client connection util: exposes the file descriptor when available.

Applies the gridconnect use select link option.
Applies the SO_SNDBUF/RCVBUF link option.

src/utils/ClientConnection.cxx

@@ -0,0 +1,77 @@
+/** \copyright
+ * Copyright (c) 2023, Balazs Racz
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are  permitted provided that the following conditions are met:
+ *
+ *  - Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ *
+ *  - Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ * \file ClientConnection.cxx
+ *
+ * Utilities for managing can-hub connections as a client application.
+ *
+ * @author Balazs Racz
+ * @date 27 Dec 2023
+ */
+
+#include "utils/ClientConnection.hxx"
+
+#include "netinet/in.h"
+#include "netinet/tcp.h"
+#include "nmranet_config.h"
+
+/** Callback from try_connect to donate the file descriptor. @param fd is
+ * the file destriptor of the connection freshly opened.  */
+void GCFdConnectionClient::connection_complete(int fd)
+{
+    struct stat statbuf;
+    fstat(fd, &statbuf);
+
+    const bool use_select =
+        (config_gridconnect_tcp_use_select() == CONSTANT_TRUE);
+    fd_ = fd;
+    const int rcvbuf = config_gridconnect_tcp_rcv_buffer_size();
+    if (rcvbuf > 1)
+    {
+        ::setsockopt(fd, SOL_SOCKET, SO_RCVBUF, &rcvbuf, sizeof(rcvbuf));
+    }
+    const int sndbuf = config_gridconnect_tcp_snd_buffer_size();
+    if (sndbuf > 1)
+    {
+        ::setsockopt(fd, SOL_SOCKET, SO_SNDBUF, &sndbuf, sizeof(sndbuf));
+        int ret = 0;
+        socklen_t retsize = sizeof(ret);
+        ::getsockopt(fd, SOL_SOCKET, SO_SNDBUF, &ret, &retsize);
+        LOG(ALWAYS, "fd %d sndbuf %d", fd, ret);
+    }
+    const int lowat = 4096;
+    if (lowat > 1 && S_ISSOCK(statbuf.st_mode))
+    {
+        ERRNOCHECK("tcp lowat",
+            ::setsockopt(
+                fd, IPPROTO_TCP, TCP_NOTSENT_LOWAT, &lowat, sizeof(lowat)));
+        int ret = 0;
+        socklen_t retsize = sizeof(ret);
+        ::getsockopt(fd, IPPROTO_TCP, TCP_NOTSENT_LOWAT, &ret, &retsize);
+        LOG(ALWAYS, "fd %d lowat %d", fd, ret);
+    }
+    create_gc_port_for_can_hub(hub_, fd, &closedNotify_, use_select);
+}

src/utils/ClientConnection.hxx

@@ -51,6 +51,10 @@ public:
      * is dead. @returns true if there is a live connection. */
     virtual bool ping() = 0;
 
+    /// @return the file descriptor, if this connection has one, or -1 if the
+    /// connection is down or doesn't have an fd.
+    virtual int fd() { return -1; }
+
     virtual ~ConnectionClient()
     {
     }
@@ -114,18 +118,19 @@ public:
         return fd_ >= 0;
     }
 
+    int fd() override
+    {
+        return fd_;
+    }
+
 protected:
     /// Abstrct base function to attempt to connect (or open device) to the
     /// destination.
     virtual void try_connect() = 0;
 
     /** Callback from try_connect to donate the file descriptor. @param fd is
      * the file destriptor of the connection freshly opened.  */
-    void connection_complete(int fd)
-    {
-        fd_ = fd;
-        create_gc_port_for_can_hub(hub_, fd, &closedNotify_);
-    }
+    void connection_complete(int fd);
 
 private:
     /// Will be called when the descriptor experiences an error (typivcally