Merge bitcoin/bitcoin#22340: p2p: Use legacy relaying to download blocks in blocks-only mode

18c5b23a0f7adf9a08bcaa967ed800badf62d90a [test] Test that -blocksonly nodes still serve compact blocks. (Niklas Gögge)
a79ad65fc2c31a2b1132a6aab389d0197a95c395 [test] Test that getdata(CMPCT) is still sent on regular low bandwidth connections. (Niklas Gögge)
5e231c116ba8165e9d8c795b85ca2833238aed54 [test] Test that -blocksonly nodes do not send getdata(CMPCT) on a low bandwidth connection. (Niklas Gögge)
5bf658745782eb571215d6c1e5fe3c655edb55b3 [test] Test that -blocksonly nodes do not request high bandwidth mode. (Niklas Gögge)
0dc8bf5b925ca876c0b1a100e426056d741aafde [net processing] Dont request compact blocks in blocks-only mode (Niklas Gögge)

Pull request description:

  A blocks-only node does not participate in transaction relay to reduce its own bandwidth usage and therefore does not have a mempool. The use of compact blocks is not beneficial to such a node since it will always have to download full blocks.

  In both high- and low-bandwidth relaying the `cmpctblock` message is sent. This represent a bandwidth overhead for blocks-only nodes because the `cmpctblock` message is several times larger in the average case than the equivalent `headers` or `inv` announcement.

  ![compact blocks](https://raw.githubusercontent.com/bitcoin/bips/master/bip-0152/protocol-flow.png)

  >**Example:**
  >A block with 2000 txs results in a `cmpctblock` with 2000*6 bytes in short ids. This is several times larger than the equivalent 82 bytes for a `headers` message or 37 bytes for an `inv`.

  ## Approach

  This PR makes blocks-only nodes always use the legacy relaying to download new blocks.
  It does so by making blocks-only nodes never initiate a high-bandwidth block relay connection by disabling the sending of `sendcmpct(1)`. Additionally a blocks-only node will never request a compact block using `getdata(CMPCT)`.

  A blocks-only node will continue to serve compact blocks to its peers in both high- and low-bandwidth mode.

ACKs for top commit:
  naumenkogs:
    ACK 18c5b23a0f7adf9a08bcaa967ed800badf62d90a
  rajarshimaitra:
    tACK https://github.com/bitcoin/bitcoin/pull/22340/commits/18c5b23a0f7adf9a08bcaa967ed800badf62d90a
  jnewbery:
    reACK 18c5b23a0f
  theStack:
    re-ACK 18c5b23a0f7adf9a08bcaa967ed800badf62d90a 🥛

Tree-SHA512: 0c78804aa397513d41f97fe314efb815efcd852d452dd903df9d4749280cd3faaa010fa9b51d7d5168b8a77e08c8ab0a491ecdbdb3202f2e9cd5137cddc74624

1 files changed, 11 insertions, 1 deletions

diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index e130272ff1..008b4d679c 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -884,6 +884,12 @@ bool PeerManagerImpl::BlockRequested(NodeId nodeid, const CBlockIndex& block, st
 void PeerManagerImpl::MaybeSetPeerAsAnnouncingHeaderAndIDs(NodeId nodeid)
 {
     AssertLockHeld(cs_main);
+
+    // Never request high-bandwidth mode from peers if we're blocks-only. Our
+    // mempool will not contain the transactions necessary to reconstruct the
+    // compact block.
+    if (m_ignore_incoming_txs) return;
+
     CNodeState* nodestate = State(nodeid);
     if (!nodestate || !nodestate->fSupportsDesiredCmpctVersion) {
         // Never ask from peers who can't provide witnesses.
@@ -2165,7 +2171,11 @@ void PeerManagerImpl::ProcessHeadersMessage(CNode& pfrom, const Peer& peer,
                             pindexLast->GetBlockHash().ToString(), pindexLast->nHeight);
                 }
                 if (vGetData.size() > 0) {
-                    if (nodestate->fSupportsDesiredCmpctVersion && vGetData.size() == 1 && mapBlocksInFlight.size() == 1 && pindexLast->pprev->IsValid(BLOCK_VALID_CHAIN)) {
+                    if (!m_ignore_incoming_txs &&
+                        nodestate->fSupportsDesiredCmpctVersion &&
+                        vGetData.size() == 1 &&
+                        mapBlocksInFlight.size() == 1 &&
+                        pindexLast->pprev->IsValid(BLOCK_VALID_CHAIN)) {
                         // In any case, we want to download using a compact block, not a regular one
                         vGetData[0] = CInv(MSG_CMPCT_BLOCK, vGetData[0].hash);
                     }