cardano-rpc: Implement FollowTip SyncService method

[carbolymer]

Context

Implements the SyncService.FollowTip method (#1219), the primary blocker of the Kupo/Ogmios replacement effort (#1229). It is the gRPC analogue of the Ouroboros ChainSync mini-protocol, built on in-process ChainDB followers (ADR-019).

Stacked on #1259; review the commits after #1259's.

FollowTip streams fully parsed blocks as the chain advances, reusing the same block-to-proto conversion FetchBlock uses. The client's intersect list is honoured in preference order, and the first streamed message is always a reset announcing the resolved start point. An empty-hash block ref denotes origin, handled as an infallible catch-all rather than a special case. An empty intersect list follows from the current tip, matching Dolos's behaviour; an intersect list that matches nothing on the chain fails with NOT_FOUND before any message is sent. Every response carries the current tip alongside the action, for lag measurement.

Rollbacks are delivered as undo messages carrying the rolled-back blocks, newest first. Consensus's RollBack is point-only, it never carries block data, so each stream tracks the points of its last k applied blocks (the security parameter, about 86 KB per stream on mainnet) and re-fetches the undone blocks from the ChainDB on rollback. The stream falls back to a reset at the rollback point in the two cases where undo is impossible: the rollback target is outside the tracked window, or a re-fetch misses because the block was garbage-collected first.

Implementation notes

• NodeKernelAccess gains withFollower, a bracket over ChainDB.newFollower that closes the follower and its resource registry on every exit path, including client disconnect, plus the ChainFollower/ChainChange types wrapping the raw follower API.
• Cardano.Api.Consensus gains the follower re-exports (newFollower, Follower, ChainType, ChainUpdate, withRegistry, ResourceRegistry).
• FollowTip is the first server-streaming handler in cardano-rpc, wired via grapesy's mkServerStreaming.
• A TraceRpcFollowTipSpan trace wraps the handler; the node-side rendering and the rpc.request.SyncService.FollowTip prometheus counter land in gRPC: fetchBlock method cardano-node#6579, the fetchblock-series node PR that already carries the earlier tracing and E2E work (there is no dedicated FollowTip node PR).
• The dead TraceRpcForkerError constructor is removed.

How to trust this PR

The unit suite (83 tests) covers the conversion layer.

The E2E test in IntersectMBO/cardano-node#6579 exercises the streaming semantics live: full sync from origin, resuming from a point, an empty intersect list resolving from the current tip, and the NOT_FOUND path for an unmatched intersect list. It also adds a live-tail scenario, being added right now in the node working tree, that submits a transaction over gRPC and asserts it arrives via FollowTip as a parsed apply action.

Run it with:

cabal test cardano-rpc-test
TASTY_PATTERN='/RPC FollowTip/' cabal test cardano-testnet-test

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff
• Changelog fragment added in .changes/

[Copilot]

Pull request overview

This PR implements the SyncService.FollowTip server-streaming RPC in cardano-rpc, providing a ChainSync-like block stream backed by in-process ChainDB followers (ADR-019), and factors shared block/point conversion logic so FollowTip and FetchBlock reuse the same proto assembly.

Changes:

• Implement followTipMethod / followTipStream with ChainDB follower intersection, streaming apply/undo/reset actions, and per-message tip reporting.
• Add NodeKernelAccess.withFollower plus ChainFollower/ChainChange wrappers; plumb consensus securityParam through NodeKernelAccess to size the undo window.
• Add conversion helpers (mkAnyChainBlock, mkTipBlockRef, chainPointToBlockRef) and unit tests for the streaming loop; wire tracing + README coverage update.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
cardano-rpc/test/cardano-rpc-test/Test/Cardano/Rpc/FollowTipStream.hs	New unit tests covering followTipStream sequencing, intersection failure, and rollback/undo vs reset fallback.
cardano-rpc/src/Cardano/Rpc/Server/NodeKernelAccess/Type.hs	Add securityParam to NodeKernelAccess to size rollback tracking.
cardano-rpc/src/Cardano/Rpc/Server/NodeKernelAccess.hs	Add follower lifecycle bracket (withFollower) and follower wrappers (ChainFollower, ChainChange).
cardano-rpc/src/Cardano/Rpc/Server/Internal/UtxoRpc/Type/ChainPoint.hs	Add shared helpers for tip/blockref projection and timestamp encoding.
cardano-rpc/src/Cardano/Rpc/Server/Internal/UtxoRpc/Type/Block.hs	New shared block-to-proto conversion (mkAnyChainBlock) used by both fetch and follow.
cardano-rpc/src/Cardano/Rpc/Server/Internal/UtxoRpc/Sync.hs	Implement FollowTip; refactor FetchBlock/ReadTip to reuse shared conversion helpers; add followTipStream.
cardano-rpc/src/Cardano/Rpc/Server/Internal/Tracing.hs	Add TraceRpcFollowTipSpan; remove dead TraceRpcForkerError.
cardano-rpc/src/Cardano/Rpc/Server.hs	Wire FollowTip into the gRPC method table as a server-streaming handler.
cardano-rpc/README.md	Mark FollowTip as supported in the SyncService coverage table.
cardano-rpc/cardano-rpc.cabal	Register new internal module and test; add test dependency needed for fixture decoding.
cardano-api/src/Cardano/Api/Consensus/Internal/Reexport.hs	Re-export follower/security-param/registry types and functions used by cardano-rpc.
cardano-api/src/Cardano/Api/Consensus.hs	Expose follower API and security-param helpers via the public consensus facade.
cardano-api/cardano-api.cabal	Add resource-registry dependency needed for withRegistry / ResourceRegistry re-exports.
.changes/20260722_cardano_rpc_follow_tip.yml	Changelog fragment for the FollowTip feature (needs kind update per review comment).
.changes/20260722_cardano_api_chaindb_follower_reexports.yml	Changelog fragment for new cardano-api re-exports.

[Jimbo4350]

LGTM. Some of the comments on your functions are difficult to decipher however I wouldn't block the PR on that. One comment for you to consider.

[Jimbo4350]

What about clients who just want the raw bytes and don't want to pay the decode cost at this point?

[carbolymer]

Great question - and I'm glad you asked, because this is exactly where I want to take this next!

Right now they can't avoid it: the AnyChainBlock response message carries both native_bytes and the parsed cardano block, so the handler has to decode every block just to populate the response, regardless of what the client wants.

The exciting part: the spec already has the answer built in - the field_mask in FollowTipRequest/FetchBlockRequest, which cardano-rpc currently ignores (like every other UTxO RPC method here).
I've just opened #1273 to fix that properly: the mask will drive response construction itself, so a mask excluding the parsed cardano.* fields selects a raw-bytes-only BlockComponent and the decode never happens at all.
Raw-bytes-only followers get dramatically cheaper streaming, practically for free.
Out of scope for this PR, but very much on the roadmap!