Implement -run-with exit-with-parent=on

Libguestfs wants to use qemu to run a captive appliance.  When the
program linked to libguestfs exits, we want qemu to be cleaned up.
Libguestfs goes to great lengths to do this at the moment: it either
forks a separate process to ensure clean-up is done, or it asks
libvirt to clean up the qemu process.  However this is complicated and
not totally reliable.

On Linux, FreeBSD and macOS, there are mechanisms to ensure a signal
or message is delivered to a process when its parent process goes
away.  The qemu test suite even uses this mechanism on Linux (see
PR_SET_PDEATHSIG in tests/qtest/libqtest.c).

In nbdkit we have long had the concept of running nbdkit captively,
and we have the nbdkit --exit-with-parent flag to help
(https://libguestfs.org/nbdkit-captive.1.html#EXIT-WITH-PARENT)

This commit adds the same mechanism.  The syntax is:

  qemu -run-with exit-with-parent=on [...]

This is not a feature that most typical users of qemu (for running
general purpose, long-lived VMs) should use, so it defaults to off.

The exit-with-parent.[ch] files are copied from nbdkit, where they
have a 3-clause BSD license which is compatible with qemu:

https://gitlab.com/nbdkit/nbdkit/-/tree/master/common/utils?ref_type=heads

Thanks: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

system/exit-with-parent.c

@@ -0,0 +1,140 @@
+/*
+ * SPDX-License-Identifier: BSD-3-Clause
+ * Originally derived from nbdkit common/utils/exit-with-parent.c
+ * Copyright Red Hat
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ *
+ * * Neither the name of Red Hat nor the names of its contributors may be
+ * used to endorse or promote products derived from this software without
+ * specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+ * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+ * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+/*
+ * Implement the --exit-with-parent feature on operating systems which
+ * support it.
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/exit-with-parent.h"
+
+#if defined(__linux__)
+
+#include <sys/prctl.h>
+
+/*
+ * Send SIGTERM to self when the parent exits.  This will cause
+ * qemu_system_killed() to be called.
+ *
+ * PR_SET_PDEATHSIG has been defined since Linux 2.1.57.
+ */
+int
+set_exit_with_parent(void)
+{
+    return prctl(PR_SET_PDEATHSIG, SIGTERM);
+}
+
+#elif defined(__FreeBSD__)
+
+#include <sys/procctl.h>
+
+/*
+ * Send SIGTERM to self when the parent exits.  This will cause
+ * qemu_system_killed() to be called.
+ *
+ * PROC_PDEATHSIG_CTL has been defined since FreeBSD 11.2.
+ */
+int
+set_exit_with_parent(void)
+{
+    const int sig = SIGTERM;
+    return procctl(P_PID, 0, PROC_PDEATHSIG_CTL, (void *) &sig);
+}
+
+#elif defined(__APPLE__)
+
+/* For macOS. */
+
+#include "qemu/thread.h"
+#include "qemu/error-report.h"
+#include "system/runstate.h"
+#include <sys/event.h>
+
+static void *
+exit_with_parent_loop(void *vp)
+{
+    const pid_t ppid = getppid();
+    int fd;
+    struct kevent kev, res[1];
+    int r;
+
+    /* Register the kevent to wait for ppid to exit. */
+    fd = kqueue();
+    if (fd == -1) {
+        error_report("exit_with_parent_loop: kqueue: %m");
+        return NULL;
+    }
+    EV_SET(&kev, ppid, EVFILT_PROC, EV_ADD | EV_ENABLE, NOTE_EXIT, 0, NULL);
+    if (kevent(fd, &kev, 1, NULL, 0, NULL) == -1) {
+        error_report("exit_with_parent_loop: kevent: %m");
+        close(fd);
+        return NULL;
+    }
+
+    /* Wait for the kevent to happen. */
+    r = kevent(fd, 0, 0, res, 1, NULL);
+    if (r == 1 && res[0].ident == ppid) {
+        /* Behave like Linux and FreeBSD above, as if SIGTERM was sent */
+        qemu_system_killed(SIGTERM, ppid);
+    }
+
+    return NULL;
+}
+
+int
+set_exit_with_parent(void)
+{
+    QemuThread exit_with_parent_thread;
+
+    /*
+     * We have to block waiting for kevent, so that requires that we
+     * start a background thread.
+     */
+    qemu_thread_create(&exit_with_parent_thread,
+                       "exit-parent",
+                       exit_with_parent_loop, NULL,
+                       QEMU_THREAD_DETACHED);
+    return 0;
+}
+
+#else /* any platform that doesn't support this function */
+
+int
+set_exit_with_parent(void)
+{
+    g_assert_not_reached();
+}
+
+#endif

system/meson.build

@@ -15,6 +15,7 @@ system_ss.add(files(
   'datadir.c',
   'dirtylimit.c',
   'dma-helpers.c',
+  'exit-with-parent.c',
   'globals.c',
   'ioport.c',
   'ram-block-attributes.c',

system/vl.c

@@ -53,6 +53,7 @@
 #include "qemu/sockets.h"
 #include "qemu/accel.h"
 #include "qemu/async-teardown.h"
+#include "qemu/exit-with-parent.h"
 #include "hw/usb.h"
 #include "hw/isa/isa.h"
 #include "hw/scsi/scsi.h"
@@ -783,6 +784,10 @@ static QemuOptsList qemu_run_with_opts = {
             .name = "chroot",
             .type = QEMU_OPT_STRING,
         },
+        {
+            .name = "exit-with-parent",
+            .type = QEMU_OPT_BOOL,
+        },
         {
             .name = "user",
             .type = QEMU_OPT_STRING,
@@ -3691,6 +3696,14 @@ void qemu_init(int argc, char **argv)
                 if (str) {
                     os_set_chroot(str);
                 }
+                if (qemu_opt_get_bool(opts, "exit-with-parent", false)) {
+                    if (!can_exit_with_parent()) {
+                        error_report("exit-with-parent is not available"
+                                     " on this platform");
+                        exit(1);
+                    }
+                    set_exit_with_parent();
+                }
                 str = qemu_opt_get(opts, "user");
                 if (str) {
                     if (!os_set_runas(str)) {