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package stm
import (
"sort"
"sync"
"unsafe"
)
// A Tx represents an atomic transaction.
type Tx struct {
reads map[*Var]uint64
writes map[*Var]interface{}
locks []*sync.Mutex
mu sync.Mutex
cond sync.Cond
}
// Check that none of the logged values have changed since the transaction began.
func (tx *Tx) verify() bool {
for v, version := range tx.reads {
changed := v.loadState().version != version
if changed {
return false
}
}
return true
}
// Writes the values in the transaction log to their respective Vars.
func (tx *Tx) commit() {
for v, val := range tx.writes {
v.changeValue(val)
}
}
// wait blocks until another transaction modifies any of the Vars read by tx.
func (tx *Tx) wait() {
for v := range tx.reads {
v.watchers.Store(tx, nil)
}
tx.mu.Lock()
for tx.verify() {
expvars.Add("waits", 1)
tx.cond.Wait()
}
tx.mu.Unlock()
for v := range tx.reads {
v.watchers.Delete(tx)
}
}
// 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
}
state := v.loadState()
// If we haven't previously read v, record its version
if _, ok := tx.reads[v]; !ok {
tx.reads[v] = state.version
}
return state.val
}
// 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) Return(v interface{}) {
panic(_return{v})
}
type _return struct {
value interface{}
}
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() {
txPool.Put(tx)
}
func (tx *Tx) lockAllVars() {
tx.resetLocks()
tx.collectAllLocks()
tx.sortLocks()
tx.lock()
}
func (tx *Tx) resetLocks() {
tx.locks = tx.locks[:0]
}
func (tx *Tx) collectReadLocks() {
for v := range tx.reads {
tx.locks = append(tx.locks, &v.mu)
}
}
func (tx *Tx) collectAllLocks() {
tx.collectReadLocks()
for v := range tx.writes {
if _, ok := tx.reads[v]; !ok {
tx.locks = append(tx.locks, &v.mu)
}
}
}
func (tx *Tx) sortLocks() {
sort.Slice(tx.locks, func(i, j int) bool {
return uintptr(unsafe.Pointer(tx.locks[i])) < uintptr(unsafe.Pointer(tx.locks[j]))
})
}
func (tx *Tx) lock() {
for _, l := range tx.locks {
l.Lock()
}
}
func (tx *Tx) unlock() {
for _, l := range tx.locks {
l.Unlock()
}
}
|
