introduce notify_socket_prepare() and use it where applicable (#36911)

This introduces notify_socket_prepare(), which creates an autobind
notify socket and IO event source for the socket. Then, use it where we
send notification messages from worker processes to their manager
process.

src/home/homed-home.c

@@ -1308,21 +1308,9 @@ static int home_start_work(
         if (r < 0)
                 return r;
         if (r == 0) {
-                _cleanup_free_ char *joined = NULL;
-                const char *suffix, *unix_path;
-
                 /* Child */
 
-                suffix = getenv("SYSTEMD_HOME_DEBUG_SUFFIX");
-                if (suffix) {
-                        joined = strjoin("/run/systemd/home/notify.", suffix);
-                        if (!joined)
-                                return log_oom();
-                        unix_path = joined;
-                } else
-                        unix_path = "/run/systemd/home/notify";
-
-                if (setenv("NOTIFY_SOCKET", unix_path, 1) < 0) {
+                if (setenv("NOTIFY_SOCKET", h->manager->notify_socket_path, /* overwrite = */ true) < 0) {
                         log_error_errno(errno, "Failed to set $NOTIFY_SOCKET: %m");
                         _exit(EXIT_FAILURE);
                 }

src/home/homed-manager.c

@@ -291,7 +291,7 @@ Manager* manager_free(Manager *m) {
         m->device_monitor = sd_device_monitor_unref(m->device_monitor);
 
         m->inotify_event_source = sd_event_source_unref(m->inotify_event_source);
-        m->notify_socket_event_source = sd_event_source_unref(m->notify_socket_event_source);
+        m->notify_socket_path = mfree(m->notify_socket_path);
         m->deferred_rescan_event_source = sd_event_source_unref(m->deferred_rescan_event_source);
         m->deferred_gc_event_source = sd_event_source_unref(m->deferred_gc_event_source);
         m->deferred_auto_login_event_source = sd_event_source_unref(m->deferred_auto_login_event_source);
@@ -1142,64 +1142,23 @@ static int on_notify_socket(sd_event_source *s, int fd, uint32_t revents, void *
 }
 
 static int manager_listen_notify(Manager *m) {
-        _cleanup_close_ int fd = -EBADF;
-        union sockaddr_union sa = {
-                .un.sun_family = AF_UNIX,
-                .un.sun_path = "/run/systemd/home/notify",
-        };
-        const char *suffix;
         int r;
 
         assert(m);
-        assert(!m->notify_socket_event_source);
+        assert(!m->notify_socket_path);
 
-        suffix = getenv("SYSTEMD_HOME_DEBUG_SUFFIX");
-        if (suffix) {
-                _cleanup_free_ char *unix_path = NULL;
-
-                unix_path = strjoin("/run/systemd/home/notify.", suffix);
-                if (!unix_path)
-                        return log_oom();
-                r = sockaddr_un_set_path(&sa.un, unix_path);
-                if (r < 0)
-                        return log_error_errno(r, "Socket path %s does not fit in sockaddr_un: %m", unix_path);
-        }
-
-        fd = socket(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
-        if (fd < 0)
-                return log_error_errno(errno, "Failed to create listening socket: %m");
-
-        (void) mkdir_parents(sa.un.sun_path, 0755);
-        (void) sockaddr_un_unlink(&sa.un);
-
-        if (bind(fd, &sa.sa, SOCKADDR_UN_LEN(sa.un)) < 0)
-                return log_error_errno(errno, "Failed to bind to socket: %m");
-
-        r = setsockopt_int(fd, SOL_SOCKET, SO_PASSCRED, true);
+        r = notify_socket_prepare(
+                        m->event,
+                        SD_EVENT_PRIORITY_NORMAL - 5, /* Make sure we process sd_notify() before SIGCHLD for
+                                                       * any worker, so that we always know the error number
+                                                       * of a client before it exits. */
+                        on_notify_socket,
+                        m,
+                        &m->notify_socket_path);
         if (r < 0)
-                return log_error_errno(r, "Failed to enable SO_PASSCRED on notify socket: %m");
+                return log_error_errno(r, "Failed to prepare notify socket: %m");
 
