While chainparams should explicilty set values for each possible
entry in vDeployments, in the past that has been missed resulting
in potential undefined behaviour due to accessing unitinitialized
data. Reduce the severity of future bugs of that nature by providing
benign default values. Adds a unit test to alert if the default value
is not overwritten for the real chains (NEVER_ACTIVE/NEVER_ACTIVE rather
than NEVER_ACTIVE/NO_TIMEOUT).
cccc1e70b8 Enforce Taproot script flags whenever WITNESS is set (MarcoFalke)
fa42299411 Remove nullptr check in GetBlockScriptFlags (MarcoFalke)
faadc606c7 refactor: Pass const reference instead of pointer to GetBlockScriptFlags (MarcoFalke)
Pull request description:
Now that Taproot is active, it makes sense to enforce its rules on all blocks, even historic ones, regardless of the deployment status.
### Benefits:
(With "script flags" I mean "taproot script verification flags".)
* Script flags are known ahead for all blocks (even blocks not yet created) and do not change. This may benefit static analysis, code review, and development of new script features that build on Taproot.
* Any future bugs introduced in the deployment code won't have any effect on the script flags, as they are independent of deployment.
* Enforcing the taproot rules regardless of the deployment status makes testing easier because invalid blocks after activation are also invalid before activation. So there is no need to differentiate the two cases.
* It gives belt-and-suspenders protection against a practically expensive and theoretically impossible IBD reorg attack where the node is eclipsed. While `nMinimumChainWork` already protects against this, the cost for a few months worth of POW might be lowered until a major version release of Bitcoin Core reaches EOL. The needed work for the attack is the difference between `nMinimumChainWork` and the work at block 709632.
For reference, previously the same was done for P2SH and WITNESS in commit 0a8b7b4b33.
### Implementation:
I found one block which fails verification with the flags applied, so I added a `TaprootException`, similar to the `BIP16Exception`.
For reference, the debug log:
```
ERROR: ConnectBlock(): CheckInputScripts on b10c007c60e14f9d087e0291d4d0c7869697c6681d979c6639dbd960792b4d41 failed with non-mandatory-script-verify-flag (Witness program was passed an empty witness)
BlockChecked: block hash=0000000000000000000f14c35b2d841e986ab5441de8c585d5ffe55ea1e395ad state=non-mandatory-script-verify-flag (Witness program was passed an empty witness)
InvalidChainFound: invalid block=0000000000000000000f14c35b2d841e986ab5441de8c585d5ffe55ea1e395ad height=692261 log2_work=92.988459 date=2021-07-23T08:24:20Z
InvalidChainFound: current best=0000000000000000000067b17a4c0ffd77c29941b15ad356ca8f980af137a25d height=692260 log2_work=92.988450 date=2021-07-23T07:47:31Z
ERROR: ConnectTip: ConnectBlock 0000000000000000000f14c35b2d841e986ab5441de8c585d5ffe55ea1e395ad failed, non-mandatory-script-verify-flag (Witness program was passed an empty witness)
```
Hint for testing, make sure to set `-noassumevalid`.
### Considerations
Obviously this change can lead to consensus splits on the network in light of massive reorgs. Currently the last block before Taproot activation, that is the last block without the Taproot script flags set, is only buried by a few days of POW. However, when and if this patch is included in the next major release, it will be buried by a few months of POW. BIP90 considerations apply when looking at reorgs this large.
ACKs for top commit:
Sjors:
tACK cccc1e70b8
achow101:
ACK cccc1e70b8
laanwj:
Code review ACK cccc1e70b8
ajtowns:
ACK cccc1e70b8 ; code review; wrote a "getblockscriptflags" rpc to quickly check that blocks just had bit 17 (taproot) added; review of earlier revisions had established non-exception blocks do validate with taproot rules enabled.
jamesob:
ACK cccc1e70b8 ([`jamesob/ackr/23536.1.MarcoFalke.enforce_taproot_script_f`](https://github.com/jamesob/bitcoin/tree/ackr/23536.1.MarcoFalke.enforce_taproot_script_f))
Tree-SHA512: 00044de68939caef6420ffd588c1291c041a8b397c80a3df1e3e3487fbeae1821d23975c51c95e44e774558db76f943b00b4e27cbd0213f64a9253116dc6edde
Adds support for versionbits deployments to DeploymentEnabled,
DeploymentActiveAfter and DeploymentActiveAt. Also moves versionbitscache
from validation to deploymentstatus.
Provides DeploymentEnabled, DeploymentActiveAt, and DeploymentActiveAfter
helpers for checking the status of buried deployments. Can be overloaded
so the same syntax works for non-buried deployments, allowing future
soft forks to be changed from signalled to buried deployments without
having to touch the implementation code.
Replaces IsWitnessEnabled and IsScriptWitnessEnabled.
Previously we used deployments that would timeout prior to Bitcoin's
invention, which allowed the deployment to still be activated in unit
tests. This switches those deployments to be truly never active.
This commit moves P2SH activation back to the genesis block, with
a hardcoded exception for the one historical block in the chain that
violated this rule.
This disentangles the script validation skipping from checkpoints.
A new option is introduced "assumevalid" which specifies a block whos
ancestors we assume all have valid scriptsigs and so we do not check
them when they are also burried under the best header by two weeks
worth of work.
Unlike checkpoints this has no influence on consensus unless you set
it to a block with an invalid history. Because of this it can be
easily be updated without risk of influencing the network consensus.
This results in a massive IBD speedup.
This approach was independently recommended by Peter Todd and Luke-Jr
since POW based signature skipping (see PR#9180) does not have the
verifiable properties of a specific hash and may create bad incentives.
The downside is that, like checkpoints, the defaults bitrot and older
releases will sync slower. On the plus side users can provide their
own value here, and if they set it to something crazy all that will
happen is more time will be spend validating signatures.
Checkblocks and checklevel are also moved to the hidden debug options:
Especially now that checkblocks has a low default there is little need
to change these settings, and users frequently misunderstand them as
influencing security or IBD speed. By hiding them we offset the
space added by this new option.
This introduces a 'minimum chain work' chainparam which is intended
to be the known amount of work in the chain for the network at the
time of software release. If you don't have this much work, you're
not yet caught up.
This is used instead of the count of blocks test from checkpoints.
This criteria is trivial to keep updated as there is no element of
subjectivity, trust, or position dependence to it. It is also a more
reliable metric of sync status than a block count.
- BIP9DeploymentInfo struct for static deployment info
- VersionBitsDeploymentInfo: Avoid C++11ism by commenting parameter names
- getblocktemplate: Make sure to set deployments in the version if it is LOCKED_IN
- In this commit, all rules are considered required for clients to support
