CHAINS-6848 Include testnet 4 support

.gitignore

@@ -46,12 +46,15 @@ btcutil/psbt/coverage.txt
 *.swo
 /.vim
 
+#IDE
+.idea
+
 # Binaries produced by "make build"
 /addblock
 /btcctl
 /btcd
 /findcheckpoint
 /gencerts
 
-#Goland
-.idea
+.DS_Store
+.aider*

blockchain/difficulty.go

@@ -191,12 +191,20 @@ func calcNextRequiredDifficulty(lastNode HeaderCtx, newBlockTime time.Time,
 		adjustedTimespan = c.MaxRetargetTimespan()
 	}
 
+	// Special difficulty rule for Testnet4
+	oldTarget := CompactToBig(lastNode.Bits())
+	if c.ChainParams().EnforceBIP94 {
+		// Here we use the first block of the difficulty period. This way
+		// the real difficulty is always preserved in the first block as
+		// it is not allowed to use the min-difficulty exception.
+		oldTarget = CompactToBig(firstNode.Bits())
+	}
+
 	// Calculate new target difficulty as:
 	//  currentDifficulty * (adjustedTimespan / targetTimespan)
 	// The result uses integer division which means it will be slightly
 	// rounded down.  Bitcoind also uses integer division to calculate this
 	// result.
-	oldTarget := CompactToBig(lastNode.Bits())
 	newTarget := new(big.Int).Mul(oldTarget, big.NewInt(adjustedTimespan))
 	targetTimeSpan := int64(c.ChainParams().TargetTimespan / time.Second)
 	newTarget.Div(newTarget, big.NewInt(targetTimeSpan))

blockchain/error.go

@@ -220,6 +220,10 @@ const (
 	// current chain tip. This is not a block validation rule, but is required
 	// for block proposals submitted via getblocktemplate RPC.
 	ErrPrevBlockNotBest
+
+	// ErrTimewarpAttack indicates a timewarp attack i.e.
+	// when block's timestamp is too early on diff adjustment block.
+	ErrTimewarpAttack
 )
 
 // Map of ErrorCode values back to their constant names for pretty printing.

blockchain/thresholdstate.go

@@ -102,6 +102,11 @@ type thresholdConditionChecker interface {
 	// not the bit associated with the condition is set, but can be more
 	// complex as needed.
 	Condition(*blockNode) (bool, error)
+
+	// ForceActive returns if the deployment should be forced to transition
+	// to the active state. This is useful on certain testnet, where we
+	// we'd like for a deployment to always be active.
+	ForceActive(*blockNode) bool
 }
 
 // thresholdStateCache provides a type to cache the threshold states of each
@@ -279,7 +284,17 @@ func thresholdStateTransition(state ThresholdState, prevNode *blockNode,
 // threshold states for previous windows are only calculated once.
 //
 // This function MUST be called with the chain state lock held (for writes).
-func (b *BlockChain) thresholdState(prevNode *blockNode, checker thresholdConditionChecker, cache *thresholdStateCache) (ThresholdState, error) {
+func (b *BlockChain) thresholdState(prevNode *blockNode,
+	checker thresholdConditionChecker,
+	cache *thresholdStateCache) (ThresholdState, error) {
+
+	// If the deployment has a nonzero AlwaysActiveHeight and the next
+	// block’s height is at or above that threshold, then force the state
+	// to Active.
+	if checker.ForceActive(prevNode) {
+		return ThresholdActive, nil
+	}
+
 	// The threshold state for the window that contains the genesis block is
 	// defined by definition.
 	confirmationWindow := int32(checker.MinerConfirmationWindow())

blockchain/thresholdstate_test.go

@@ -175,6 +175,10 @@ func (c customDeploymentChecker) Condition(_ *blockNode) (bool, error) {
 	return c.conditionTrue, nil
 }
 
+func (c customDeploymentChecker) ForceActive(_ *blockNode) bool {
+	return false
+}
+
 // TestThresholdStateTransition tests that the thresholdStateTransition
 // properly implements the BIP 009 state machine, along with the speedy trial
 // augments.

