feat: Add new GetNumberOfConnectedPeers() API

fairmq/Channel.h

@@ -186,6 +186,14 @@ class Channel
     /// @return true/false, true if automatic binding is enabled
     bool GetAutoBind() const { return fAutoBind; }
 
+    /// @par Thread Safety
+    /// * @e Distinct @e objects: Safe.@n
+    /// * @e Shared @e objects: Unsafe.
+    auto GetNumberOfConnectedPeers() const
+    {
+        return fSocket ? fSocket->GetNumberOfConnectedPeers() : 0;
+    }
+
     /// Set channel name
     /// @param name Arbitrary channel name
     void UpdateName(const std::string& name) { fName = name; Invalidate(); }

fairmq/Socket.h

@@ -72,6 +72,8 @@ struct Socket
     virtual unsigned long GetMessagesTx() const = 0;
     virtual unsigned long GetMessagesRx() const = 0;
 
+    virtual unsigned long GetNumberOfConnectedPeers() const = 0;
+
     TransportFactory* GetTransport() { return fTransport; }
     void SetTransport(TransportFactory* transport) { fTransport = transport; }
 

fairmq/ofi/Socket.h

@@ -74,6 +74,11 @@ class Socket final : public fair::mq::Socket
     auto GetMessagesTx() const -> unsigned long override { return fMessagesTx; }
     auto GetMessagesRx() const -> unsigned long override { return fMessagesRx; }
 
+    auto GetNumberOfConnectedPeers() const -> unsigned long override
+    {
+        throw SocketError("not yet implemented");
+    }
+
     static auto GetConstant(const std::string& constant) -> int;
 
     ~Socket() override;

fairmq/shmem/Socket.h

@@ -53,14 +53,16 @@ class Socket final : public fair::mq::Socket
   public:
     Socket(Manager& manager, const std::string& type, const std::string& name, const std::string& id, void* context, FairMQTransportFactory* fac = nullptr)
         : fair::mq::Socket(fac)
-        , fSocket(nullptr)
         , fManager(manager)
         , fId(id + "." + name + "." + type)
+        , fSocket(nullptr)
+        , fMonitorSocket(nullptr)
         , fBytesTx(0)
         , fBytesRx(0)
         , fMessagesTx(0)
         , fMessagesRx(0)
         , fTimeout(100)
+        , fConnectedPeersCount(0)
     {
         assert(context);
 
@@ -70,6 +72,7 @@ class Socket final : public fair::mq::Socket
         }
 
         fSocket = zmq_socket(context, zmq::getConstant(type));
+        fMonitorSocket = zmq::makeMonitorSocket(context, fSocket, fId);
 
         if (fSocket == nullptr) {
             LOG(error) << "Failed creating socket " << fId << ", reason: " << zmq_strerror(errno);
@@ -349,15 +352,17 @@ class Socket final : public fair::mq::Socket
     {
         // LOG(debug) << "Closing socket " << fId;
 
-        if (fSocket == nullptr) {
-            return;
+        if (fSocket && zmq_close(fSocket) != 0) {
+            LOG(error) << "Failed closing data socket " << fId
+                       << ", reason: " << zmq_strerror(errno);
         }
-
-        if (zmq_close(fSocket) != 0) {
-            LOG(error) << "Failed closing socket " << fId << ", reason: " << zmq_strerror(errno);
-        }
-
         fSocket = nullptr;
+
+        if (fMonitorSocket && zmq_close(fMonitorSocket) != 0) {
+            LOG(error) << "Failed closing monitor socket " << fId
+                       << ", reason: " << zmq_strerror(errno);
+        }
+        fMonitorSocket = nullptr;
     }
 
     void SetOption(const std::string& option, const void* value, size_t valueSize) override
@@ -465,6 +470,12 @@ class Socket final : public fair::mq::Socket
         return value;
     }
 
