Continue receiving data for the current messages, even when we select to send

Author: psgreco

src/net.cpp

@@ -1425,6 +1425,9 @@ bool CConnman::GenerateSelectSet(const std::vector<CNode*>& nodes,
         //   write buffer in this case before receiving more. This avoids
         //   needlessly queueing received data, if the remote peer is not themselves
         //   receiving data. This means properly utilizing TCP flow control signalling.
+        //   This logic can put both nodes in deadlock if they are both "not receiving",
+        //   so there is a special case where we only stop receiving new messages, but
+        //   keep processing the in-progress ones.
         // * Otherwise, if there is space left in the receive buffer, select() for
         //   receiving data.
         // * Hand off all complete messages to the processor, to be handled without
@@ -1445,7 +1448,9 @@ bool CConnman::GenerateSelectSet(const std::vector<CNode*>& nodes,
         error_set.insert(pnode->m_sock->Get());
         if (select_send) {
             send_set.insert(pnode->m_sock->Get());
-            continue;
+            // Only stop receiving new messages, but keep processing incomplete ones
+            if (pnode->m_deserializer->IsEmpty())
+                continue;
         }
         if (select_recv) {
             recv_set.insert(pnode->m_sock->Get());

src/net.h

@@ -313,6 +313,8 @@ class CNetMessage {
  */
 class TransportDeserializer {
 public:
+    // returns true if the current deserialization is empty
+    virtual bool IsEmpty() const = 0;
     // returns true if the current deserialization is complete
     virtual bool Complete() const = 0;
     // set the serialization context version
@@ -363,6 +365,10 @@ class V1TransportDeserializer final : public TransportDeserializer
         Reset();
     }
 
+    bool IsEmpty() const override
+    {
+        return (nHdrPos == 0);
+    }
     bool Complete() const override
     {
         if (!in_data)