Testnet 5 (BIP95) #35861

BIP54 counts sigops differently from existing sigops-based checks. Since
we are overloading the sigops term, make clear the constant refers to
BIP54-sigops, not other kinds of pre-existing sigops.

Also update the functional test framework's constant that has
"standardness" in its name, since we are about to make it consensus
critical (func test scripted-diff courtesy of Anthony Towns).

-BEGIN VERIFY SCRIPT-
sed -i 's/MAX_TX_LEGACY_SIGOPS/MAX_TX_BIP54_SIGOPS/g' $(git grep -l MAX_TX_LEGACY_SIGOPS src/)
sed -i 's/MAX_STD_LEGACY_SIGOPS/MAX_TX_BIP54_SIGOPS/g' $(git grep -l MAX_STD_LEGACY_SIGOPS)
sed -i 's/signature operations in validating a transaction./signature operations in a single transaction, per BIP54./' test/functional/test_framework/script_util.py
-END VERIFY SCRIPT-

Co-Authored-by: Anthony Towns <aj@erisian.com.au>

Move the function that checks whether a transaction respects the BIP54 sigops rule to the
consensus folder (along with the accompanying constant), as it will be made consensus-critical
in the next commit. Can be reviewed with git's --color-moved option.

When BIP54 is active, enforce that block transactions do not violate the BIP54 limit on the
number of legacy sigops present in Scripts that get executed during block validation.

In Taproot the signature commits to the list of spent outputs.

Test the newly introduced limit with various combinations of inputs and outputs types,
historical transactions, and exercise some implementation-specific edge cases. Record
each test case and optionally write them to disk as JSON to generate the BIP test vectors.

The fuzz target was specifically crafted to support seeding it with the BIP54 test vectors
generated by the unit test in the previous commit.

We are going to introduce the timewarp fix for mainnet with a greater grace period. Rename
the MAX_TIMEWARP value for testnet to differentiate them.

-BEGIN VERIFY SCRIPT-

for f in $(git grep -l MAX_TIMEWARP); do sed -i "s/MAX_TIMEWARP/MAX_TIMEWARP_TESTNET4/g" "$f"; done

-END VERIFY SCRIPT-

Documentation about the test vectors, including about their structure and content, as well as
reproduction instructions, is available here: https://github.com/bitcoin/bips/tree/master/bip-0054/test_vectors

When BIP 54 is active, coinbase transactions must have their nLockTime field set to the block height
minus 1 (since it encodes the last height at which the transaction is invalid), and their nSequence
field may be anything but the maximum value (which indicates "final", bypassing timelock
validation).

Documentation about the test vectors' structure and content, as well as instructions for generating
them is available at https://github.com/bitcoin/bips/tree/master/bip-0054/test_vectors .

64-byte transactions are also now treated as a consensus failure in
PreChecks, like BIP54-sigops check failures. Note this only changes the
RPC error, and not the disconnection behaviour in P2P since
266dd0e10d08c0bfde63205db15d6c210a021b90.

This adds tests exercising the bounds of the checks on the invalid transaction size, for various
types of transactions (legacy, Segwit, bytes in input/output to get to 64 bytes) as well as
sanity checking against some known historical violations.

Thanks to Chris Stewart for digging up the historical violations to this rule.

The previously introduced unit tests extensively test the specific implementation of each
mitigation. This functional test complements them by end-to-end testing all mitigations.
For the added timestamp constraints, it mimicks how they would get exploited (by implementing pseudo
timewarp and Murch-Zawy attacks) and demonstrates those exploits are not possible anymore after
BIP54 activates.

Testnet 5 is specified in BIP95. It drops the 20-minute minimum-difficulty
exception of Testnet 3/4 and raises the minimum difficulty to nBits
0x1a0fffff.

BIP95 enforces the BIP54 rules on Testnet 5 from block 1. This also covers
timewarp, so BIP94 is not enforced.