chainreg: accommodate buried taproot deployment in Bitcoin Core v31

[pinheadmz]

A PR merged since Bitcoin Core release v30 breaks how lnd detects taproot support:

error creating wallet config: unable to create partial chain control: node backend does not support taproot

This PR removes "taproot" from "deployments" in RPC getdeploymentinfo: bitcoin/bitcoin#26201

Fortunately a second PR adds a new "script_flags" field to RPC getdeploymentinfo: bitcoin/bitcoin#32998

This PR adds a (third) taproot-support check compatible with Bitcoin Core current master and eventually v31.

Note that taproot is active for all blocks since segregated witness is enforced. There is only one exception which is hard-coded into the software.

For your next bitcoin trivia night:

https://github.com/bitcoin/bitcoin/blob/4f8bd396f8ab04f7ea8bde0dce3829c345d6c54b/src/kernel/chainparams.cpp#L87-L88

https://b10c.me/blog/007-spending-p2tr-pre-activation/

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a breaking change in how Bitcoin Core reports taproot support via RPC. By introducing a fallback mechanism that checks for the newly added 'script_flags' field, the system ensures continued compatibility with upcoming Bitcoin Core releases where taproot is no longer listed as a standard BIP9 deployment.

Highlights

• Compatibility Update: Updated the taproot support check to remain compatible with Bitcoin Core v31 and later versions.
• New Verification Method: Implemented a check for the new 'script_flags' field in the getdeploymentinfo RPC response to detect taproot support.

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[gemini-code-assist]

Code Review

This pull request updates the backendSupportsTaproot function to maintain compatibility with Bitcoin Core v31 and later by adding a ScriptFlags field to the deployment info structure and checking for the 'TAPROOT' flag. The review feedback points out an improvement opportunity to remove a redundant nil check before ranging over the ScriptFlags slice, which is idiomatic in Go.

[erickcestari]

Thanks! I tested it locally with and without the fix, and it works! This PR indeed fixes the issue.

By the way, it's amusing to read the b10c blog post about spending from P2TR outputs before activation, knowing that we now have one more hardcoded block hash in bitcoin nodes. 🫠

There's only one small nit, and then it should be good to go!

[erickcestari]

LGTM!

[Roasbeef]

@pinheadmz can you update the release notes here? That this is the first release that has taproot activation detection compatible with bitcoind v30?