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switch-to-configuration-ng: wait for NameOwnerChanged after systemd Reexecute

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To re-execute a systemd manager interface, the
current switch-to-configuration-ng code calls the
org.freedesktop.systemd1.Manager.Reexecute method on D-Bus.

As noted in a comment, the systemd manager does not reply to that
method call (it has the org.freedesktop.DBus.Method.NoReply attribute).
Instead, to signal when it is done re-executing, it passes its old D-Bus
connection socket to the new systemd manager and once the new systemd
manager is ready to receive D-Bus messages on the new D-Bus connection
socket, it will close the old connection socket. As the old connection
socket is closed, the D-Bus daemon assigns the org.freedesktop.systemd1
bus name to the new D-Bus connection [1].

The dbus-codegen crate does not support the
org.freedesktop.DBus.Method.NoReply attribute. What currently
happens in switch-to-configuration-ng is that it blocks until the
old connection socket is closed which makes the D-Bus daemon send a
org.freedesktop.DBus.Error.NoReply error to switch-to-configuration-ng
which then unblocks it.

If the systemd manager takes a long time to re-execute (more than 10
seconds), then the method call can also timeout which means that
switch-to-configuration-ng will proceed with the next systemd manager
method calls, leading to the following error:

> Error: Failed to restart nixos-activation.service
>
> Caused by:
>     Message recipient disconnected from message bus without replying

This was observed on a server with a very large number of
filesystem mounts that make re-execution take more than 10
seconds.

Use the org.freedesktop.DBus.NameOwnerChanged signal [2] to determine
when systemd is done reexecuting and is ready to receive new D-Bus
messages again.

