Trait EventFloorTracksCheckpoints 

pub trait EventFloorTracksCheckpoints: SequentialChainState { }

Invariant (A stream’s readable floor is its first index since the last checkpoint). For every event stream of a chain, StreamCounts::first_index is the index of the first event published to that stream since the most recent checkpoint, and the indices from there up to StreamCounts::next_index are contiguous. Events below the floor may have been pruned and are unavailable to a node that bootstrapped from that checkpoint; events at or above it are untouched by checkpointing.

This is the sense in which pre-checkpoint events are summarized rather than merely dropped: what a checkpoint discards is bounded, and the boundary is a published number rather than something a reader has to infer.

Proof. ChainStateView’s process_emitted_events maintains both fields whenever a block is processed — executed or merely preprocessed — and branches on whether the block records a predecessor for the stream in previous_event_blocks, the per-stream map holding the hash and height of the last block that emitted to it. That map is built during execution and is covered by the block hash through previous_event_blocks_hash, so every node branches the same way on the same block.

• A predecessor is recorded. The stream has published since the last checkpoint, so its events must continue contiguously. If they do not — lo != counts.next_index — the tracker is left untouched, so a gap never advances next_index over a missing index and never moves the floor.
• No predecessor, and lo >= first_index. This block is the first to emit to the stream since a checkpoint. lo becomes the new floor and next_index advances to max(next_index, hi + 1).
• No predecessor, and lo < first_index. An earlier checkpoint era, already superseded by a later one this chain has recorded. It is ignored, so the floor does not move backwards. ∎

The floor only ever rises, which is what makes it safe to publish. The >= guard in the second branch exists for checkpoints seen out of order — an earlier one preprocessed after a later one — and without it a stale checkpoint would lower a floor that readers had already relied on.

What a reader may conclude. A cross-chain read of an event through OracleResponse::Event is guaranteed to find it only at an index at or above that stream’s floor. Below the floor, availability depends on some node not having pruned, which no statement here provides.

Implementors