feat: Add testnet4 chain support (recreated)

[bullet-tooth]

This PR is a recreation of the previous PR because I incidentally dropped a brunch and the PR was closed automatically.

Description

This PR adds support of the Bitcoin testnet version 4 chain (testnet3 is in a way of deprecation).

Reference: https://bips.dev/94/

Testnet4 genesis block: https://mempool.space/testnet4/block/00000000da84f2bafbbc53dee25a72ae507ff4914b867c565be350b0da8bf043

[guggero]

Thanks for the updates. Definitely looks better now.

Have a couple of fixes that are required, otherwise things crash quite immediately (please test things before requesting review in the future).

After those fixes I was able to sync up to block height 70513

[guggero]

nit: line length. just do this?

	expectedHash := "00000000da84f2bafbbc53dee25a72ae507ff4914b867c565be3" +
		"50b0da8bf043"
	require.Equal(t, expectedHash, hash.String())

[guggero]

Could add it like this to be consistent with the rest:

var testNet4GenesisBlockBytes = []byte{
	0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, /* |........| */
	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, /* |........| */
	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, /* |........| */
	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, /* |........| */
	0x00, 0x00, 0x00, 0x00, 0x4e, 0x7b, 0x2b, 0x91, /* |....N{+.| */
	0x28, 0xfe, 0x02, 0x91, 0xdb, 0x06, 0x93, 0xaf, /* |(.......| */
	0x2a, 0xe4, 0x18, 0xb7, 0x67, 0xe6, 0x57, 0xcd, /* |*...g.W.| */
	0x40, 0x7e, 0x80, 0xcb, 0x14, 0x34, 0x22, 0x1e, /* |@~...4".| */
	0xae, 0xa7, 0xa0, 0x7a, 0x04, 0x6f, 0x35, 0x66, /* |...z.o5f| */
	0xff, 0xff, 0x00, 0x1d, 0xbb, 0x0c, 0x78, 0x17, /* |......x.| */
	0x01, 0x01, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, /* |........| */
	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, /* |........| */
	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, /* |........| */
	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, /* |........| */
	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xff, 0xff, /* |........| */
	0xff, 0xff, 0x55, 0x04, 0xff, 0xff, 0x00, 0x1d, /* |..U.....| */
	0x01, 0x04, 0x4c, 0x4c, 0x30, 0x33, 0x2f, 0x4d, /* |..LL03/M| */
	0x61, 0x79, 0x2f, 0x32, 0x30, 0x32, 0x34, 0x20, /* |ay/2024 | */
	0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, /* |00000000| */
	0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, /* |00000000| */
	0x30, 0x30, 0x30, 0x30, 0x31, 0x65, 0x62, 0x64, /* |00001ebd| */
	0x35, 0x38, 0x63, 0x32, 0x34, 0x34, 0x39, 0x37, /* |58c24497| */
	0x30, 0x62, 0x33, 0x61, 0x61, 0x39, 0x64, 0x37, /* |0b3aa9d7| */
	0x38, 0x33, 0x62, 0x62, 0x30, 0x30, 0x31, 0x30, /* |83bb0010| */
	0x31, 0x31, 0x66, 0x62, 0x65, 0x38, 0x65, 0x61, /* |11fbe8ea| */
	0x38, 0x65, 0x39, 0x38, 0x65, 0x30, 0x30, 0x65, /* |8e98e00e| */
	0xff, 0xff, 0xff, 0xff, 0x01, 0x00, 0xf2, 0x05, /* |........| */
	0x2a, 0x01, 0x00, 0x00, 0x00, 0x23, 0x21, 0x00, /* |*....#!.| */
	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, /* |........| */
	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, /* |........| */
	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, /* |........| */
	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, /* |........| */
	0xac, 0x00, 0x00, 0x00, 0x00, /* |.....   | */
}

[guggero]

This is a 4th level if nested block. That's just not readable IMO. This is consensus critical code, readability should be the second-highest priority (after correctness).

Especially if it can be un-nested with early returns in a function.

My proposal:

		// Testnet4 only: Check timestamp against prev for
		// difficulty-adjustment blocks to prevent timewarp attacks.
		if c.ChainParams().EnforceBIP94 {
			err := assertNoTimeWarp(
				blockHeight, c.BlocksPerRetarget(),
				header.Timestamp,
				time.Unix(prevNode.Timestamp(), 0),
			)
			if err != nil {
				return err
			}
		}

...


// 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
}

[guggero]

Looks like we use time.Time{} in other places, which has a different semantic.

Should replace with:

			DeploymentStarter: NewMedianTimeDeploymentStarter(
				time.Time{}, // Always available for vote
			),
			DeploymentEnder: NewMedianTimeDeploymentEnder(
				time.Time{}, // Never expires
			),

[guggero]

This has incorrect endianness and should be 0x283f161c.

[guggero]

Seems like we need the CSV and SegWit deployments here as well, otherwise things panicked on my machine:

			DeploymentTestDummyMinActivation: {
				BitNumber:                 22,
				CustomActivationThreshold: 1815,    // Only needs 90% hash rate.
				MinActivationHeight:       10_0000, // Can only activate after height 10k.
				DeploymentStarter: NewMedianTimeDeploymentStarter(
					time.Time{}, // Always available for vote
				),
				DeploymentEnder: NewMedianTimeDeploymentEnder(
					time.Time{}, // Never expires
				),
			},
			DeploymentCSV: {
				BitNumber: 29,
				DeploymentStarter: NewMedianTimeDeploymentStarter(
					time.Time{}, // Always available for vote
				),
				DeploymentEnder: NewMedianTimeDeploymentEnder(
					time.Time{}, // Never expires
				),
			},
			DeploymentSegwit: {
				BitNumber: 29,
				DeploymentStarter: NewMedianTimeDeploymentStarter(
					time.Time{}, // Always available for vote
				),
				DeploymentEnder: NewMedianTimeDeploymentEnder(
					time.Time{}, // Never expires
				),
			},