feat: Add function for sending fd

Signed-off-by: ComixHe <ComixHe1895@outlook.com>

Author: ComixHe

libs/common/CMakeLists.txt

@@ -16,6 +16,8 @@ pfl_add_library(
   src/linglong/common/display.h
   src/linglong/common/error.h
   src/linglong/common/error.cpp
+  src/linglong/common/socket.cpp
+  src/linglong/common/socket.h
   src/linglong/common/strings.cpp
   src/linglong/common/strings.h
   src/linglong/common/xdg.cpp

libs/common/src/linglong/common/socket.cpp

@@ -0,0 +1,142 @@
+// SPDX-FileCopyrightText: 2026 UnionTech Software Technology Co., Ltd.
+//
+// SPDX-License-Identifier: LGPL-3.0-or-later
+
+#include "linglong/common/socket.h"
+
+#include "linglong/common/error.h"
+
+#include <sys/ioctl.h>
+
+#include <cstring>
+
+#include <sys/socket.h>
+#include <unistd.h>
+
+namespace linglong::common::socket {
+
+tl::expected<SocketData, std::string> recvFdWithPayload(int socketFd, std::size_t bufSize)
+{
+    if (socketFd < 0) {
+        return tl::make_unexpected("Invalid file descriptor");
+    }
+
+    std::string buffer(bufSize, '\0');
+    struct iovec iov{};
+    iov.iov_base = buffer.data();
+    iov.iov_len = buffer.size();
+
+    alignas(struct cmsghdr) std::array<char, CMSG_SPACE(sizeof(int))> control_buf{};
+
+    struct msghdr msg{};
+    msg.msg_iov = &iov;
+    msg.msg_iovlen = 1;
+    msg.msg_control = control_buf.data();
+    msg.msg_controllen = sizeof(control_buf);
+
+    ssize_t n;
+    while (true) {
+        n = recvmsg(socketFd, &msg, 0);
+        if (n < 0 && errno == EINTR) {
+            continue;
+        }
+
+        break;
+    }
+
+    if (n < 0) {
+        return tl::make_unexpected("recvmsg failed: " + error::errorString(errno));
+    }
+
+    if (n == 0) {
+        return tl::make_unexpected("Connection closed");
+    }
+
+    int received_fd{ -1 };
+    struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+
+    if (cmsg != nullptr && cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SCM_RIGHTS) {
+        std::memcpy(&received_fd, CMSG_DATA(cmsg), sizeof(int));
+    }
+
+    if ((static_cast<std::size_t>(msg.msg_flags) & MSG_CTRUNC) != 0) {
+        if (received_fd != -1) {
+            ::close(received_fd);
+        }
+
+        return tl::make_unexpected("Control data truncated");
+    }
+
+    if (received_fd == -1) {
+        return tl::make_unexpected("No file descriptor received");
+    }
+
+    bool isTruncated{ false };
+    if ((static_cast<std::size_t>(msg.msg_flags) & MSG_TRUNC) != 0) {
+        isTruncated = true;
+    } else if (static_cast<std::size_t>(n) == bufSize) {
+        int bytes_available{ 0 };
+        if (ioctl(socketFd, FIONREAD, &bytes_available) < 0) {
+            return tl::make_unexpected("FIONREAD socket failed: " + error::errorString(errno));
+        }
+
+        if (bytes_available > 0) {
+            isTruncated = true;
+        }
+    }
+
+    buffer.resize(n);
+    return SocketData{ isTruncated, received_fd, std::move(buffer) };
+}
+
+tl::expected<void, std::string> sendFdWithPayload(int socketFd, int fd, const std::string &payload)
+{
+    if (socketFd < 0) {
+        return tl::make_unexpected("Invalid socket file descriptor");
+    }
+
+    std::size_t totalSent{ 0 };
+    bool fdSent{ false };
+
+    while (totalSent < payload.size() || !fdSent) {
+        struct msghdr msg{};
+        struct iovec iov{};
+
+        iov.iov_base = const_cast<char *>(payload.data() + totalSent);
+        iov.iov_len = payload.size() - totalSent;
+        msg.msg_iov = &iov;
+        msg.msg_iovlen = 1;
+
+        alignas(struct cmsghdr) std::array<std::byte, CMSG_SPACE(sizeof(int))> control_buf{};
+        if (!fdSent) {
+            msg.msg_control = control_buf.data();
+            msg.msg_controllen = control_buf.size();
+
+            struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+            cmsg->cmsg_level = SOL_SOCKET;
+            cmsg->cmsg_type = SCM_RIGHTS;
+            cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+            std::memcpy(CMSG_DATA(cmsg), &fd, sizeof(int));
+        }
+
+        const auto n = sendmsg(socketFd, &msg, 0);
+        if (n < 0) {
+            if (errno == EINTR) {
+                continue;
+            }
+
+            return tl::make_unexpected("sendmsg failed: " + error::errorString(errno));
+        }
+
+        fdSent = true;
+        totalSent += n;
+
+        if (payload.empty()) {
+            break;
+        }
+    }
+
+    return {};
+}
+
+} // namespace linglong::common::socket

libs/common/src/linglong/common/socket.h

@@ -0,0 +1,24 @@
+// SPDX-FileCopyrightText: 2026 UnionTech Software Technology Co., Ltd.
+//
+// SPDX-License-Identifier: LGPL-3.0-or-later
+
+#pragma once
+
+#include <tl/expected.hpp>
+
+#include <string>
+
+namespace linglong::common::socket {
+
+struct SocketData
+{
+    bool more_data{false};
+    int fd;
+    std::string payload;
+};
+
+tl::expected<SocketData, std::string> recvFdWithPayload(int socketFd, std::size_t bufSize = 4096);
+
+tl::expected<void, std::string> sendFdWithPayload(int socketFd, int fd, const std::string &payload);
+
+} // namespace linglong::common::socket

libs/linglong/tests/ll-tests/CMakeLists.txt

@@ -17,6 +17,7 @@ pfl_add_executable(
   src/linglong/builder/source_fetcher_test.cpp
   src/linglong/cli/cli_test.cpp
   src/linglong/common/strings_test.cpp
+  src/linglong/common/socket_test.cpp
   src/linglong/common/xdg_test.cpp
   src/linglong/mocks/layer_packager_mock.h
   src/linglong/mocks/linglong_builder_mock.h
@@ -59,6 +60,7 @@ pfl_add_executable(
   src/linglong/utils/xdg/directory_test.cpp
   src/main.cpp
 
+
   # ll-driver-detect tests
   ${PROJECT_SOURCE_DIR}/apps/ll-driver-detect/src/nvidia_driver_detector.cpp
   ${PROJECT_SOURCE_DIR}/apps/ll-driver-detect/src/driver_detection_config.cpp