p2p: Start downloading historical blocks from common ancestor

Otherwise, if the background tip is not an ancestor of the snapshot, blocks in between that ancestor up to the height of the background tip will never be requested.

Co-authored-by: Martin Zumsande <[email protected]>
Co-authored-by: Alfonso Roman Zubeldia <[email protected]>

Author: mzumsande

src/net_processing.cpp

@@ -6293,10 +6293,13 @@ bool PeerManagerImpl::SendMessages(CNode* pto)
             // before the background chainstate to prioritize getting to network tip.
             FindNextBlocksToDownload(*peer, get_inflight_budget(), vToDownload, staller);
             if (m_chainman.BackgroundSyncInProgress() && !IsLimitedPeer(*peer)) {
+                // If the background tip is not an ancestor of the snapshot block,
+                // we need to start requesting blocks from their last common ancestor.
+                const CBlockIndex *from_tip = LastCommonAncestor(m_chainman.GetBackgroundSyncTip(), m_chainman.GetSnapshotBaseBlock());
                 TryDownloadingHistoricalBlocks(
                     *peer,
                     get_inflight_budget(),
-                    vToDownload, m_chainman.GetBackgroundSyncTip(),
+                    vToDownload, from_tip,
                     Assert(m_chainman.GetSnapshotBaseBlock()));
             }
             for (const CBlockIndex *pindex : vToDownload) {