+    unsigned long GetNumberOfConnectedPeers() const override
+    {
+        fConnectedPeersCount = zmq::updateNumberOfConnectedPeers(fConnectedPeersCount, fMonitorSocket);
+        return fConnectedPeersCount;
+    }
+
     unsigned long GetBytesTx() const override { return fBytesTx; }
     unsigned long GetBytesRx() const override { return fBytesRx; }
     unsigned long GetMessagesTx() const override { return fMessagesTx; }
@@ -476,15 +487,17 @@ class Socket final : public fair::mq::Socket
     ~Socket() override { Close(); }
 
   private:
-    void* fSocket;
     Manager& fManager;
     std::string fId;
+    void* fSocket;
+    void* fMonitorSocket;
     std::atomic<unsigned long> fBytesTx;
     std::atomic<unsigned long> fBytesRx;
     std::atomic<unsigned long> fMessagesTx;
     std::atomic<unsigned long> fMessagesRx;
 
     int fTimeout;
+    mutable unsigned long fConnectedPeersCount;
 };
 
 } // namespace fair::mq::shmem

fairmq/zeromq/Common.h

@@ -8,6 +8,8 @@
 #ifndef FAIR_MQ_ZMQ_COMMON_H
 #define FAIR_MQ_ZMQ_COMMON_H
 
+#include <fairlogger/Logger.h>
+#include <fairmq/Error.h>
 #include <fairmq/tools/Strings.h>
 #include <stdexcept>
 #include <string_view>
@@ -53,6 +55,119 @@ inline auto getConstant(std::string_view constant) -> int
     throw Error(tools::ToString("getConstant called with an invalid argument: ", constant));
 }
 
+/// Create a zmq event monitor socket pair, and configure/connect the reading socket
+/// @return reading monitor socket
+inline auto makeMonitorSocket(void* zmqCtx, void* socketToMonitor, std::string_view id) -> void*
+{
+    assertm(zmqCtx, "Given zmq context exists");   // NOLINT
+
+    if (!socketToMonitor) {   // nothing to do in this case
+        return nullptr;
+    }
+
+    auto const address(tools::ToString("inproc://", id));
+    {   // Create writing monitor socket on socket to be monitored and subscribe
+        // to all relevant events needed to compute connected peers
+        // from http://api.zeromq.org/master:zmq-socket-monitor:
+        //
+        // ZMQ_EVENT_CONNECTED - The socket has successfully connected to a remote peer. The event
+        //   value is the file descriptor (FD) of the underlying network socket. Warning: there is
+        //   no guarantee that the FD is still valid by the time your code receives this event.
+        // ZMQ_EVENT_ACCEPTED - The socket has accepted a connection from a remote peer. The event
+        //   value is the FD of the underlying network socket. Warning: there is no guarantee that
+        //   the FD is still valid by the time your code receives this event.
+        // ZMQ_EVENT_DISCONNECTED - The socket was disconnected unexpectedly. The event value is the
+        //   FD of the underlying network socket. Warning: this socket will be closed.
+        auto const rc =
+            zmq_socket_monitor(socketToMonitor,
+                               address.c_str(),
+                               ZMQ_EVENT_CONNECTED | ZMQ_EVENT_ACCEPTED | ZMQ_EVENT_DISCONNECTED);
+        assertm(rc == 0, "Creating writing monitor socket succeeded");   // NOLINT
+    }
+    // Create reading monitor socket
+    auto mon(zmq_socket(zmqCtx, ZMQ_PAIR));
+    assertm(mon, "Creating reading monitor socker succeeded");   // NOLINT
+    {   // Set receive queue size to unlimited on reading socket
+        // Rationale: In the current implementation this is needed for correctness, because
+        //            we do not have any thread that emptys the receive queue regularly.
+        //            Progress only happens, when a user calls GetNumberOfConnectedPeers()`.
+        //            The assumption here is, that not too many events will pile up anyways.
+        int const unlimited(0);
+        auto const rc = zmq_setsockopt(mon, ZMQ_RCVHWM, &unlimited, sizeof(unlimited));
+        assertm(rc == 0, "Setting rcv queue size to unlimited succeeded");   // NOLINT
+    }
+    {   // Connect the reading monitor socket
+        auto const rc = zmq_connect(mon, address.c_str());
+        assertm(rc == 0, "Connecting reading monitor socket succeeded");   // NOLINT
+    }
+    return mon;
+}
+
+/// Read pending zmq monitor event in a non-blocking fashion.
+/// @return event id or -1 for no event pending
+inline auto getMonitorEvent(void* monitorSocket) -> int
+{
+    assertm(monitorSocket, "zmq monitor socket exists");   // NOLINT
+
+    // First frame in message contains event id
+    zmq_msg_t msg;
+    zmq_msg_init(&msg);
+    {
+        auto const size = zmq_msg_recv(&msg, monitorSocket, ZMQ_DONTWAIT);
+        if (size == -1) {
+            return -1;   // no event pending
+        }
+        assertm(size >= 2, "At least two bytes were received");   // NOLINT
+    }
+
+    // Unpack event id
+    auto const event = *static_cast<uint16_t*>(zmq_msg_data(&msg));
+
+    // No unpacking of the event value needed for now
+
+    // Second frame in message contains event address
+    assertm(zmq_msg_more(&msg), "A second frame is pending");   // NOLINT
+    zmq_msg_init(&msg);
+    {
+        auto const rc = zmq_msg_recv(&msg, monitorSocket, 0);
+        assertm(rc >= 0, "second monitor event frame successfully received");   // NOLINT
+    }
+    assertm(!zmq_msg_more(&msg), "No more frames are pending");   // NOLINT
+    // No unpacking of the event address needed for now
+
+    return event;
+}
+
+/// Compute updated connected peers count by consuming pending events from a zmq monitor socket
+/// @return updated connected peers count
+inline auto updateNumberOfConnectedPeers(unsigned long count, void* monitorSocket) -> unsigned long
+{
+    if (monitorSocket == nullptr) {
+        return count;
+    }
+
+    int event = getMonitorEvent(monitorSocket);
+    while (event >= 0) {
+        switch (event) {
+            case ZMQ_EVENT_CONNECTED:
+            case ZMQ_EVENT_ACCEPTED:
+                ++count;
+                break;
+            case ZMQ_EVENT_DISCONNECTED:
+                if (count > 0) {
+                    --count;
+                } else {
+                    LOG(warn) << "Computing connected peers would result in negative count! Some event was missed!";
+                }
+                break;
+            default:
+                break;
+        }
+        event = getMonitorEvent(monitorSocket);
+    }
+    return count;
+}
+
 } // namespace fair::mq::zmq
 
