Merge bitcoin/bitcoin#29519: p2p: For assumeutxo, download snapshot chain before background chain

49d569c p2p: For assumeutxo, download snapshot chain before background chain (Martin Zumsande)
7a88551 p2p: Restrict downloading of blocks for snapshot chain (Martin Zumsande)

Pull request description:

  After loading a snapshot, `pindexLastCommonBlock` is usually already set to some block for existing peers. That means we'd continue syncing the background chain from those peers instead of prioritising the snapshot chain, which defeats the purpose of doing assumeutxo in the first place. Only existing peers are affected by this bug.

ACKs for top commit:
  fjahr:
    re-ACK 49d569c
  achow101:
    ACK 49d569c
  Sjors:
    tACK 49d569c

Tree-SHA512: 0eaebe1c29a8510d5ced57e14c09b128ccb34b491692815291df68bf12e2a15b52b1e7bf8d9f34808904e7f7bc20f70b0ad0f7e14df93bbdf456bd12cc02a5d2

Author: achow101

src/net_processing.cpp

@@ -1518,9 +1518,20 @@ void PeerManagerImpl::FindNextBlocksToDownload(const Peer& peer, unsigned int co
         return;
     }
 
-    if (state->pindexLastCommonBlock == nullptr) {
-        // Bootstrap quickly by guessing a parent of our best tip is the forking point.
-        // Guessing wrong in either direction is not a problem.
+    // When we sync with AssumeUtxo and discover the snapshot is not in the peer's best chain, abort:
+    // We can't reorg to this chain due to missing undo data until the background sync has finished,
+    // so downloading blocks from it would be futile.
+    const CBlockIndex* snap_base{m_chainman.GetSnapshotBaseBlock()};
+    if (snap_base && state->pindexBestKnownBlock->GetAncestor(snap_base->nHeight) != snap_base) {
+        LogDebug(BCLog::NET, "Not downloading blocks from peer=%d, which doesn't have the snapshot block in its best chain.\n", peer.m_id);
+        return;
+    }
+
+    // Bootstrap quickly by guessing a parent of our best tip is the forking point.
+    // Guessing wrong in either direction is not a problem.
+    // Also reset pindexLastCommonBlock after a snapshot was loaded, so that blocks after the snapshot will be prioritised for download.
+    if (state->pindexLastCommonBlock == nullptr ||
+        (snap_base && state->pindexLastCommonBlock->nHeight < snap_base->nHeight)) {
         state->pindexLastCommonBlock = m_chainman.ActiveChain()[std::min(state->pindexBestKnownBlock->nHeight, m_chainman.ActiveChain().Height())];
     }