-        r = setsockopt_int(fd, SOL_SOCKET, SO_PASSPIDFD, true);
-        if (r < 0)
-                log_warning_errno(r, "Failed to enable SO_PASSPIDFD on notify socket, ignoring: %m");
-
-        r = sd_event_add_io(m->event, &m->notify_socket_event_source, fd, EPOLLIN, on_notify_socket, m);
-        if (r < 0)
-                return log_error_errno(r, "Failed to allocate event source for notify socket: %m");
-
-        (void) sd_event_source_set_description(m->notify_socket_event_source, "notify-socket");
-
-        /* Make sure we process sd_notify() before SIGCHLD for any worker, so that we always know the error
-         * number of a client before it exits. */
-        r = sd_event_source_set_priority(m->notify_socket_event_source, SD_EVENT_PRIORITY_NORMAL - 5);
-        if (r < 0)
-                return log_error_errno(r, "Failed to alter priority of NOTIFY_SOCKET event source: %m");
-
-        r = sd_event_source_set_io_fd_own(m->notify_socket_event_source, true);
-        if (r < 0)
-                return log_error_errno(r, "Failed to pass ownership of notify socket: %m");
-
-        return TAKE_FD(fd);
+        return 0;
 }
 
 static int manager_add_device(Manager *m, sd_device *d) {

src/home/homed-manager.h

@@ -42,8 +42,7 @@ struct Manager {
 
         sd_event_source *inotify_event_source;
 
-        /* An event source we receive sd_notify() messages from our worker from */
-        sd_event_source *notify_socket_event_source;
+        char *notify_socket_path;
 
         sd_device_monitor *device_monitor;
 

src/import/importd.c

@@ -107,9 +107,7 @@ struct Manager {
 
         Hashmap *polkit_registry;
 
-        int notify_fd;
-
-        sd_event_source *notify_event_source;
+        char *notify_socket_path;
 
         bool use_btrfs_subvol;
         bool use_btrfs_quota;
@@ -467,7 +465,7 @@ static int transfer_start(Transfer *t) {
                 /* Child */
 
                 if (setenv("SYSTEMD_LOG_TARGET", "console-prefixed", 1) < 0 ||
-                    setenv("NOTIFY_SOCKET", "/run/systemd/import/notify", 1) < 0) {
+                    setenv("NOTIFY_SOCKET", t->manager->notify_socket_path, 1) < 0) {
                         log_error_errno(errno, "setenv() failed: %m");
                         _exit(EXIT_FAILURE);
                 }
@@ -624,8 +622,7 @@ static Manager *manager_unref(Manager *m) {
         if (!m)
                 return NULL;
 
-        sd_event_source_unref(m->notify_event_source);
-        safe_close(m->notify_fd);
+        free(m->notify_socket_path);
 
         while ((t = hashmap_first(m->transfers)))
                 transfer_unref(t);
@@ -687,10 +684,6 @@ static int manager_on_notify(sd_event_source *s, int fd, uint32_t revents, void
 
 static int manager_new(Manager **ret) {
         _cleanup_(manager_unrefp) Manager *m = NULL;
-        static const union sockaddr_union sa = {
-                .un.sun_family = AF_UNIX,
-                .un.sun_path = "/run/systemd/import/notify",
-        };
         int r;
 
         assert(ret);
@@ -725,26 +718,12 @@ static int manager_new(Manager **ret) {
         if (r < 0)
                 log_debug_errno(r, "Failed to enable watchdog logic, ignoring: %m");
 