 #endif /* FAIR_MQ_ZMQ_COMMON_H */

fairmq/zeromq/Socket.h

@@ -20,7 +20,9 @@
 #include <zmq.h>
 
 #include <atomic>
+#include <functional>
 #include <memory> // unique_ptr, make_unique
+#include <string_view>
 
 namespace fair::mq::zmq
 {
@@ -31,13 +33,15 @@ class Socket final : public fair::mq::Socket
     Socket(Context& ctx, const std::string& type, const std::string& name, const std::string& id, FairMQTransportFactory* factory = nullptr)
         : fair::mq::Socket(factory)
         , fCtx(ctx)
-        , fSocket(zmq_socket(fCtx.GetZmqCtx(), getConstant(type)))
         , fId(id + "." + name + "." + type)
+        , fSocket(zmq_socket(fCtx.GetZmqCtx(), getConstant(type)))
+        , fMonitorSocket(makeMonitorSocket(fCtx.GetZmqCtx(), fSocket, fId))
         , fBytesTx(0)
         , fBytesRx(0)
         , fMessagesTx(0)
         , fMessagesRx(0)
         , fTimeout(100)
+        , fConnectedPeersCount(0)
     {
         if (fSocket == nullptr) {
             LOG(error) << "Failed creating socket " << fId << ", reason: " << zmq_strerror(errno);
@@ -301,15 +305,17 @@ class Socket final : public fair::mq::Socket
     {
         // LOG(debug) << "Closing socket " << fId;
 
