Release v0.1.0

vendor/github.com/btcsuite/btcd/blockchain/mediantime.go

@@ -0,0 +1,218 @@
+// 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"
+)
+
+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!  btcd 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),
+	}
+}