-        m->notify_fd = socket(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
-        if (m->notify_fd < 0)
-                return -errno;
-
-        (void) mkdir_parents_label(sa.un.sun_path, 0755);
-        (void) sockaddr_un_unlink(&sa.un);
-
-        if (bind(m->notify_fd, &sa.sa, SOCKADDR_UN_LEN(sa.un)) < 0)
-                return -errno;
-
-        r = setsockopt_int(m->notify_fd, SOL_SOCKET, SO_PASSCRED, true);
-        if (r < 0)
-                return r;
-
-        r = setsockopt_int(m->notify_fd, SOL_SOCKET, SO_PASSPIDFD, true);
-        if (r < 0)
-                log_debug_errno(r, "Failed to enable SO_PASSPIDFD on notification socket, ignoring. %m");
-
-        r = sd_event_add_io(m->event, &m->notify_event_source,
-                            m->notify_fd, EPOLLIN, manager_on_notify, m);
+        r = notify_socket_prepare(
+                        m->event,
+                        SD_EVENT_PRIORITY_NORMAL,
+                        manager_on_notify,
+                        m,
+                        &m->notify_socket_path);
         if (r < 0)
                 return r;
 

src/notify/notify.c

@@ -494,28 +494,28 @@ static int action_fork(char *const *_command) {
         if (!command)
                 return log_oom();
 
-        _cleanup_close_ int socket_fd = socket(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
-        if (socket_fd < 0)
-                return log_error_errno(errno, "Failed to allocate AF_UNIX socket for notifications: %m");
-
-        r = setsockopt_int(socket_fd, SOL_SOCKET, SO_PASSCRED, true);
-        if (r < 0)
-                return log_error_errno(r, "Failed to enable SO_PASSCRED: %m");
-
-        r = setsockopt_int(socket_fd, SOL_SOCKET, SO_PASSPIDFD, true);
-        if (r < 0)
-                log_debug_errno(r, "Failed to enable SO_PASSPIDFD, ignoring: %m");
-
-        _cleanup_free_ char *addr_string = NULL;
-        r = socket_autobind(socket_fd, &addr_string);
-        if (r < 0)
-                return log_error_errno(r, "Failed to bind notify socket: %m");
-
         _cleanup_free_ char *c = strv_join(command, " ");
         if (!c)
                 return log_oom();
 
+        _cleanup_(sd_event_unrefp) sd_event *event = NULL;
+        r = sd_event_new(&event);
+        if (r < 0)
+                return log_error_errno(r, "Failed to allocate event loop: %m");
+
         _cleanup_(pidref_done) PidRef child = PIDREF_NULL;
+        _cleanup_free_ char *addr_string = NULL;
+        r = notify_socket_prepare(
+                        event,
+                        SD_EVENT_PRIORITY_NORMAL - 10, /* If we receive both the sd_notify() message and a
+                                                        * SIGCHLD always process sd_notify() first, it's the
+                                                        * more interesting, "positive" information. */
+                        on_notify_socket,
+                        &child,
+                        &addr_string);
+        if (r < 0)
+                return log_error_errno(r, "Failed to prepare notify socket: %m");
+
         r = pidref_safe_fork_full(
                         "(notify)",
                         /* stdio_fds= */ (const int[]) { -EBADF, -EBADF, STDERR_FILENO },
@@ -526,11 +526,6 @@ static int action_fork(char *const *_command) {
         if (r < 0)
                 return log_error_errno(r, "Failed to fork child in order to execute '%s': %m", c);
         if (r == 0) {
-                /* Let's explicitly close the fds we just opened. Not because it was necessary (we should be
-                 * setting O_CLOEXEC after all on all of them), but mostly to make debugging nice */
-                socket_fd = safe_close(socket_fd);
-                pidref_done(&child);
-
                 if (setenv("NOTIFY_SOCKET", addr_string, /* overwrite= */ true) < 0) {
                         log_error_errno(errno, "Failed to set $NOTIFY_SOCKET: %m");
                         _exit(EXIT_MEMORY);
@@ -549,22 +544,6 @@ static int action_fork(char *const *_command) {
 
         BLOCK_SIGNALS(SIGCHLD);
 
