package systemd
import (
"context"
"errors"
"fmt"
"sync"
systemdDbus "github.com/coreos/go-systemd/v22/dbus"
dbus "github.com/godbus/dbus/v5"
)
var (
dbusC *systemdDbus.Conn
dbusMu sync.RWMutex
dbusInited bool
dbusRootless bool
)
type dbusConnManager struct{}
// newDbusConnManager initializes systemd dbus connection manager.
func newDbusConnManager(rootless bool) *dbusConnManager {
dbusMu.Lock()
defer dbusMu.Unlock()
if dbusInited && rootless != dbusRootless {
panic("can't have both root and rootless dbus")
}
dbusInited = true
dbusRootless = rootless
return &dbusConnManager{}
}
// getConnection lazily initializes and returns systemd dbus connection.
func (d *dbusConnManager) getConnection() (*systemdDbus.Conn, error) {
// In the case where dbusC != nil
// Use the read lock the first time to ensure
// that Conn can be acquired at the same time.
dbusMu.RLock()
if conn := dbusC; conn != nil {
dbusMu.RUnlock()
return conn, nil
}
dbusMu.RUnlock()
// In the case where dbusC == nil
// Use write lock to ensure that only one
// will be created
dbusMu.Lock()
defer dbusMu.Unlock()
if conn := dbusC; conn != nil {
return conn, nil
}
conn, err := d.newConnection()
if err != nil {
// When dbus-user-session is not installed, we can't detect whether we should try to connect to user dbus or system dbus, so d.dbusRootless is set to false.
// This may fail with a cryptic error "read unix @->/run/systemd/private: read: connection reset by peer: unknown."
// https://github.com/moby/moby/issues/42793
return nil, fmt.Errorf("failed to connect to dbus (hint: for rootless containers, maybe you need to install dbus-user-session package, see https://github.com/opencontainers/runc/blob/master/docs/cgroup-v2.md): %w", err)
}
dbusC = conn
return conn, nil
}
func (d *dbusConnManager) newConnection() (*systemdDbus.Conn, error) {
if dbusRootless {
return newUserSystemdDbus()
}
return systemdDbus.NewWithContext(context.TODO())
}
// resetConnection resets the connection to its initial state
// (so it can be reconnected if necessary).
func (d *dbusConnManager) resetConnection(conn *systemdDbus.Conn) {
dbusMu.Lock()
defer dbusMu.Unlock()
if dbusC != nil && dbusC == conn {
dbusC.Close()
dbusC = nil
}
}
// retryOnDisconnect calls op, and if the error it returns is about closed dbus
// connection, the connection is re-established and the op is retried. This helps
// with the situation when dbus is restarted and we have a stale connection.
func (d *dbusConnManager) retryOnDisconnect(op func(*systemdDbus.Conn) error) error {
for {
conn, err := d.getConnection()
if err != nil {
return err
}
err = op(conn)
if err == nil {
return nil
}
if !errors.Is(err, dbus.ErrClosed) {
return err
}
d.resetConnection(conn)
}
}