forked from moby/libnetwork
/
lamport.go
45 lines (38 loc) · 1.29 KB
/
lamport.go
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package serf
import (
"sync/atomic"
)
// LamportClock is a thread safe implementation of a lamport clock. It
// uses efficient atomic operations for all of its functions, falling back
// to a heavy lock only if there are enough CAS failures.
type LamportClock struct {
counter uint64
}
// LamportTime is the value of a LamportClock.
type LamportTime uint64
// Time is used to return the current value of the lamport clock
func (l *LamportClock) Time() LamportTime {
return LamportTime(atomic.LoadUint64(&l.counter))
}
// Increment is used to increment and return the value of the lamport clock
func (l *LamportClock) Increment() LamportTime {
return LamportTime(atomic.AddUint64(&l.counter, 1))
}
// Witness is called to update our local clock if necessary after
// witnessing a clock value received from another process
func (l *LamportClock) Witness(v LamportTime) {
WITNESS:
// If the other value is old, we do not need to do anything
cur := atomic.LoadUint64(&l.counter)
other := uint64(v)
if other < cur {
return
}
// Ensure that our local clock is at least one ahead.
if !atomic.CompareAndSwapUint64(&l.counter, cur, other+1) {
// The CAS failed, so we just retry. Eventually our CAS should
// succeed or a future witness will pass us by and our witness
// will end.
goto WITNESS
}
}