blockchain/validate.go

@@ -46,6 +46,11 @@ const (
 	// coinbaseHeightAllocSize is the amount of bytes that the
 	// ScriptBuilder will allocate when validating the coinbase height.
 	coinbaseHeightAllocSize = 5
+
+	// maxTimeWarp is a maximum number of seconds that the timestamp of the first
+	// block of a difficulty adjustment period is allowed to
+	// be earlier than the last block of the previous period (BIP94).
+	maxTimeWarp = 600 * time.Second
 )
 
 var (
@@ -684,6 +689,12 @@ func compareScript(height int32, script []byte) error {
 func CheckBlockHeaderContext(header *wire.BlockHeader, prevNode HeaderCtx,
 	flags BehaviorFlags, c ChainCtx, skipCheckpoint bool) error {
 
+	// The height of this block is one more than the referenced previous
+	// block.
+	blockHeight := prevNode.Height() + 1
+
+	params := c.ChainParams()
+
 	fastAdd := flags&BFFastAdd == BFFastAdd
 	if !fastAdd {
 		// Ensure the difficulty specified in the block header matches
@@ -710,16 +721,24 @@ func CheckBlockHeaderContext(header *wire.BlockHeader, prevNode HeaderCtx,
 			str = fmt.Sprintf(str, header.Timestamp, medianTime)
 			return ruleError(ErrTimeTooOld, str)
 		}
-	}
 
-	// The height of this block is one more than the referenced previous
-	// block.
-	blockHeight := prevNode.Height() + 1
+		// Testnet4 only: Check timestamp against prev for
+		// difficulty-adjustment blocks to prevent timewarp attacks.
+		if params.EnforceBIP94 {
+			err := assertNoTimeWarp(
+				blockHeight, c.BlocksPerRetarget(),
+				header.Timestamp,
+				time.Unix(prevNode.Timestamp(), 0),
+			)
+			if err != nil {
+				return err
+			}
+		}
+	}
 
 	// Reject outdated block versions once a majority of the network
 	// has upgraded.  These were originally voted on by BIP0034,
 	// BIP0065, and BIP0066.
-	params := c.ChainParams()
 	if header.Version < 2 && blockHeight >= params.BIP0034Height ||
 		header.Version < 3 && blockHeight >= params.BIP0066Height ||
 		header.Version < 4 && blockHeight >= params.BIP0065Height {
@@ -761,6 +780,30 @@ func CheckBlockHeaderContext(header *wire.BlockHeader, prevNode HeaderCtx,
 	return nil
 }
 
+// assertNoTimeWarp checks the timestamp of the block against the previous
+// block's timestamp for the first block of each difficulty adjustment interval
+// to prevent timewarp attacks. This is defined in BIP-0094.
+func assertNoTimeWarp(blockHeight, blocksPerReTarget int32, headerTimestamp,
+	prevBlockTimestamp time.Time) error {
+
+	// If this isn't the first block of the difficulty adjustment interval,
+	// then we can exit early.
+	if blockHeight%blocksPerReTarget != 0 {
+		return nil
+	}
+
+	// Check timestamp for the first block of each difficulty adjustment
+	// interval, except the genesis block.
+	if headerTimestamp.Before(prevBlockTimestamp.Add(-maxTimeWarp)) {
+		str := "block's timestamp %v is too early on diff adjustment " +
+			"block %v"
+		str = fmt.Sprintf(str, headerTimestamp, prevBlockTimestamp)
+		return ruleError(ErrTimewarpAttack, str)
+	}
+
+	return nil
+}
+
 // checkBlockContext performs several validation checks on the block which depend
 // on its position within the block chain.
 //
@@ -1230,7 +1273,7 @@ func (b *BlockChain) checkConnectBlock(node *blockNode, block *btcutil.Block, vi
 	if csvState == ThresholdActive {
 		// If the CSV soft-fork is now active, then modify the
 		// scriptFlags to ensure that the CSV op code is properly
-		// validated during the script checks bleow.
+		// validated during the script checks below.
 		scriptFlags |= txscript.ScriptVerifyCheckSequenceVerify
 
 		// We obtain the MTP of the *previous* block in order to