detect UDP RX buffer overflows

On Linux, use SO_RXQ_OVFL to snoop on the OS dropped packet
counter for our socket buffer.  Does not detect drops on
ingress, or internal to, to OS IP stack.

src/util.cpp

@@ -15,6 +15,7 @@
 
 #include <ctype.h>
 
+#include <pvxs/log.h>
 #include <pvxs/util.h>
 #include <pvxs/sharedArray.h>
 #include <pvxs/data.h>
@@ -25,6 +26,8 @@
 
 namespace pvxs {
 
+DEFINE_LOGGER(log, "pvxs.util");
+
 #define stringifyX(X) #X
 #define stringify(X) stringifyX(X)
 
@@ -286,6 +289,54 @@ SigInt::~SigInt()
 
 #endif // !defined(__rtems__) && !defined(vxWorks)
 
+void enable_SO_RXQ_OVFL(SOCKET sock)
+{
+#ifdef SO_RXQ_OVFL
+    // Linux specific feature exposes OS dropped packet count
+    {
+        int val = 1;
+        if(setsockopt(sock, SOL_SOCKET, SO_RXQ_OVFL, (char*)&val, sizeof(val)))
+            log_warn_printf(log, "Unable to set SO_RXQ_OVFL: %d\n", SOCKERRNO);
+    }
+#endif
+}
+
+int recvfromx(SOCKET sock, void *buf, size_t buflen, sockaddr* peer, osiSocklen_t* peerlen, uint32_t *ndrop)
+{
+#ifdef SO_RXQ_OVFL
+    alignas (alignof (msghdr)) char cbuf[CMSG_SPACE(4u)];
+    iovec iov = {buf, buflen};
+    msghdr msg = {};
+    msg.msg_iov = &iov;
+    msg.msg_iovlen = 1u;
+    msg.msg_name = peer;
+    msg.msg_namelen = peerlen ? *peerlen : 0;
+    msg.msg_control = cbuf;
+    msg.msg_controllen = sizeof(cbuf);
+
+    int ret = recvmsg(sock, &msg, 0);
+
+    if(ret>=0 && peerlen)
+        *peerlen = msg.msg_namelen;
+
+    if(msg.msg_flags & MSG_CTRUNC)
+        log_debug_printf(log, "MSG_CTRUNC %zu, %zu\n", msg.msg_controllen, sizeof(cbuf));
+
+    if(ndrop) {
+        for(cmsghdr *hdr = CMSG_FIRSTHDR(&msg); hdr ; hdr = CMSG_NXTHDR(&msg, hdr)) {
+            if(hdr->cmsg_level==SOL_SOCKET && hdr->cmsg_type==SO_RXQ_OVFL && hdr->cmsg_len>=CMSG_LEN(4u)) {
+                memcpy(ndrop, CMSG_DATA(hdr), 4u);
+            }
+        }
+    }
+
+    return ret;
+
+#else
+    return recvfrom(sock, (char*)buf, buflen, 0, peer, peerlen);
+#endif
+}
+
 SockAddr::SockAddr(int af)
 {
     memset(&store, 0, sizeof(store));