-        if (fSocket == nullptr) {
-            return;
+        if (fSocket && zmq_close(fSocket) != 0) {
+            LOG(error) << "Failed closing data socket " << fId
+                       << ", reason: " << zmq_strerror(errno);
         }
-
-        if (zmq_close(fSocket) != 0) {
-            LOG(error) << "Failed closing socket " << fId << ", reason: " << zmq_strerror(errno);
-        }
-
         fSocket = nullptr;
+
+        if (fMonitorSocket && zmq_close(fMonitorSocket) != 0) {
+            LOG(error) << "Failed closing monitor socket " << fId
+                       << ", reason: " << zmq_strerror(errno);
+        }
+        fMonitorSocket = nullptr;
     }
 
     void SetOption(const std::string& option, const void* value, size_t valueSize) override
@@ -417,6 +423,12 @@ class Socket final : public fair::mq::Socket
         return value;
     }
 
+    unsigned long GetNumberOfConnectedPeers() const override
+    {
+        fConnectedPeersCount = updateNumberOfConnectedPeers(fConnectedPeersCount, fMonitorSocket);
+        return fConnectedPeersCount;
+    }
+
     unsigned long GetBytesTx() const override { return fBytesTx; }
     unsigned long GetBytesRx() const override { return fBytesRx; }
     unsigned long GetMessagesTx() const override { return fMessagesTx; }
@@ -429,14 +441,16 @@ class Socket final : public fair::mq::Socket
 
   private:
     Context& fCtx;
-    void* fSocket;
     std::string fId;
+    void* fSocket;
+    void* fMonitorSocket;
     std::atomic<unsigned long> fBytesTx;
     std::atomic<unsigned long> fBytesRx;
     std::atomic<unsigned long> fMessagesTx;
     std::atomic<unsigned long> fMessagesRx;
 
     int fTimeout;
+    mutable unsigned long fConnectedPeersCount;
 };
 
 } // namespace fair::mq::zmq

test/channel/_channel.cxx

@@ -6,9 +6,14 @@
  *                  copied verbatim in the file "LICENSE"                       *
  ********************************************************************************/
 
+#include <chrono>
 #include <fairmq/Channel.h>
+#include <fairmq/ProgOptions.h>
+#include <fairmq/Tools.h>
+#include <fairmq/TransportFactory.h>
 #include <gtest/gtest.h>
 #include <string>
+#include <thread>
 
 namespace
 {
@@ -91,4 +96,42 @@ TEST(Channel, Validation)
     ASSERT_EQ(channel2.Validate(), true);
 }
 
+auto testConnectedPeers(std::string const& transport)
+{
+    using namespace std::chrono_literals;
+
+    ProgOptions config;
+    config.SetProperty<string>("session", tools::Uuid());
+    string const address(tools::ToString("ipc://", config.GetProperty<string>("session")));
+    unsigned long constexpr zero(0), one(1);
+    auto factory(TransportFactory::CreateTransportFactory(transport, tools::Uuid(), &config));
+
+    Channel ch1("ch1", "pair", factory);
+    ASSERT_EQ(ch1.GetNumberOfConnectedPeers(), zero);
+    ch1.Bind(address);
+    ASSERT_EQ(ch1.GetNumberOfConnectedPeers(), zero);
+
+    {
+        Channel ch2("ch2", "pair", factory);
+        ASSERT_EQ(ch2.GetNumberOfConnectedPeers(), zero);
+        ch2.Connect(address);
+        std::this_thread::sleep_for(10ms);
+        ASSERT_EQ(ch1.GetNumberOfConnectedPeers(), one);
+        ASSERT_EQ(ch2.GetNumberOfConnectedPeers(), one);
+    }
+
+    std::this_thread::sleep_for(10ms);
+    ASSERT_EQ(ch1.GetNumberOfConnectedPeers(), zero);
+}
+
+TEST(Channel, GetNumberOfConnectedPeers_zeromq)
+{
+    testConnectedPeers("zeromq");
+}
+
+TEST(Channel, GetNumberOfConnectedPeers_shmem)
+{
+    testConnectedPeers("shmem");
+}
+
 } /* namespace */