Guard rebranch_to against empty target_chain#3703
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viquezclaudio
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`verify_inferior_chain_macro_block_proposal` calls `fork_chain.remove(0)` to strip the proposed block before invoking `rebranch_to`. If the proposal's parent is the common ancestor, `fork_chain` becomes empty, and `rebranch_to` would panic on `target_chain.last().unwrap()` when updating `main_chain_successor`. This state is not reachable under normal protocol invariants (the non-inferior path would be taken, or `verify_macro_successor` would reject the proposal), but guard defensively so a broken invariant elsewhere cannot turn into a validator-wide crash. Returning `Err(vec![])` funnels into the existing error handling in both callers, which abort the write transaction and surface `PushError::InvalidFork`.
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verify_inferior_chain_macro_block_proposalcallsfork_chain.remove(0)to strip the proposed block before invokingrebranch_to. If the proposal's parent is the common ancestor,fork_chainbecomes empty, andrebranch_towould panic ontarget_chain.last().unwrap()when updatingmain_chain_successor.This state is not reachable under normal protocol invariants (the non-inferior path would be taken, or
verify_macro_successorwould reject the proposal), but guard defensively so a broken invariant elsewhere cannot turn into a validator-wide crash. ReturningErr(vec![])funnels into the existing error handling in both callers, which abort the write transaction and surfacePushError::InvalidFork.Pull request checklist
clippyandrustfmtwarnings.