Fix zero-byte datagram handling and kqueue SO_NOSIGPIPE guard

- Preserve source endpoint for zero-byte datagrams on both
  reactor inline path (n >= 0) and IOCP completion handler
- Allow zero-length UDP sends instead of short-circuiting them
- Guard SO_NOSIGPIPE with #ifdef on kqueue UDP service

Author: sgerbino

include/boost/corosio/native/detail/iocp/win_udp_service.hpp

@@ -194,7 +194,7 @@ recv_from_op::do_complete(
     // Extract source endpoint on success
     bool success =
         (op->dwError == 0 && !op->cancelled.load(std::memory_order_acquire));
-    if (success && op->source_out && op->bytes_transferred > 0)
+    if (success && op->source_out)
     {
         *op->source_out = from_sockaddr(op->source_storage);
     }
@@ -382,13 +382,6 @@ win_udp_socket_internal::send_to(
     op.wsabuf_count =
         static_cast<DWORD>(param.copy_to(bufs, send_to_op::max_buffers));
 
-    // Handle empty buffer: complete immediately with 0 bytes
-    if (op.wsabuf_count == 0 || (op.wsabuf_count == 1 && bufs[0].size() == 0))
-    {
-        svc_.on_completion(&op, 0, 0);
-        return std::noop_coroutine();
-    }
-
     for (DWORD i = 0; i < op.wsabuf_count; ++i)
     {
         op.wsabufs[i].buf = static_cast<char*>(bufs[i].data());
@@ -553,12 +546,6 @@ win_udp_socket_internal::send(
     op.wsabuf_count =
         static_cast<DWORD>(param.copy_to(bufs, udp_send_op::max_buffers));
 
-    if (op.wsabuf_count == 0 || (op.wsabuf_count == 1 && bufs[0].size() == 0))
-    {
-        svc_.on_completion(&op, 0, 0);
-        return std::noop_coroutine();
-    }
-
     for (DWORD i = 0; i < op.wsabuf_count; ++i)
     {
         op.wsabufs[i].buf = static_cast<char*>(bufs[i].data());

include/boost/corosio/native/detail/kqueue/kqueue_udp_service.hpp

@@ -264,14 +264,13 @@ kqueue_udp_service::open_datagram_socket(
         return make_err(errn);
     }
 
-    // SO_NOSIGPIPE for macOS/BSD
-    int one = 1;
-    if (::setsockopt(fd, SOL_SOCKET, SO_NOSIGPIPE, &one, sizeof(one)) != 0)
+    // SO_NOSIGPIPE on macOS (where MSG_NOSIGNAL doesn't exist)
+#ifdef SO_NOSIGPIPE
     {
-        int errn = errno;
-        ::close(fd);
-        return make_err(errn);
+        int one = 1;
+        ::setsockopt(fd, SOL_SOCKET, SO_NOSIGPIPE, &one, sizeof(one));
     }
+#endif
 
     kq_impl->fd_ = fd;
 

include/boost/corosio/native/detail/reactor/reactor_datagram_socket.hpp

@@ -272,19 +272,6 @@ reactor_datagram_socket<
     op.iovec_count =
         static_cast<int>(param.copy_to(bufs, SendToOp::max_buffers));
 
-    if (op.iovec_count == 0 || (op.iovec_count == 1 && bufs[0].size() == 0))
-    {
-        op.h         = h;
-        op.ex        = ex;
-        op.ec_out    = ec;
-        op.bytes_out = bytes_out;
-        op.start(token, static_cast<Derived*>(this));
-        op.impl_ptr = this->shared_from_this();
-        op.complete(0, 0);
-        this->svc_.post(&op);
-        return std::noop_coroutine();
-    }
-
     for (int i = 0; i < op.iovec_count; ++i)
     {
         op.iovecs[i].iov_base = bufs[i].data();
@@ -433,7 +420,7 @@ reactor_datagram_socket<
         {
             *ec        = err ? make_err(err) : std::error_code{};
             *bytes_out = bytes;
-            if (source && !err && n > 0)
+            if (source && !err && n >= 0)
                 *source = from_sockaddr(op.source_storage);
             return dispatch_coro(ex, h);
         }
@@ -580,19 +567,6 @@ reactor_datagram_socket<
     capy::mutable_buffer bufs[SendOp::max_buffers];
     op.iovec_count = static_cast<int>(param.copy_to(bufs, SendOp::max_buffers));
 
-    if (op.iovec_count == 0 || (op.iovec_count == 1 && bufs[0].size() == 0))
-    {
-        op.h         = h;
-        op.ex        = ex;
-        op.ec_out    = ec;
-        op.bytes_out = bytes_out;
-        op.start(token, static_cast<Derived*>(this));
-        op.impl_ptr = this->shared_from_this();
-        op.complete(0, 0);
-        this->svc_.post(&op);
-        return std::noop_coroutine();
-    }
-
     for (int i = 0; i < op.iovec_count; ++i)
     {
         op.iovecs[i].iov_base = bufs[i].data();