From 9c47b1527eb2debae32bcae2c73a26ccc84973cc Mon Sep 17 00:00:00 2001
From: Yu Watanabe <watanabe.yu+github@gmail.com>
Date: Mon, 10 Mar 2025 22:44:02 +0900
Subject: [PATCH] udev: scan partitions and trigger synthetic change events in
 child process

Rereading partition table may take longer on slow disk. The main process
should not be blocked by the operation. Let's fork a child process and
do that on the child.

Prompted by #36624 and #36269.
---
 src/udev/udev-manager.c | 48 +++++++++++++++++++++++++++++++++++++----
 src/udev/udev-manager.h |  1 +
 2 files changed, 45 insertions(+), 4 deletions(-)

diff --git a/src/udev/udev-manager.c b/src/udev/udev-manager.c
index a696f6a69e..adfe8540b2 100644
--- a/src/udev/udev-manager.c
+++ b/src/udev/udev-manager.c
@@ -153,6 +153,7 @@ Manager* manager_free(Manager *manager) {
         sd_varlink_server_unref(manager->varlink_server);
 
         sd_event_source_unref(manager->inotify_event);
+        set_free(manager->synthesize_change_child_event_sources);
         sd_event_source_unref(manager->kill_workers_event);
         sd_event_unref(manager->event);
 
@@ -895,9 +896,18 @@ static int synthesize_change_all(sd_device *dev) {
         return r;
 }
 
-static int synthesize_change(sd_device *dev) {
+static int synthesize_change_child_handler(sd_event_source *s, const siginfo_t *si, void *userdata) {
+        Manager *manager = ASSERT_PTR(userdata);
+        assert(s);
+
+        sd_event_source_unref(set_remove(manager->synthesize_change_child_event_sources, s));
+        return 0;
+}
+
+static int synthesize_change(Manager *manager, sd_device *dev) {
         int r;
 
+        assert(manager);
         assert(dev);
 
         const char *sysname;
@@ -908,7 +918,37 @@ static int synthesize_change(sd_device *dev) {
         if (startswith(sysname, "dm-") || block_device_is_whole_disk(dev) <= 0)
                 return synthesize_change_one(dev, dev);
 
-        return synthesize_change_all(dev);
+        _cleanup_(pidref_done) PidRef pidref = PIDREF_NULL;
+        r = pidref_safe_fork(
+                        "(udev-synth)",
+                        FORK_RESET_SIGNALS|FORK_CLOSE_ALL_FDS|FORK_DEATHSIG_SIGTERM|FORK_LOG|FORK_RLIMIT_NOFILE_SAFE,
+                        &pidref);
+        if (r < 0)
+                return r;
+        if (r == 0) {
+                /* child */
+                (void) synthesize_change_all(dev);
+                _exit(EXIT_SUCCESS);
+        }
+
+        _cleanup_(sd_event_source_unrefp) sd_event_source *s = NULL;
+        r = event_add_child_pidref(manager->event, &s, &pidref, WEXITED, synthesize_change_child_handler, manager);
+        if (r < 0) {
+                log_debug_errno(r, "Failed to add child event source for "PID_FMT", ignoring: %m", pidref.pid);
+                return 0;
+        }
+
+        r = sd_event_source_set_child_pidfd_own(s, true);
+        if (r < 0)
+                return r;
+        TAKE_PIDREF(pidref);
+
+        r = set_ensure_put(&manager->synthesize_change_child_event_sources, &event_source_hash_ops, s);
+        if (r < 0)
+                return r;
+        TAKE_PTR(s);
+
+        return 0;
 }
 
 static int on_inotify(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
@@ -955,7 +995,7 @@ static int on_inotify(sd_event_source *s, int fd, uint32_t revents, void *userda
                 log_device_debug(dev, "Received inotify event for %s.", devnode);
 
                 (void) event_queue_assume_block_device_unlocked(manager, dev);
-                (void) synthesize_change(dev);
+                (void) synthesize_change(manager, dev);
         }
 
         return 0;
@@ -1055,7 +1095,7 @@ static int on_post(sd_event_source *s, void *userdata) {
         if (manager->exit)
                 return sd_event_exit(manager->event, 0);
 
-        if (manager->cgroup)
+        if (manager->cgroup && set_isempty(manager->synthesize_change_child_event_sources))
                 /* cleanup possible left-over processes in our cgroup */
                 (void) cg_kill(manager->cgroup, SIGKILL, CGROUP_IGNORE_SELF, /* set=*/ NULL, /* kill_log= */ NULL, /* userdata= */ NULL);
 
diff --git a/src/udev/udev-manager.h b/src/udev/udev-manager.h
index 808f75d3e1..1681560533 100644
--- a/src/udev/udev-manager.h
+++ b/src/udev/udev-manager.h
@@ -35,6 +35,7 @@ typedef struct Manager {
         /* used by udev-watch */
         int inotify_fd;
         sd_event_source *inotify_event;
+        Set *synthesize_change_child_event_sources;
 
         sd_event_source *kill_workers_event;