Merge bitcoin/bitcoin#20295: rpc: getblockfrompeer

dce8c4c381 rpc: getblockfrompeer (Sjors Provoost)
b884ababc2 rpc: move Ensure* helpers to server_util.h (Sjors Provoost)

Pull request description:

  This adds an RPC method to fetch a block directly from a peer. This can used to fetch stale blocks with lower proof of work that are normally ignored by the node (`headers-only` in `getchaintips`).

  Usage:
  ```
  bitcoin-cli getblockfrompeer HASH peer_n
  ```

  Closes #20155

  Limitations:
  * you have to specify which peer to fetch the block from
  * the node must already have the header

ACKs for top commit:
  jnewbery:
    ACK dce8c4c381
  fjahr:
     re-ACK dce8c4c381

Tree-SHA512: 843ba2b7a308f640770d624d0aa3265fdc5c6ea48e8db32269b96a082b7420f7953d1d8d1ef2e6529392c7172dded9d15639fbc9c24e7bfa5cfb79e13a5498c8

src/net_processing.cpp

@@ -312,6 +312,7 @@ public:
     /** Implement PeerManager */
     void StartScheduledTasks(CScheduler& scheduler) override;
     void CheckForStaleTipAndEvictPeers() override;
+    bool FetchBlock(NodeId id, const uint256& hash, const CBlockIndex& index) override;
     bool GetNodeStateStats(NodeId nodeid, CNodeStateStats& stats) const override;
     bool IgnoresIncomingTxs() override { return m_ignore_incoming_txs; }
     void SendPings() override;
@@ -1427,6 +1428,41 @@ bool PeerManagerImpl::BlockRequestAllowed(const CBlockIndex* pindex)
            (GetBlockProofEquivalentTime(*pindexBestHeader, *pindex, *pindexBestHeader, m_chainparams.GetConsensus()) < STALE_RELAY_AGE_LIMIT);
 }
 
+bool PeerManagerImpl::FetchBlock(NodeId id, const uint256& hash, const CBlockIndex& index)
+{
+    if (fImporting || fReindex) return false;
+
+    LOCK(cs_main);
+    // Ensure this peer exists and hasn't been disconnected
+    CNodeState* state = State(id);
+    if (state == nullptr) return false;
+    // Ignore pre-segwit peers
+    if (!state->fHaveWitness) return false;
+
+    // Mark block as in-flight unless it already is
+    if (!BlockRequested(id, index)) return false;
+
+    // Construct message to request the block
+    std::vector<CInv> invs{CInv(MSG_BLOCK | MSG_WITNESS_FLAG, hash)};
+
+    // Send block request message to the peer
+    bool success = m_connman.ForNode(id, [this, &invs](CNode* node) {
+        const CNetMsgMaker msgMaker(node->GetCommonVersion());
+        this->m_connman.PushMessage(node, msgMaker.Make(NetMsgType::GETDATA, invs));
+        return true;
+    });
+
+    if (success) {
+        LogPrint(BCLog::NET, "Requesting block %s from peer=%d\n",
+                 hash.ToString(), id);
+    } else {
+        RemoveBlockRequest(hash);
+        LogPrint(BCLog::NET, "Failed to request block %s from peer=%d\n",
+                 hash.ToString(), id);
+    }
+    return success;
+}
+
 std::unique_ptr<PeerManager> PeerManager::make(const CChainParams& chainparams, CConnman& connman, AddrMan& addrman,
                                                BanMan* banman, ChainstateManager& chainman,
                                                CTxMemPool& pool, bool ignore_incoming_txs)