always set socket to blocking mode before read_message_with_fds's rec…

…vmsg()

libraries/chain/webassembly/runtimes/eos-vm-oc/ipc_helpers.cpp

@@ -7,6 +7,16 @@ static constexpr size_t max_message_size = 8192;
 static constexpr size_t max_num_fds = 4;
 
 std::tuple<bool, eosvmoc_message, std::vector<wrapped_fd>> read_message_with_fds(boost::asio::local::datagram_protocol::socket& s) {
+   // read_message_with_fds() is intended to be blocking, and sockets it is used with are never explicitly set to non-blocking mode.
+   // But when doing an async_wait() on an asio socket, asio will set the underlying file descriptor to non-blocking mode.
+   // It's not clear why, but in some cases after async_wait() indicates readiness, a recvmsg() on the socket can fail with
+   // EAGAIN if the file descriptor is still in non-blocking mode (as asio had set it to).
+   // Always set the file descriptor to blocking mode before performing the recvmsg()
+   boost::system::error_code ec;
+   s.native_non_blocking(false, ec);
+   if(ec)
+      wlog("Failed to set socket's native blocking mode");
+
    return read_message_with_fds(s.native_handle());
 }