dbus: hold node-message-bus, and never restart the bus live (hq ADR 0215)

A live restart of the system bus during an upgrade hung every login on a workstation until a reboot. The module owns the bus's packages, declares the bus running with no restart or reload trigger, publishes only curated events (health, services, denials; never traffic) and serves tools to look at both buses.
2026-10-05 11:50:52 +02:00
parent 4224ac7252
commit 7b5e1d3362
19 changed files with 3106 additions and 0 deletions
@@ -0,0 +1,133 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"encoding/xml"
+	"fmt"
+	"regexp"
+	"strings"
+
+	"github.com/godbus/dbus/v5/introspect"
+)
+
+// A bus name and an object path as the specification allows them; anything else is refused before
+// it reaches busctl, so no argument is ever taken for an option.
+var (
+	busNameRE    = regexp.MustCompile(`^(:[A-Za-z0-9_-]+(\.[A-Za-z0-9_-]+)+|[A-Za-z_-][A-Za-z0-9_-]*(\.[A-Za-z_-][A-Za-z0-9_-]*)+)$`)
+	objectPathRE = regexp.MustCompile(`^/([A-Za-z0-9_]+(/[A-Za-z0-9_]+)*)?$`)
+)
+
+// Member is one method or signal, with its arguments as "name type".
+type Member struct {
+	Name string   `json:"name"`
+	In   []string `json:"in,omitempty"`
+	Out  []string `json:"out,omitempty"`
+	Args []string `json:"args,omitempty"`
+}
+
+// Property is one property's name, type and access, never its value: a value can be anything a
+// service holds, and reading it is a call of its own.
+type Property struct {
+	Name   string `json:"name"`
+	Type   string `json:"type"`
+	Access string `json:"access"`
+}
+
+// Interface is one interface of an object.
+type Interface struct {
+	Name       string     `json:"name"`
+	Methods    []Member   `json:"methods,omitempty"`
+	Properties []Property `json:"properties,omitempty"`
+	Signals    []Member   `json:"signals,omitempty"`
+}
+
+// Object is dbus_introspect's answer.
+type Object struct {
+	Bus        string      `json:"bus"`
+	Service    string      `json:"service"`
+	Path       string      `json:"path"`
+	Interfaces []Interface `json:"interfaces"`
+	Children   []string    `json:"children,omitempty"`
+}
+
+// ParseIntrospection reads `busctl call … Introspect --json=short` ({"type":"s","data":["<xml>"]}).
+func ParseIntrospection(out string) (introspect.Node, error) {
+	var reply struct {
+		Data []string `json:"data"`
+	}
+	var node introspect.Node
+	if err := json.Unmarshal([]byte(strings.TrimSpace(out)), &reply); err != nil || len(reply.Data) != 1 {
+		return node, fmt.Errorf("the introspection answer is not a single string")
+	}
+	if err := xml.Unmarshal([]byte(reply.Data[0]), &node); err != nil {
+		return node, fmt.Errorf("the introspection document does not parse: %w", err)
+	}
+	return node, nil
+}
+
+// Shape turns an introspection document into the tool's answer.
+func Shape(bus, service, path string, node introspect.Node) Object {
+	o := Object{Bus: bus, Service: service, Path: path, Interfaces: []Interface{}}
+	for _, i := range node.Interfaces {
+		iface := Interface{Name: i.Name}
+		for _, m := range i.Methods {
+			mem := Member{Name: m.Name}
+			for _, a := range m.Args {
+				s := strings.TrimSpace(a.Name + " " + a.Type)
+				if a.Direction == "out" {
+					mem.Out = append(mem.Out, s)
+				} else {
+					mem.In = append(mem.In, s)
+				}
+			}
+			iface.Methods = append(iface.Methods, mem)
+		}
+		for _, p := range i.Properties {
+			iface.Properties = append(iface.Properties, Property{Name: p.Name, Type: p.Type, Access: p.Access})
+		}
+		for _, s := range i.Signals {
+			mem := Member{Name: s.Name}
+			for _, a := range s.Args {
+				mem.Args = append(mem.Args, strings.TrimSpace(a.Name+" "+a.Type))
+			}
+			iface.Signals = append(iface.Signals, mem)
+		}
+		o.Interfaces = append(o.Interfaces, iface)
+	}
+	for _, c := range node.Children {
+		if len(o.Children) >= AnswerCap {
+			break
+		}
+		o.Children = append(o.Children, c.Name)
+	}
+	return o
+}
+
+// Introspect asks one object what it offers, as this account, without starting a service that is not
+// running (--auto-start=no): looking must not change what runs.
+func (m *Machine) Introspect(ctx context.Context, bus, service, path string) (Object, error) {
+	if !busNameRE.MatchString(service) {
+		return Object{}, fmt.Errorf("%q is not a bus name", service)
+	}
+	if path == "" {
+		path = "/"
+	}
+	if !objectPathRE.MatchString(path) {
+		return Object{}, fmt.Errorf("%q is not an object path", path)
+	}
+	args, err := m.busArgs(bus)
+	if err != nil {
+		return Object{}, err
+	}
+	out, err := m.Run(ctx, "busctl", append(args, "--json=short", "--no-pager", "--auto-start=no", "call",
+		service, path, "org.freedesktop.DBus.Introspectable", "Introspect")...)
+	if err != nil {
+		return Object{}, err
+	}
+	node, err := ParseIntrospection(out)
+	if err != nil {
+		return Object{}, err
+	}
+	return Shape(orWord(bus, "system"), service, path, node), nil
+}