fix(auth): release daemon descriptors in session child

Close inherited daemon file descriptors in the forked session leader
so old D-Bus connections cannot retain the display manager service
names. Fail fast when D-Bus registration is unavailable and restore
the original service start limit.

Log: DDM 通过 fork 创建 session leader 时，子进程若继承 system D-Bus
fd，旧 DDM 主进程退出后仍可能保留 org.deepin.DisplayManager 服务名，阻
止新 DDM 注册。session child 仅保留标准流和与父进程通信的 pipe，并关闭
其余 daemon fd；DDM 注册 D-Bus 服务或对象失败时立即退出，避免 systemd
显示 active 而 D-Bus 实际不可用。该修复不处理 systemd start-limit-hit；
DDM 被限流停止时必须 reset-failed 后重新启动。
PMS:
Influence: Affects session startup and DDM D-Bus ownership across daemon restarts.

Author: zorowk

src/daemon/Auth.cpp

@@ -17,11 +17,54 @@
 #include <security/pam_appl.h>
 #include <signal.h>
 #include <unistd.h>
+#include <climits>
+#include <sys/syscall.h>
 #include <utmp.h>
 #include <utmpx.h>
 
 namespace DDM {
 
+#ifdef SYS_close_range
+    bool tryCloseInheritedWithCloseRange(int preservedFd)
+    {
+        const unsigned int first = STDERR_FILENO + 1;
+        const unsigned int last  = UINT_MAX;
+
+        // No preserved FD in range: just close everything
+        if (preservedFd <= static_cast<int>(first) || preservedFd > static_cast<int>(last))
+            return syscall(SYS_close_range, first, last, 0) == 0;
+
+        // Close [first, preservedFd - 1]
+        if (syscall(SYS_close_range,
+                    first,
+                    static_cast<unsigned int>(preservedFd - 1),
+                    0) == -1)
+            return false;
+
+        // Close [preservedFd + 1, last]
+        if (syscall(SYS_close_range,
+                    static_cast<unsigned int>(preservedFd + 1),
+                    last,
+                    0) == -1)
+            return false;
+
+        return true;
+    }
+#endif
+
+    void closeInheritedFileDescriptors(int preservedFd)
+    {
+#ifdef SYS_close_range
+        if (tryCloseInheritedWithCloseRange(preservedFd))
+            return;
+#endif
+        const long maxFd = sysconf(_SC_OPEN_MAX);
+        for (int fd = STDERR_FILENO + 1; fd < maxFd; ++fd) {
+            if (fd != preservedFd)
+                close(fd);
+        }
+    }
+
     ///////////////////////////
     // utmp helper functions //
     ///////////////////////////
@@ -255,6 +298,10 @@ namespace DDM {
             // Delete old signal handlers, in order to close old fds
             // which are shared with the parent process.
             delete daemonApp->signalHandler();
+            // The session leader must not keep daemon-owned sockets alive.
+            // In particular, an inherited system bus fd would retain DDM's
+            // well-known names after the daemon exits.
+            closeInheritedFileDescriptors(pipefd[1]);
 
             // Restore default SIGINT and SIGTERM handlers. We need
             // the signal hander to terminate ourself, since we're