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|
package stm
import (
"fmt"
"sort"
"sync"
"unsafe"
)
// A Tx represents an atomic transaction.
type Tx struct {
reads map[*Var]VarValue
writes map[*Var]interface{}
watching map[*Var]struct{}
locks txLocks
mu sync.Mutex
cond sync.Cond
waiting bool
completed bool
tries int
}
// Check that none of the logged values have changed since the transaction began.
func (tx *Tx) inputsChanged() bool {
for v, read := range tx.reads {
if read.Changed(v.value.Load().(VarValue)) {
return true
}
}
return false
}
// Writes the values in the transaction log to their respective Vars.
func (tx *Tx) commit() {
for v, val := range tx.writes {
v.changeValue(val)
}
}
func (tx *Tx) updateWatchers() {
for v := range tx.watching {
if _, ok := tx.reads[v]; !ok {
delete(tx.watching, v)
v.watchers.Delete(tx)
}
}
for v := range tx.reads {
if _, ok := tx.watching[v]; !ok {
v.watchers.Store(tx, nil)
tx.watching[v] = struct{}{}
}
}
}
// wait blocks until another transaction modifies any of the Vars read by tx.
func (tx *Tx) wait() {
if len(tx.reads) == 0 {
panic("not waiting on anything")
}
tx.updateWatchers()
tx.mu.Lock()
firstWait := true
for !tx.inputsChanged() {
if !firstWait {
expvars.Add("wakes for unchanged versions", 1)
}
expvars.Add("waits", 1)
tx.waiting = true
tx.cond.Broadcast()
tx.cond.Wait()
tx.waiting = false
firstWait = false
}
tx.mu.Unlock()
}
// Get returns the value of v as of the start of the transaction.
func (tx *Tx) Get(v *Var) interface{} {
// If we previously wrote to v, it will be in the write log.
if val, ok := tx.writes[v]; ok {
return val
}
// If we haven't previously read v, record its version
vv, ok := tx.reads[v]
if !ok {
vv = v.value.Load().(VarValue)
tx.reads[v] = vv
}
return vv.Get()
}
// Set sets the value of a Var for the lifetime of the transaction.
func (tx *Tx) Set(v *Var, val interface{}) {
if v == nil {
panic("nil Var")
}
tx.writes[v] = val
}
// Retry aborts the transaction and retries it when a Var changes.
func (tx *Tx) Retry() {
panic(Retry)
}
// Assert is a helper function that retries a transaction if the condition is
// not satisfied.
func (tx *Tx) Assert(p bool) {
if !p {
tx.Retry()
}
}
func (tx *Tx) reset() {
for k := range tx.reads {
delete(tx.reads, k)
}
for k := range tx.writes {
delete(tx.writes, k)
}
tx.resetLocks()
}
func (tx *Tx) recycle() {
for v := range tx.watching {
delete(tx.watching, v)
v.watchers.Delete(tx)
}
txPool.Put(tx)
}
func (tx *Tx) lockAllVars() {
tx.resetLocks()
tx.collectAllLocks()
tx.sortLocks()
tx.lock()
}
func (tx *Tx) resetLocks() {
tx.locks.clear()
}
func (tx *Tx) collectReadLocks() {
for v := range tx.reads {
tx.locks.append(&v.mu)
}
}
func (tx *Tx) collectAllLocks() {
tx.collectReadLocks()
for v := range tx.writes {
if _, ok := tx.reads[v]; !ok {
tx.locks.append(&v.mu)
}
}
}
func (tx *Tx) sortLocks() {
sort.Sort(&tx.locks)
}
func (tx *Tx) lock() {
for _, l := range tx.locks.mus {
l.Lock()
}
}
func (tx *Tx) unlock() {
for _, l := range tx.locks.mus {
l.Unlock()
}
}
func (tx *Tx) String() string {
return fmt.Sprintf("%[1]T %[1]p", tx)
}
// Dedicated type avoids reflection in sort.Slice.
type txLocks struct {
mus []*sync.Mutex
}
func (me txLocks) Len() int {
return len(me.mus)
}
func (me txLocks) Less(i, j int) bool {
return uintptr(unsafe.Pointer(me.mus[i])) < uintptr(unsafe.Pointer(me.mus[j]))
}
func (me txLocks) Swap(i, j int) {
me.mus[i], me.mus[j] = me.mus[j], me.mus[i]
}
func (me *txLocks) clear() {
me.mus = me.mus[:0]
}
func (me *txLocks) append(mu *sync.Mutex) {
me.mus = append(me.mus, mu)
}
|