-        _cleanup_(sd_event_unrefp) sd_event *event = NULL;
-        r = sd_event_new(&event);
-        if (r < 0)
-                return log_error_errno(r, "Failed to allocate event loop: %m");
-
-        _cleanup_(sd_event_source_disable_unrefp) sd_event_source *socket_event_source = NULL;
-        r = sd_event_add_io(event, &socket_event_source, socket_fd, EPOLLIN, on_notify_socket, &child);
-        if (r < 0)
-                return log_error_errno(r, "Failed to allocate IO source: %m");
-
-        /* If we receive both the sd_notify() message and a SIGCHLD always process sd_notify() first, it's
-         * the more interesting, "positive" information. */
-        r = sd_event_source_set_priority(socket_event_source, SD_EVENT_PRIORITY_NORMAL - 10);
-        if (r < 0)
-                return log_error_errno(r, "Failed to change child event source priority: %m");
-
         _cleanup_(sd_event_source_disable_unrefp) sd_event_source *child_event_source = NULL;
         r = event_add_child_pidref(event, &child_event_source, &child, WEXITED, on_child, /* userdata= */ NULL);
         if (r < 0)

src/shared/notify-recv.c

@@ -7,6 +7,61 @@
 #include "strv.h"
 #include "user-util.h"
 
+int notify_socket_prepare(
+                sd_event *event,
+                int64_t priority,
+                sd_event_io_handler_t handler,
+                void *userdata,
+                char **ret_path) {
+
+        int r;
+
+        assert(event);
+
+        _cleanup_close_ int fd = socket(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
+        if (fd < 0)
+                return log_debug_errno(errno, "Failed to create notification socket: %m");
+
+        _cleanup_free_ char *path = NULL;
+        r = socket_autobind(fd, &path);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to bind notification socket: %m");
+
+        r = setsockopt_int(fd, SOL_SOCKET, SO_PASSCRED, true);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to enable SO_PASSCRED on notification socket: %m");
+
+        /* SO_PASSPIDFD is supported since kernel v6.5. */
+        r = setsockopt_int(fd, SOL_SOCKET, SO_PASSPIDFD, true);
+        if (r < 0)
+                log_debug_errno(r, "Failed to enable SO_PASSPIDFD on notification socket, ignoring. %m");
+
+        _cleanup_(sd_event_source_unrefp) sd_event_source *s = NULL;
+        r = sd_event_add_io(event, &s, fd, EPOLLIN, handler, userdata);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to create notification event source: %m");
+
+        r = sd_event_source_set_priority(s, priority);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to set priority to notification event source: %m");
+
+        r = sd_event_source_set_io_fd_own(s, true);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to make notification event source own file descriptor: %m");
+        TAKE_FD(fd);
+
+        (void) sd_event_source_set_description(s, "notify-socket");
+
+        r = sd_event_source_set_floating(s, true);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to make notification event source floating: %m");
+
+        if (ret_path)
+                *ret_path = TAKE_PTR(path);
+
+        return 0;
+}
+
 int notify_recv_with_fds(
                 int fd,
                 char **ret_text,

src/shared/notify-recv.h

@@ -3,9 +3,18 @@
 
 #include <sys/socket.h>
 
+#include "sd-event.h"
+
 #include "fdset.h"
 #include "pidref.h"
 
+int notify_socket_prepare(
+                sd_event *event,
+                int64_t priority,
+                sd_event_io_handler_t handler,
+                void *userdata,
+                char **ret_path);
+
 int notify_recv_with_fds(
                 int fd,
                 char **ret_text,

src/sysupdate/sysupdate-transfer.c

@@ -945,15 +945,13 @@ static int callout_context_new(const Transfer *t, const Instance *i, TransferPro
 }
 
 static int helper_on_exit(sd_event_source *s, const siginfo_t *si, void *userdata) {
-        _cleanup_(callout_context_freep) CalloutContext *ctx = ASSERT_PTR(userdata);
+        CalloutContext *ctx = ASSERT_PTR(userdata);
         int r;
 
         assert(s);
         assert(si);
         assert(ctx);
 
