphore given the
// API client, key prefix, session name, and slot holder limit.
func (c *LockCommand) setupSemaphore(client *api.Client, limit int, prefix, name string) (*LockUnlock, error) {
	if c.verbose {
		c.Ui.Info(fmt.Sprintf("Setting up semaphore (limit %d) at prefix: %s", limit, prefix))
	}
	opts := api.SemaphoreOptions{
		Prefix:      prefix,
		Limit:       limit,
		SessionName: name,
	}
	s, err := client.SemaphoreOpts(&opts)
	if err != nil {
		return nil, err
	}
	lu := &LockUnlock{
		lockFn:    s.Acquire,
		unlockFn:  s.Release,
		cleanupFn: s.Destroy,
		inUseErr:  api.ErrSemaphoreInUse,
	}
	return lu, nil
}

// startChild is a long running routine used to start and
// wait for the child process to exit.
func (c *LockCommand) startChild(script string, doneCh chan struct{}, passStdin bool) error {
	defer close(doneCh)
	if c.verbose {
		c.Ui.Info(fmt.Sprintf("Starting handler '%s'", script))
	}
	// Create the command
	cmd, err := agent.ExecScript(script)
	if err != nil {
		c.Ui.Error(fmt.Sprintf("Error executing handler: %s", err))
		return err
	}

	// Setup the command streams
	cmd.Env = append(os.Environ(),
		"CONSUL_LOCK_HELD=true",
	)
	if passStdin {
		if c.verbose {
			c.Ui.Info("Stdin passed to handler process")
		}
		cmd.Stdin = os.Stdin
	} else {
		cmd.Stdin = nil
	}
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr

	// Start the child process
	c.childLock.Lock()
	if err := cmd.Start(); err != nil {
		c.Ui.Error(fmt.Sprintf("Error starting handler: %s", err))
		c.childLock.Unlock()
		return err
	}

	// Setup the child info
	c.child = cmd.Process
	c.childLock.Unlock()

	// Wait for the child process
	if err := cmd.Wait(); err != nil {
		c.Ui.Error(fmt.Sprintf("Error running handler: %s", err))
		return err
	}
	return nil
}

// killChild is used to forcefully kill the child, first using SIGTERM
// to allow for a graceful cleanup and then using SIGKILL for a hard
// termination.
// On Windows, the child is always hard terminated with a SIGKILL, even
// on the first attempt.
func (c *LockCommand) killChild(childDone chan struct{}) error {
	// Get the child process
	child := c.child

	// If there is no child process (failed to start), we can quit early
	if child == nil {
		if c.verbose {
			c.Ui.Info("No child process to kill")
		}
		return nil
	}

	// Attempt termination first
	if c.verbose {
		c.Ui.Info(fmt.Sprintf("Terminating child pid %d", child.Pid))
	}
	if err := signalPid(child.Pid, syscall.SIGTERM); err != nil {
		return fmt.Errorf("Failed to terminate %d: %v", child.Pid, err)
	}

	// Wait for termination, or until a timeout
	select {
	case <-childDone:
		if c.verbose {
			c.Ui.Info("Child terminated")
		}
		return nil
	case <-time.After(lockKillGracePeriod):
		if c.verbose {
			c.Ui.Info(fmt.Sprintf("Child did not exit after grace period of %v",
				lockKillGracePeriod))
		}
	}

	// Send a final SIGKILL
	if c.verbose {
		c.Ui.Info(fmt.Sprintf("Killing child pid %d", child.Pid))
	}
	if err := signalPid(child.Pid, syscall.SIGKILL); err != nil {
		return fmt.Errorf("Failed to kill %d: %v", child.Pid, err)
	}
	return nil
}

func (c *LockCommand) Synopsis() string {
	return "Execute a command holding a lock"
}

// LockUnlock is used to abstract over the differences between
// a lock and a semaphore.
type LockUnlock struct {
	lockFn    func(<-chan struct{}) (<-chan struct{}, error)
	unlockFn  func() error
	cleanupFn func() error
	inUseErr  error
}