[1]: https://dbus.freedesktop.org/doc/dbus-specification.html#message-bus-overview
[2]: https://dbus.freedesktop.org/doc/dbus-specification.html#bus-messages-name-owner-changed
This commit is contained in:
beviu
2025-09-20 11:33:38 +02:00
parent 3b301b1170
commit 67b8817f26
3 changed files with 91 additions and 6 deletions
Download Patch File Download Diff File Expand all files Collapse all files
pkgs/by-name/sw/switch-to-configuration-ng/src/build.rs
+4
View File
@@ -18,6 +18,10 @@ fn main() {
let out_path = std::path::PathBuf::from(std::env::var("OUT_DIR").unwrap());
let fdo_dbus_code = code_for_dbus_xml("org.freedesktop.DBus.xml");
let mut file = std::fs::File::create(out_path.join("fdo_dbus.rs")).unwrap();
file.write_all(fdo_dbus_code.as_bytes()).unwrap();
let systemd_manager_code =
code_for_dbus_xml(systemd_dbus_interface_dir.join("org.freedesktop.systemd1.Manager.xml"));
let mut file = std::fs::File::create(out_path.join("systemd_manager.rs")).unwrap();
pkgs/by-name/sw/switch-to-configuration-ng/src/org.freedesktop.DBus.xml
+12
View File
@@ -0,0 +1,12 @@
<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
"https://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
<node>
<!-- From https://dbus.freedesktop.org/doc/dbus-specification.html#message-bus-messages. -->
<interface name="org.freedesktop.DBus">
<signal name="NameOwnerChanged">
<arg type="s" direction="out" name="name"/>
<arg type="s" direction="out" name="old_owner"/>
<arg type="s" direction="out" name="new_owner"/>
</signal>
</interface>
</node>
pkgs/by-name/sw/switch-to-configuration-ng/src/src/main.rs
+75 -6
View File
@@ -17,6 +17,8 @@ use std::{
use anyhow::{anyhow, bail, Context, Result};
use dbus::{
blocking::{stdintf::org_freedesktop_dbus::Properties, LocalConnection, Proxy},
channel::Sender,
strings::{BusName, Interface, Member},
Message,
};
use glob::glob;
@@ -32,6 +34,15 @@ use nix::{
use regex::Regex;
use syslog::Facility;
mod fdo_dbus {
#![allow(non_upper_case_globals)]
#![allow(non_camel_case_types)]
#![allow(non_snake_case)]
#![allow(unused)]
#![allow(clippy::all)]
include!(concat!(env!("OUT_DIR"), "/fdo_dbus.rs"));
}
mod systemd_manager {
#![allow(non_upper_case_globals)]
#![allow(non_camel_case_types)]
@@ -50,7 +61,9 @@ mod logind_manager {
include!(concat!(env!("OUT_DIR"), "/logind_manager.rs"));
}
use crate::systemd_manager::OrgFreedesktopSystemd1Manager;
use crate::{
fdo_dbus::OrgFreedesktopDBusNameOwnerChanged, systemd_manager::OrgFreedesktopSystemd1Manager,
};
use crate::{
logind_manager::OrgFreedesktopLogin1Manager,
systemd_manager::{
@@ -894,6 +907,14 @@ impl std::fmt::Display for Job {
}
}
fn fdo_dbus_proxy(conn: &LocalConnection) -> Proxy<'_, &LocalConnection> {
conn.with_proxy(
"org.freedesktop.DBus",
"/org/freedesktop/DBus",
Duration::from_millis(500),
)
}
fn systemd1_proxy(conn: &LocalConnection) -> Proxy<'_, &LocalConnection> {
conn.with_proxy(
"org.freedesktop.systemd1",
@@ -930,6 +951,54 @@ fn remove_file_if_exists(p: impl AsRef<Path>) -> std::io::Result<()> {
}
}
fn reexecute_systemd_manager(
dbus_conn: &LocalConnection,
fdo_dbus: &Proxy<'_, &LocalConnection>,
) -> anyhow::Result<()> {
let reexecute_done = Rc::new(RefCell::new(false));
let _reexecute_done = reexecute_done.clone();
let owner_changed_token = fdo_dbus
.match_signal(
move |signal: OrgFreedesktopDBusNameOwnerChanged, _: &LocalConnection, _: &Message| {
if signal.name.as_str() == "org.freedesktop.systemd1" {
*_reexecute_done.borrow_mut() = true;
}
true
},
)
.context("Failed to add signal match for DBus name owner changes")?;
let bus_name = BusName::from("org.freedesktop.systemd1");
let object_path = dbus::Path::from("/org/freedesktop/systemd1");
let interface = Interface::new("org.freedesktop.systemd1.Manager")
.expect("the org.freedesktop.systemd1.Manager interface name should be valid");
let method_name = Member::new("Reexecute").expect("the Reexecute method name should be valid");
// Systemd does not reply to the Reexecute method.
let _serial = dbus_conn
.send(Message::method_call(
&bus_name,
&object_path,
&interface,
&method_name,
))
.map_err(|_err| anyhow!("Failed to send org.freedesktop.systemd1.Manager.Reexecute"))?;
log::debug!("waiting for systemd to finish reexecuting");
while !*reexecute_done.borrow() {
_ = dbus_conn
.process(Duration::from_secs(500))
.context("Failed to process dbus messages")?;
}
dbus_conn
.remove_match(owner_changed_token)
.context("Failed to remove jobs token")?;
Ok(())
}
/// Performs switch-to-configuration functionality for a single non-root user
fn do_user_switch(parent_exe: String) -> anyhow::Result<()> {
if Path::new(&parent_exe)
@@ -945,8 +1014,11 @@ fn do_user_switch(parent_exe: String) -> anyhow::Result<()> {
}
let dbus_conn = LocalConnection::new_session().context("Failed to open dbus connection")?;
let fdo_dbus = fdo_dbus_proxy(&dbus_conn);
let systemd = systemd1_proxy(&dbus_conn);
reexecute_systemd_manager(&dbus_conn, &fdo_dbus)?;
let nixos_activation_done = Rc::new(RefCell::new(false));
let _nixos_activation_done = nixos_activation_done.clone();
let jobs_token = systemd
@@ -963,10 +1035,6 @@ fn do_user_switch(parent_exe: String) -> anyhow::Result<()> {
)
.context("Failed to add signal match for systemd removed jobs")?;
// The systemd user session seems to not send a Reloaded signal, so we don't have anything to
// wait on here.
_ = systemd.reexecute();
systemd
.restart_unit("nixos-activation.service", "replace")
.context("Failed to restart nixos-activation.service")?;
@@ -1139,6 +1207,7 @@ won't take effect until you reboot the system.
let mut units_to_reload = map_from_list_file(RELOAD_LIST_FILE);
let dbus_conn = LocalConnection::new_system().context("Failed to open dbus connection")?;
let fdo_dbus = fdo_dbus_proxy(&dbus_conn);
let systemd = systemd1_proxy(&dbus_conn);
let logind = login1_proxy(&dbus_conn);
@@ -1664,7 +1733,7 @@ won't take effect until you reboot the system.
// just in case the new one has trouble communicating with the running pid 1.
if restart_systemd {
eprintln!("restarting systemd...");
_ = systemd.reexecute(); // we don't get a dbus reply here
reexecute_systemd_manager(&dbus_conn, &fdo_dbus)?;
log::debug!("waiting for systemd restart to finish");
while !*systemd_reload_status.borrow() {