Add CaptureMessage

This commit adds the CaptureMessage function.  This will later be called
when any message is sent or received.  The capture directory is fixed,
in a new folder "message_capture" in the datadir.  Peers will then have
their own subfolders, named with their IP address and port, replacing
colons with underscores to keep compatibility with Windows.  Inside,
received and sent messages will be captured into two binary files,
msgs_recv.dat and msgs_sent.dat.

e.g.
message_capture/203.0.113.7_56072/msgs_recv.dat
message_capture/203.0.113.7_56072/msgs_sent.dat

The format has been designed as to result in a minimal performance
impact.  A parsing script is added in a later commit.

src/init.cpp

@@ -1042,16 +1042,17 @@ bool AppInitParameterInteraction(const ArgsManager& args)
 
     // Trim requested connection counts, to fit into system limitations
     // <int> in std::min<int>(...) to work around FreeBSD compilation issue described in #2695
-    nFD = RaiseFileDescriptorLimit(nMaxConnections + MIN_CORE_FILEDESCRIPTORS + MAX_ADDNODE_CONNECTIONS + nBind);
+    nFD = RaiseFileDescriptorLimit(nMaxConnections + MIN_CORE_FILEDESCRIPTORS + MAX_ADDNODE_CONNECTIONS + nBind + NUM_FDS_MESSAGE_CAPTURE);
+
 #ifdef USE_POLL
     int fd_max = nFD;
 #else
     int fd_max = FD_SETSIZE;
 #endif
-    nMaxConnections = std::max(std::min<int>(nMaxConnections, fd_max - nBind - MIN_CORE_FILEDESCRIPTORS - MAX_ADDNODE_CONNECTIONS), 0);
+    nMaxConnections = std::max(std::min<int>(nMaxConnections, fd_max - nBind - MIN_CORE_FILEDESCRIPTORS - MAX_ADDNODE_CONNECTIONS - NUM_FDS_MESSAGE_CAPTURE), 0);
     if (nFD < MIN_CORE_FILEDESCRIPTORS)
         return InitError(_("Not enough file descriptors available."));
-    nMaxConnections = std::min(nFD - MIN_CORE_FILEDESCRIPTORS - MAX_ADDNODE_CONNECTIONS, nMaxConnections);
+    nMaxConnections = std::min(nFD - MIN_CORE_FILEDESCRIPTORS - MAX_ADDNODE_CONNECTIONS - NUM_FDS_MESSAGE_CAPTURE, nMaxConnections);
 
     if (nMaxConnections < nUserMaxConnections)
         InitWarning(strprintf(_("Reducing -maxconnections from %d to %d, because of system limitations."), nUserMaxConnections, nMaxConnections));

src/net.cpp

@@ -2929,3 +2929,31 @@ uint64_t CConnman::CalculateKeyedNetGroup(const CAddress& ad) const
 
     return GetDeterministicRandomizer(RANDOMIZER_ID_NETGROUP).Write(vchNetGroup.data(), vchNetGroup.size()).Finalize();
 }
+
+void CaptureMessage(const CAddress& addr, const std::string& msg_type, const Span<const unsigned char>& data, bool is_incoming)
+{
+    // Note: This function captures the message at the time of processing,
+    // not at socket receive/send time.
+    // This ensures that the messages are always in order from an application
+    // layer (processing) perspective.
+    auto now = GetTime<std::chrono::microseconds>();
+
+    // Windows folder names can not include a colon
+    std::string clean_addr = addr.ToString();
+    std::replace(clean_addr.begin(), clean_addr.end(), ':', '_');
+
+    fs::path base_path = GetDataDir() / "message_capture" / clean_addr;
+    fs::create_directories(base_path);
+
+    fs::path path = base_path / (is_incoming ? "msgs_recv.dat" : "msgs_sent.dat");
+    CAutoFile f(fsbridge::fopen(path, "ab"), SER_DISK, CLIENT_VERSION);
+
+    ser_writedata64(f, now.count());
+    f.write(msg_type.data(), msg_type.length());
+    for (auto i = msg_type.length(); i < CMessageHeader::COMMAND_SIZE; ++i) {
+        f << '\0';
+    }
+    uint32_t size = data.size();
+    ser_writedata32(f, size);
+    f.write((const char*)data.data(), data.size());
+}

src/net.h

@@ -20,6 +20,7 @@
 #include <policy/feerate.h>
 #include <protocol.h>
 #include <random.h>
+#include <span.h>
 #include <streams.h>
 #include <sync.h>
 #include <threadinterrupt.h>
@@ -75,6 +76,8 @@ static constexpr uint64_t DEFAULT_MAX_UPLOAD_TARGET = 0;
 static const bool DEFAULT_BLOCKSONLY = false;
 /** -peertimeout default */
 static const int64_t DEFAULT_PEER_CONNECT_TIMEOUT = 60;
+/** Number of file descriptors required for message capture **/
+static const int NUM_FDS_MESSAGE_CAPTURE = 1;
 
 static const bool DEFAULT_FORCEDNSSEED = false;
 static const size_t DEFAULT_MAXRECEIVEBUFFER = 5 * 1000;
@@ -1239,6 +1242,9 @@ inline std::chrono::microseconds PoissonNextSend(std::chrono::microseconds now,
     return std::chrono::microseconds{PoissonNextSend(now.count(), average_interval.count())};
 }
 
+/** Dump binary message to file, with timestamp */
+void CaptureMessage(const CAddress& addr, const std::string& msg_type, const Span<const unsigned char>& data, bool is_incoming);
+
 struct NodeEvictionCandidate
 {
     NodeId id;