Freeze the pivot header for partial state nodes to ensure stable state
sync progress:
- Suppress pivot movement in fetchHeaders() (beaconsync.go)
- Suppress pivot movement in processSnapSyncContent() (downloader.go)
- Reuse existing pivot across sync cycle restarts in syncToHead()
After initial snap sync completes, bridge the gap from pivot to HEAD:
- Import post-pivot blocks with receipts (no execution needed since
untracked contracts have empty storage tries)
- Run second state sync to download HEAD state root
- Add AdvancePartialHead to update currentBlock without re-execution
Guard the backfiller for partial state mode:
- suspend() skips Cancel() during active snap sync to prevent
constant cancel/restart cycles from beacon head updates
- resume() skips new sync cycles after partial sync completes
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