terraform/states/statemgr/locker.go

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package statemgr
import (
"bytes"
"context"
"encoding/json"
"errors"
"fmt"
"math/rand"
"os"
"os/user"
"strings"
"text/template"
"time"
uuid "github.com/hashicorp/go-uuid"
"github.com/hashicorp/terraform/version"
)
var rngSource = rand.New(rand.NewSource(time.Now().UnixNano()))
// Locker is the interface for state managers that are able to manage
// mutual-exclusion locks for state.
//
// Implementing Locker alongside Persistent relaxes some of the usual
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// implementation constraints for implementations of Refresher and Persister,
// under the assumption that the locking mechanism effectively prevents
// multiple Terraform processes from reading and writing state concurrently.
// In particular, a type that implements both Locker and Persistent is only
// required to that the Persistent implementation is concurrency-safe within
// a single Terraform process.
//
// A Locker implementation must ensure that another processes with a
// similarly-configured state manager cannot successfully obtain a lock while
// the current process is holding it, or vice-versa, assuming that both
// processes agree on the locking mechanism.
//
// A Locker is not required to prevent non-cooperating processes from
// concurrently modifying the state, but is free to do so as an extra
// protection. If a mandatory locking mechanism of this sort is implemented,
// the state manager must ensure that RefreshState and PersistState calls
// can succeed if made through the same manager instance that is holding the
// lock, such has by retaining some sort of lock token that the Persistent
// methods can then use.
type Locker interface {
// Lock attempts to obtain a lock, using the given lock information.
//
// The result is an opaque id that can be passed to Unlock to release
// the lock, or an error if the lock cannot be acquired. Lock returns
// an instance of LockError immediately if the lock is already held,
// and the helper function LockWithContext uses this to automatically
// retry lock acquisition periodically until a timeout is reached.
Lock(info *LockInfo) (string, error)
// Unlock releases a lock previously acquired by Lock.
//
// If the lock cannot be released -- for example, if it was stolen by
// another user with some sort of administrative override privilege --
// then an error is returned explaining the situation in a way that
// is suitable for returning to an end-user.
Unlock(id string) error
}
// test hook to verify that LockWithContext has attempted a lock
var postLockHook func()
// LockWithContext locks the given state manager using the provided context
// for both timeout and cancellation.
//
// This method has a built-in retry/backoff behavior up to the context's
// timeout.
func LockWithContext(ctx context.Context, s Locker, info *LockInfo) (string, error) {
delay := time.Second
maxDelay := 16 * time.Second
for {
id, err := s.Lock(info)
if err == nil {
return id, nil
}
le, ok := err.(*LockError)
if !ok {
// not a lock error, so we can't retry
return "", err
}
if le == nil || le.Info == nil || le.Info.ID == "" {
// If we don't have a complete LockError then there's something
// wrong with the lock.
return "", err
}
if postLockHook != nil {
postLockHook()
}
// there's an existing lock, wait and try again
select {
case <-ctx.Done():
// return the last lock error with the info
return "", err
case <-time.After(delay):
if delay < maxDelay {
delay *= 2
}
}
}
}
// LockInfo stores lock metadata.
//
// Only Operation and Info are required to be set by the caller of Lock.
// Most callers should use NewLockInfo to create a LockInfo value with many
// of the fields populated with suitable default values.
type LockInfo struct {
// Unique ID for the lock. NewLockInfo provides a random ID, but this may
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// be overridden by the lock implementation. The final value of ID will be
// returned by the call to Lock.
ID string
// Terraform operation, provided by the caller.
Operation string
// Extra information to store with the lock, provided by the caller.
Info string
// user@hostname when available
Who string
// Terraform version
Version string
// Time that the lock was taken.
Created time.Time
// Path to the state file when applicable. Set by the Lock implementation.
Path string
}
// NewLockInfo creates a LockInfo object and populates many of its fields
// with suitable default values.
func NewLockInfo() *LockInfo {
// this doesn't need to be cryptographically secure, just unique.
// Using math/rand alleviates the need to check handle the read error.
// Use a uuid format to match other IDs used throughout Terraform.
buf := make([]byte, 16)
rngSource.Read(buf)
id, err := uuid.FormatUUID(buf)
if err != nil {
// this of course shouldn't happen
panic(err)
}
// don't error out on user and hostname, as we don't require them
userName := ""
if userInfo, err := user.Current(); err == nil {
userName = userInfo.Username
}
host, _ := os.Hostname()
info := &LockInfo{
ID: id,
Who: fmt.Sprintf("%s@%s", userName, host),
Version: version.Version,
Created: time.Now().UTC(),
}
return info
}
// Err returns the lock info formatted in an error
func (l *LockInfo) Err() error {
return errors.New(l.String())
}
// Marshal returns a string json representation of the LockInfo
func (l *LockInfo) Marshal() []byte {
js, err := json.Marshal(l)
if err != nil {
panic(err)
}
return js
}
// String return a multi-line string representation of LockInfo
func (l *LockInfo) String() string {
tmpl := `Lock Info:
ID: {{.ID}}
Path: {{.Path}}
Operation: {{.Operation}}
Who: {{.Who}}
Version: {{.Version}}
Created: {{.Created}}
Info: {{.Info}}
`
t := template.Must(template.New("LockInfo").Parse(tmpl))
var out bytes.Buffer
if err := t.Execute(&out, l); err != nil {
panic(err)
}
return out.String()
}
// LockError is a specialization of type error that is returned by Locker.Lock
// to indicate that the lock is already held by another process and that
// retrying may be productive to take the lock once the other process releases
// it.
type LockError struct {
Info *LockInfo
Err error
}
func (e *LockError) Error() string {
var out []string
if e.Err != nil {
out = append(out, e.Err.Error())
}
if e.Info != nil {
out = append(out, e.Info.String())
}
return strings.Join(out, "\n")
}