Constant MAX_BLOCK_REOBTAIN_RETRIES 

const MAX_BLOCK_REOBTAIN_RETRIES: u8 = 3;

Controls how many times the syncer will re-request a block whose download failed because no peer delivered it (a NotFound), before giving up and letting the normal tip re-walk handle it.

Without this re-request, a single missing block at the checkpoint frontier is dropped and never re-fetched, wedging the whole verify pipeline until the 8-minute BLOCK_VERIFY_TIMEOUT fires (#5709). Each attempt already goes through the tower-level BLOCK_DOWNLOAD_RETRY_LIMIT (and hedging), so this is a coarse, hash-scoped retry on top of an exhausted per-request retry.