-        pidref_done(&ctx->pid);
-
         if (si->si_code == CLD_EXITED) {
                 if (si->si_status == EXIT_SUCCESS) {
                         r = 0;
@@ -1031,11 +1029,7 @@ static int run_callout(
                 const Instance *instance,
                 TransferProgress callback,
                 void *userdata) {
-        _cleanup_(sd_event_unrefp) sd_event *event = NULL;
-        _cleanup_(sd_event_source_unrefp) sd_event_source *exit_source = NULL, *notify_source = NULL;
-        _cleanup_close_ int fd = -EBADF;
-        _cleanup_free_ char *bind_name = NULL;
-        union sockaddr_union bsa;
+
         int r;
 
         assert(name);
@@ -1043,11 +1037,11 @@ static int run_callout(
         assert(cmdline[0]);
 
         _cleanup_(callout_context_freep) CalloutContext *ctx = NULL;
-
         r = callout_context_new(transfer, instance, callback, name, userdata, &ctx);
         if (r < 0)
                 return log_oom();
 
+        _cleanup_(sd_event_unrefp) sd_event *event = NULL;
         r = sd_event_new(&event);
         if (r < 0)
                 return log_error_errno(r, "Failed to create event: %m");
@@ -1060,27 +1054,15 @@ static int run_callout(
         if (r < 0)
                 return log_error_errno(r, "Failed to register signal to event: %m");
 
-        fd = socket(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
-        if (fd < 0)
-                return log_error_errno(errno, "Failed to create UNIX socket for notification: %m");
-
-        if (asprintf(&bind_name, "@%" PRIx64 "/sysupdate/" PID_FMT "/notify", random_u64(), getpid_cached()) < 0)
-                return log_oom();
-
-        r = sockaddr_un_set_path(&bsa.un, bind_name);
+        _cleanup_free_ char *bind_name = NULL;
+        r = notify_socket_prepare(
+                        event,
+                        SD_EVENT_PRIORITY_NORMAL - 5,
+                        helper_on_notify,
+                        ctx,
+                        &bind_name);
         if (r < 0)
-                return log_error_errno(r, "Failed to set socket path: %m");
-
-        if (bind(fd, &bsa.sa, r) < 0)
-                return log_error_errno(errno, "Failed to bind to notification socket: %m");
-
-        r = setsockopt_int(fd, SOL_SOCKET, SO_PASSCRED, true);
-        if (r < 0)
-                return log_error_errno(r, "Failed to enable SO_PASSCRED: %m");
-
-        r = setsockopt_int(fd, SOL_SOCKET, SO_PASSPIDFD, true);
-        if (r < 0)
-                log_debug_errno(r, "Failed to enable SO_PASSPIDFD, ignoring: %m");
+                return log_error_errno(r, "Failed to prepare notify socket: %m");
 
         r = pidref_safe_fork(ctx->name, FORK_RESET_SIGNALS|FORK_DEATHSIG_SIGTERM|FORK_LOG, &ctx->pid);
         if (r < 0)
@@ -1097,7 +1079,8 @@ static int run_callout(
         }
 
         /* Quit the loop w/ when child process exits */
-        r = event_add_child_pidref(event, &exit_source, &ctx->pid, WEXITED, helper_on_exit, (void*) ctx);
+        _cleanup_(sd_event_source_unrefp) sd_event_source *exit_source = NULL;
+        r = event_add_child_pidref(event, &exit_source, &ctx->pid, WEXITED, helper_on_exit, ctx);
         if (r < 0)
                 return log_error_errno(r, "Failed to add child process to event loop: %m");
 
@@ -1105,20 +1088,6 @@ static int run_callout(
         if (r < 0)
                 return log_error_errno(r, "Failed to take ownership of child process: %m");
 
