// Copyright (c) 2013-2014 The btcsuite developers
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.
package blockchain
import (
"math"
"sort"
"sync"
"time"
"github.com/pkt-cash/pktd/pktlog/log"
)
const (
// maxAllowedOffsetSeconds is the maximum number of seconds in either
// direction that local clock will be adjusted. When the median time
// of the network is outside of this range, no offset will be applied.
maxAllowedOffsetSecs = 70 * 60 // 1 hour 10 minutes
// similarTimeSecs is the number of seconds in either direction from the
// local clock that is used to determine that it is likely wrong and
// hence to show a warning.
similarTimeSecs = 5 * 60 // 5 minutes
)
var (
// maxMedianTimeEntries is the maximum number of entries allowed in the
// median time data. This is a variable as opposed to a constant so the
// test code can modify it.
maxMedianTimeEntries = 200
)
// MedianTimeSource provides a mechanism to add several time samples which are
// used to determine a median time which is then used as an offset to the local
// clock.
type MedianTimeSource interface {
// AdjustedTime returns the current time adjusted by the median time
// offset as calculated from the time samples added by AddTimeSample.
AdjustedTime() time.Time
// AddTimeSample adds a time sample that is used when determining the
// median time of the added samples.
AddTimeSample(id string, timeVal time.Time)
// Offset returns the number of seconds to adjust the local clock based
// upon the median of the time samples added by AddTimeData.
Offset() time.Duration
}
// int64Sorter implements sort.Interface to allow a slice of 64-bit integers to
// be sorted.
type int64Sorter []int64
// Len returns the number of 64-bit integers in the slice. It is part of the
// sort.Interface implementation.
func (s int64Sorter) Len() int {
return len(s)
}
// Swap swaps the 64-bit integers at the passed indices. It is part of the
// sort.Interface implementation.
func (s int64Sorter) Swap(i, j int) {
s[i], s[j] = s[j], s[i]
}
// Less returns whether the 64-bit integer with index i should sort before the
// 64-bit integer with index j. It is part of the sort.Interface
// implementation.
func (s int64Sorter) Less(i, j int) bool {
return s[i] < s[j]
}
// medianTime provides an implementation of the MedianTimeSource interface.
// It is limited to maxMedianTimeEntries includes the same buggy behavior as
// the time offset mechanism in Bitcoin Core. This is necessary because it is
// used in the consensus code.
type medianTime struct {
mtx sync.Mutex
knownIDs map[string]struct{}
offsets []int64
offsetSecs int64
invalidTimeChecked bool
}
// Ensure the medianTime type implements the MedianTimeSource interface.
var _ MedianTimeSource = (*medianTime)(nil)
// AdjustedTime returns the current time adjusted by the median time offset as
// calculated from the time samples added by AddTimeSample.
//
// This function is safe for concurrent access and is part of the
// MedianTimeSource interface implementation.
func (m *medianTime) AdjustedTime() time.Time {
m.mtx.Lock()
defer m.mtx.Unlock()
// Limit the adjusted time to 1 second precision.
now := time.Unix(time.Now().Unix(), 0)
return now.Add(time.Duration(m.offsetSecs) * time.Second)
}
// AddTimeSample adds a time sample that is used when determining the median
// time of the added samples.
//
// This function is safe for concurrent access and is part of the
// MedianTimeSource interface implementation.
func (m *medianTime) AddTimeSample(sourceID string, timeVal time.Time) {
m.mtx.Lock()
defer m.mtx.Unlock()
// Don't add time data from the same source.
if _, exists := m.knownIDs[sourceID]; exists {
return
}
m.knownIDs[sourceID] = struct{}{}
// Truncate the provided offset to seconds and append it to the slice
// of offsets while respecting the maximum number of allowed entries by
// replacing the oldest entry with the new entry once the maximum number
// of entries is reached.
now := time.Unix(time.Now().Unix(), 0)
offsetSecs := int64(timeVal.Sub(now).Seconds())
numOffsets := len(m.offsets)
if numOffsets == maxMedianTimeEntries && maxMedianTimeEntries > 0 {
m.offsets = m.offsets[1:]
numOffsets--
}
m.offsets = append(m.offsets, offsetSecs)
numOffsets++
// Sort the offsets so the median can be obtained as needed later.
sortedOffsets := make([]int64, numOffsets)
copy(sortedOffsets, m.offsets)
sort.Sort(int64Sorter(sortedOffsets))
offsetDuration := time.Duration(offsetSecs) * time.Second
log.Debugf("Added time sample of %v (total: %v)", offsetDuration,
numOffsets)
// NOTE: The following code intentionally has a bug to mirror the
// buggy behavior in Bitcoin Core since the median time is used in the
// consensus rules.
//
// In particular, the offset is only updated when the number of entries
// is odd, but the max number of entries is 200, an even number. Thus,
// the offset will never be updated again once the max number of entries
// is reached.
// The median offset is only updated when there are enough offsets and
// the number of offsets is odd so the middle value is the true median.
// Thus, there is nothing to do when those conditions are not met.
if numOffsets < 5 || numOffsets&0x01 != 1 {
return
}
// At this point the number of offsets in the list is odd, so the
// middle value of the sorted offsets is the median.
median := sortedOffsets[numOffsets/2]
// Set the new offset when the median offset is within the allowed
// offset range.
if math.Abs(float64(median)) < maxAllowedOffsetSecs {
m.offsetSecs = median
} else {
// The median offset of all added time data is larger than the
// maximum allowed offset, so don't use an offset. This
// effectively limits how far the local clock can be skewed.
m.offsetSecs = 0
if !m.invalidTimeChecked {
m.invalidTimeChecked = true
// Find if any time samples have a time that is close
// to the local time.
var remoteHasCloseTime bool
for _, offset := range sortedOffsets {
if math.Abs(float64(offset)) < similarTimeSecs {
remoteHasCloseTime = true
break
}
}
// Warn if none of the time samples are close.
if !remoteHasCloseTime {
log.Warnf("Please check your date and time " +
"are correct! cjdcoind will not work " +
"properly with an invalid time")
}
}
}
medianDuration := time.Duration(m.offsetSecs) * time.Second
log.Debugf("New time offset: %v", medianDuration)
}
// Offset returns the number of seconds to adjust the local clock based upon the
// median of the time samples added by AddTimeData.
//
// This function is safe for concurrent access and is part of the
// MedianTimeSource interface implementation.
func (m *medianTime) Offset() time.Duration {
m.mtx.Lock()
defer m.mtx.Unlock()
return time.Duration(m.offsetSecs) * time.Second
}
// NewMedianTime returns a new instance of concurrency-safe implementation of
// the MedianTimeSource interface. The returned implementation contains the
// rules necessary for proper time handling in the chain consensus rules and
// expects the time samples to be added from the timestamp field of the version
// message received from remote peers that successfully connect and negotiate.
func NewMedianTime() MedianTimeSource {
return &medianTime{
knownIDs: make(map[string]struct{}),
offsets: make([]int64, 0, maxMedianTimeEntries),
}
}