[net] Don't return an optional from TransportDeserializer::GetMessage()

Also, access mapRecvBytesPerMsgCmd with `at()` not `find()`. This
throws an error if COMMAND_OTHER doesn't exist, which should never
happen. `find()` instead just accessed the last element, which could make
debugging more difficult.

Resolves review comments from PR19107:

- https://github.com/bitcoin/bitcoin/pull/19107#discussion_r478718436
- https://github.com/bitcoin/bitcoin/pull/19107#discussion_r478714497

src/net.cpp

@@ -641,25 +641,26 @@ bool CNode::ReceiveMsgBytes(Span<const uint8_t> msg_bytes, bool& complete)
 
         if (m_deserializer->Complete()) {
             // decompose a transport agnostic CNetMessage from the deserializer
-            uint32_t out_err_raw_size{0};
-            std::optional<CNetMessage> result{m_deserializer->GetMessage(time, out_err_raw_size)};
-            if (!result) {
+            bool reject_message{false};
+            CNetMessage msg = m_deserializer->GetMessage(time, reject_message);
+            if (reject_message) {
                 // Message deserialization failed.  Drop the message but don't disconnect the peer.
                 // store the size of the corrupt message
-                mapRecvBytesPerMsgCmd.find(NET_MESSAGE_COMMAND_OTHER)->second += out_err_raw_size;
+                mapRecvBytesPerMsgCmd.at(NET_MESSAGE_COMMAND_OTHER) += msg.m_raw_message_size;
                 continue;
             }
 
-            //store received bytes per message command
+            // Store received bytes per message command
             // to prevent a memory DOS, only allow valid commands
-            mapMsgCmdSize::iterator i = mapRecvBytesPerMsgCmd.find(result->m_command);
-            if (i == mapRecvBytesPerMsgCmd.end())
+            auto i = mapRecvBytesPerMsgCmd.find(msg.m_command);
+            if (i == mapRecvBytesPerMsgCmd.end()) {
                 i = mapRecvBytesPerMsgCmd.find(NET_MESSAGE_COMMAND_OTHER);
+            }
             assert(i != mapRecvBytesPerMsgCmd.end());
-            i->second += result->m_raw_message_size;
+            i->second += msg.m_raw_message_size;
 
             // push the message to the process queue,
-            vRecvMsg.push_back(std::move(*result));
+            vRecvMsg.push_back(std::move(msg));
 
             complete = true;
         }
@@ -733,16 +734,18 @@ const uint256& V1TransportDeserializer::GetMessageHash() const
     return data_hash;
 }
 
-std::optional<CNetMessage> V1TransportDeserializer::GetMessage(const std::chrono::microseconds time, uint32_t& out_err_raw_size)
+CNetMessage V1TransportDeserializer::GetMessage(const std::chrono::microseconds time, bool& reject_message)
 {
+    // Initialize out parameter
+    reject_message = false;
     // decompose a single CNetMessage from the TransportDeserializer
-    std::optional<CNetMessage> msg(std::move(vRecv));
+    CNetMessage msg(std::move(vRecv));
 
     // store command string, time, and sizes
-    msg->m_command = hdr.GetCommand();
-    msg->m_time = time;
-    msg->m_message_size = hdr.nMessageSize;
-    msg->m_raw_message_size = hdr.nMessageSize + CMessageHeader::HEADER_SIZE;
+    msg.m_command = hdr.GetCommand();
+    msg.m_time = time;
+    msg.m_message_size = hdr.nMessageSize;
+    msg.m_raw_message_size = hdr.nMessageSize + CMessageHeader::HEADER_SIZE;
 
     uint256 hash = GetMessageHash();
 
@@ -752,17 +755,15 @@ std::optional<CNetMessage> V1TransportDeserializer::GetMessage(const std::chrono
     // Check checksum and header command string
     if (memcmp(hash.begin(), hdr.pchChecksum, CMessageHeader::CHECKSUM_SIZE) != 0) {
         LogPrint(BCLog::NET, "Header error: Wrong checksum (%s, %u bytes), expected %s was %s, peer=%d\n",
-                 SanitizeString(msg->m_command), msg->m_message_size,
+                 SanitizeString(msg.m_command), msg.m_message_size,
                  HexStr(Span<uint8_t>(hash.begin(), hash.begin() + CMessageHeader::CHECKSUM_SIZE)),
                  HexStr(hdr.pchChecksum),
                  m_node_id);
-        out_err_raw_size = msg->m_raw_message_size;
-        msg = std::nullopt;
+        reject_message = true;
     } else if (!hdr.IsCommandValid()) {
         LogPrint(BCLog::NET, "Header error: Invalid message type (%s, %u bytes), peer=%d\n",
-                 SanitizeString(hdr.GetCommand()), msg->m_message_size, m_node_id);
-        out_err_raw_size = msg->m_raw_message_size;
-        msg.reset();
+                 SanitizeString(hdr.GetCommand()), msg.m_message_size, m_node_id);
+        reject_message = true;
     }
 
     // Always reset the network deserializer (prepare for the next message)

src/net.h

@@ -309,7 +309,7 @@ public:
     /** read and deserialize data, advances msg_bytes data pointer */
     virtual int Read(Span<const uint8_t>& msg_bytes) = 0;
     // decomposes a message from the context
-    virtual std::optional<CNetMessage> GetMessage(std::chrono::microseconds time, uint32_t& out_err) = 0;
+    virtual CNetMessage GetMessage(std::chrono::microseconds time, bool& reject_message) = 0;
     virtual ~TransportDeserializer() {}
 };
 
@@ -373,7 +373,7 @@ public:
         }
         return ret;
     }
-    std::optional<CNetMessage> GetMessage(std::chrono::microseconds time, uint32_t& out_err_raw_size) override;
+    CNetMessage GetMessage(std::chrono::microseconds time, bool& reject_message) override;
 };
 
 /** The TransportSerializer prepares messages for the network transport

src/test/fuzz/p2p_transport_serialization.cpp

@@ -68,18 +68,16 @@ FUZZ_TARGET_INIT(p2p_transport_serialization, initialize_p2p_transport_serializa
         }
         if (deserializer.Complete()) {
             const std::chrono::microseconds m_time{std::numeric_limits<int64_t>::max()};
-            uint32_t out_err_raw_size{0};
-            std::optional<CNetMessage> result{deserializer.GetMessage(m_time, out_err_raw_size)};
-            if (result) {
-                assert(result->m_command.size() <= CMessageHeader::COMMAND_SIZE);
-                assert(result->m_raw_message_size <= mutable_msg_bytes.size());
-                assert(result->m_raw_message_size == CMessageHeader::HEADER_SIZE + result->m_message_size);
-                assert(result->m_time == m_time);
+            bool reject_message{false};
+            CNetMessage msg = deserializer.GetMessage(m_time, reject_message);
+            assert(msg.m_command.size() <= CMessageHeader::COMMAND_SIZE);
+            assert(msg.m_raw_message_size <= mutable_msg_bytes.size());
+            assert(msg.m_raw_message_size == CMessageHeader::HEADER_SIZE + msg.m_message_size);
+            assert(msg.m_time == m_time);
 
             std::vector<unsigned char> header;
-                auto msg = CNetMsgMaker{result->m_recv.GetVersion()}.Make(result->m_command, MakeUCharSpan(result->m_recv));
-                serializer.prepareForTransport(msg, header);
-            }
+            auto msg2 = CNetMsgMaker{msg.m_recv.GetVersion()}.Make(msg.m_command, MakeUCharSpan(msg.m_recv));
+            serializer.prepareForTransport(msg2, header);
         }
     }
 }