-        /* Propagate sd_notify calls */
-        r = sd_event_add_io(event, &notify_source, fd, EPOLLIN, helper_on_notify, TAKE_PTR(ctx));
-        if (r < 0)
-                return log_error_errno(r, "Failed to add notification propagation to event loop: %m");
-
-        (void) sd_event_source_set_description(notify_source, "notify-socket");
-
-        (void) sd_event_source_set_priority(notify_source, SD_EVENT_PRIORITY_NORMAL - 5);
-
-        r = sd_event_source_set_io_fd_own(notify_source, true);
-        if (r < 0)
-                return log_error_errno(r, "Event loop failed to take ownership of notification source: %m");
-        TAKE_FD(fd);
-
         /* Process events until the helper quits */
         return sd_event_loop(event);
 }

src/sysupdate/sysupdated.c

@@ -46,7 +46,7 @@ typedef struct Manager {
 
         Hashmap *polkit_registry;
 
-        sd_event_source *notify_event;
+        char *notify_socket_path;
 
         RuntimeScope runtime_scope; /* For now only RUNTIME_SCOPE_SYSTEM */
 } Manager;
@@ -456,7 +456,7 @@ static int job_start(Job *j) {
                 };
                 size_t k = 2;
 
-                if (setenv("NOTIFY_SOCKET", "/run/systemd/sysupdate/notify", /* overwrite= */ 1) < 0) {
+                if (setenv("NOTIFY_SOCKET", j->manager->notify_socket_path, /* overwrite= */ 1) < 0) {
                         log_error_errno(errno, "setenv() failed: %m");
                         _exit(EXIT_FAILURE);
                 }
@@ -1637,7 +1637,7 @@ static Manager *manager_free(Manager *m) {
         hashmap_free(m->jobs);
 
         m->bus = sd_bus_flush_close_unref(m->bus);
-        sd_event_source_unref(m->notify_event);
+        free(m->notify_socket_path);
         sd_event_unref(m->event);
 
         return mfree(m);
@@ -1698,11 +1698,6 @@ static int manager_on_notify(sd_event_source *s, int fd, uint32_t revents, void
 
 static int manager_new(Manager **ret) {
         _cleanup_(manager_freep) Manager *m = NULL;
-        _cleanup_close_ int notify_fd = -EBADF;
-        static const union sockaddr_union sa = {
-                .un.sun_family = AF_UNIX,
-                .un.sun_path = "/run/systemd/sysupdate/notify",
-        };
         int r;
 
         assert(ret);
@@ -1738,35 +1733,15 @@ static int manager_new(Manager **ret) {
         if (r < 0)
                 return r;
 
-        notify_fd = socket(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
-        if (notify_fd < 0)
-                return -errno;
-
-        (void) mkdir_parents_label(sa.un.sun_path, 0755);
-        (void) sockaddr_un_unlink(&sa.un);
-
-        if (bind(notify_fd, &sa.sa, SOCKADDR_UN_LEN(sa.un)) < 0)
-                return -errno;
-
-        r = setsockopt_int(notify_fd, SOL_SOCKET, SO_PASSCRED, true);
+        r = notify_socket_prepare(
+                        m->event,
+                        SD_EVENT_PRIORITY_NORMAL,
+                        manager_on_notify,
+                        m,
+                        &m->notify_socket_path);
         if (r < 0)
                 return r;
 
-        r = setsockopt_int(notify_fd, SOL_SOCKET, SO_PASSPIDFD, true);
-        if (r < 0)
-                log_debug_errno(r, "Failed to enable SO_PASSPIDFD, ignoring: %m");
-
-        r = sd_event_add_io(m->event, &m->notify_event, notify_fd, EPOLLIN, manager_on_notify, m);
-        if (r < 0)
-                return r;
-
-        (void) sd_event_source_set_description(m->notify_event, "notify-socket");
-
-        r = sd_event_source_set_io_fd_own(m->notify_event, true);
-        if (r < 0)
-                return r;
-        TAKE_FD(notify_fd);
-
         *ret = TAKE_PTR(m);
         return 0;
 }

src/test/meson.build

@@ -132,6 +132,7 @@ simple_tests += files(
         'test-modhex.c',
         'test-mountpoint-util.c',
         'test-net-naming-scheme.c',
+        'test-notify-recv.c',
         'test-nsresource.c',
         'test-nulstr-util.c',
         'test-open-file.c',

