db3c25cfae index: add explicit early exit in NextSyncBlock() when the input is the chain tip (Hao Xu)
Pull request description:
When `pindex_prev` is the chain tip, `NextSyncBlock()` should return `nullptr` (there's no next block to sync). Currently this works, but relies on the fork/reorg path to produce the result: `chain.Next(tip)` returns `nullptr`, falling through to `FindFork(tip)` which returns `tip` itself, then `chain.Next(tip)` returns `nullptr` again.
Add an explicit early return for this case. This makes `NextSyncBlock()`'s three cases self-documenting:
1. next block exists on the active chain — return it
2. at the chain tip — return `nullptr`
3. on a stale branch — find the fork point and return the block after it
It also makes the existing comment ("Since block is not in the chain") accurate — the tip is in the chain, so it shouldn't reach that path.
ACKs for top commit:
optout21:
crACK db3c25cfae
furszy:
ACK db3c25cfae
stickies-v:
ACK db3c25cfae
Tree-SHA512: c1633bb3d3ffed2643c8e174c2b2283deaeffd0a06eb3fb2cf03eed097bfdc5cf6fbd7ebdcdb44fd56f712be8004ad46c730a33990fb1fc6cfa629c9b14f8cd9
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