src/test/test-notify-recv.c

@@ -0,0 +1,91 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <unistd.h>
+
+#include "sd-daemon.h"
+
+#include "event-util.h"
+#include "notify-recv.h"
+#include "path-util.h"
+#include "process-util.h"
+#include "rm-rf.h"
+#include "tests.h"
+#include "tmpfile-util.h"
+
+typedef struct Context {
+        unsigned data;
+        PidRef pidref;
+} Context;
+
+static void context_done(Context *c) {
+        assert(c);
+
+        pidref_done(&c->pidref);
+}
+
+static int on_recv(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
+        Context *c = ASSERT_PTR(userdata);
+
+        _cleanup_(pidref_done) PidRef sender = PIDREF_NULL;
+        _cleanup_strv_free_ char **l = NULL;
+        ASSERT_OK(notify_recv_strv(fd, &l, /* ret_ucred= */ NULL, &sender));
+
+        ASSERT_TRUE(pidref_equal(&c->pidref, &sender));
+
+        _cleanup_free_ char *joined = strv_join(l, ", ");
+        ASSERT_NOT_NULL(joined);
+        log_info("Received message: %s", joined);
+
+        if (strv_contains(l, "FIRST_MESSAGE=1")) {
+                ASSERT_STREQ(l[0], "FIRST_MESSAGE=1");
+                ASSERT_NULL(l[1]);
+                ASSERT_EQ(c->data, 0u);
+        } else if (strv_contains(l, "SECOND_MESSAGE=1")) {
+                ASSERT_STREQ(l[0], "SECOND_MESSAGE=1");
+                ASSERT_STREQ(l[1], "ADDITIONAL_DATA=hoge");
+                ASSERT_EQ(c->data, 1u);
+        }
+
+        c->data++;
+
+        return 0;
+}
+
+static int on_sigchld(sd_event_source *s, const siginfo_t *si, void *userdata) {
+        Context *c = ASSERT_PTR(userdata);
+
+        ASSERT_EQ(si->si_code, CLD_EXITED);
+        ASSERT_EQ(si->si_status, EXIT_SUCCESS);
+
+        ASSERT_EQ(c->data, 2u);
+
+        return sd_event_exit(sd_event_source_get_event(s), 0);
+}
+
+TEST(notify_socket_prepare) {
+        int r;
+
+        _cleanup_(sd_event_unrefp) sd_event *e = NULL;
+        ASSERT_OK(sd_event_new(&e));
+
+        _cleanup_(context_done) Context c = {
+                .pidref = PIDREF_NULL,
+        };
+        _cleanup_free_ char *path = NULL;
+        ASSERT_OK(notify_socket_prepare(e, SD_EVENT_PRIORITY_NORMAL - 10, on_recv, &c, &path) >= 0);
+
+        ASSERT_OK(sigprocmask_many(SIG_BLOCK, NULL, SIGCHLD));
+
+        ASSERT_OK(r = pidref_safe_fork("(test-notify-recv-child)", FORK_DEATHSIG_SIGTERM|FORK_LOG, &c.pidref));
+        if (r == 0) {
+                ASSERT_OK_ERRNO(setenv("NOTIFY_SOCKET", path, /* overwrite = */ true));
+                ASSERT_OK_POSITIVE(sd_notify(/* unset_environment = */ false, "FIRST_MESSAGE=1"));
+                ASSERT_OK_POSITIVE(sd_notify(/* unset_environment = */ false, "FIRST_MESSAGE=2\nADDITIONAL_DATA=hoge"));
+                _exit(EXIT_SUCCESS);
+        }
+
+        ASSERT_OK(event_add_child_pidref(e, NULL, &c.pidref, WEXITED, on_sigchld, &c));
+        ASSERT_OK(sd_event_loop(e));
+}
+
+DEFINE_TEST_MAIN(LOG_DEBUG);