msa-agent/internal/config/live.go

package config

import (
	"crypto/ed25519"
	"encoding/base64"
	"fmt"
	"os"
	"sync"
	"time"
)

// LiveDevices is the node's paired-device set, kept in step with the config file.
//
// The daemon used to load its device list once at startup and hold it in memory forever. That made
// the documented revocation path not work: the operator runs `--pair`, POSTs /destroy, the file on
// disk is emptied — and the running daemon keeps answering with the list it read at boot, then
// re-pushes that list to the relay on its next handshake. So a revoked phone stayed authorised at
// the node and stayed routable at the relay, and the only way to make the revocation real was to
// restart the service, which the deployment document does not say.
//
// The design anticipated this: the Devices interface in internal/agent notes that the first
// implementation is a static list and the intended one is a snapshot pulled from the relay. This is
// the smaller step that makes revocation work without a new wire protocol — the file is already the
// authority, it just was not being re-read.
//
// Reloading is driven by the file's modification time and size rather than a timer, so a poll cycle
// pays one stat and only re-reads when something actually changed. A read that fails leaves the
// previous set in place: a transient filesystem error must not turn into "nobody is paired", which
// would lock the user out of a node whose configuration is fine.
type LiveDevices struct {
	path string

	mu      sync.RWMutex
	keys    map[string]ed25519.PublicKey
	devices []Device
	stamp   fileStamp
}

// fileStamp is what a change is detected by. Modification time alone is not enough: two writes
// inside the same clock tick are possible, and a pairing followed immediately by a destroy is
// exactly that. The size catches those, and the inode catches a rename over the file — which is how
// Save writes, so every legitimate change is a rename.
type fileStamp struct {
	modTime time.Time
	size    int64
	inode   uint64
}

// NewLiveDevices reads the device list and remembers how to tell when it changes.
func NewLiveDevices(path string) (*LiveDevices, error) {
	d := &LiveDevices{path: path}
	if err := d.reload(); err != nil {
		return nil, err
	}
	return d, nil
}

// SigningKey returns the phone's public key, or nil when it is not paired.
//
// The read is on the hot path — every envelope is verified through it — so it takes the read lock
// and does no I/O. Reloading happens in Refresh, which the loop calls once per cycle.
func (d *LiveDevices) SigningKey(app string) ed25519.PublicKey {
	d.mu.RLock()
	defer d.mu.RUnlock()
	return d.keys[app]
}

// Len reports how many phones are trusted.
func (d *LiveDevices) Len() int {
	d.mu.RLock()
	defer d.mu.RUnlock()
	return len(d.keys)
}

// Devices returns a copy of the current paired-device list, for the handshake's registration push.
func (d *LiveDevices) Devices() []Device {
	d.mu.RLock()
	defer d.mu.RUnlock()
	return append([]Device(nil), d.devices...)
}

// Refresh re-reads the config if it changed on disk, and reports whether it did.
//
// The caller uses the report to decide whether to re-assert the set at the relay: a destroy that
// only took effect locally would leave the revoked bearer routable, and the phone would get
// "unknown device" from a node that had stopped trusting it rather than a clean refusal.
func (d *LiveDevices) Refresh() (bool, error) {
	stamp, err := stampOf(d.path)
	if err != nil {
		return false, err
	}
	d.mu.RLock()
	unchanged := stamp == d.stamp
	d.mu.RUnlock()
	if unchanged {
		return false, nil
	}
	if err := d.reload(); err != nil {
		return false, err
	}
	return true, nil
}

func (d *LiveDevices) reload() error {
	cfg, err := loadFile(d.path)
	if err != nil {
		return err
	}
	// Pair mode is the right validation here: a node whose list was just destroyed has zero devices
	// and must still be able to load, because that state is the whole point of a revocation.
	if err := cfg.Validate(true); err != nil {
		return fmt.Errorf("config: %s: %w", d.path, err)
	}
	keys := make(map[string]ed25519.PublicKey, len(cfg.Devices))
	for _, device := range cfg.Devices {
		raw, err := base64.StdEncoding.Strict().DecodeString(device.SignKey)
		if err != nil || len(raw) != ed25519.PublicKeySize {
			return fmt.Errorf("config: device %s has an unusable signKey", device.ID)
		}
		keys[device.ID] = ed25519.PublicKey(raw)
	}
	stamp, err := stampOf(d.path)
	if err != nil {
		return err
	}
	d.mu.Lock()
	d.keys = keys
	d.devices = append([]Device(nil), cfg.Devices...)
	d.stamp = stamp
	d.mu.Unlock()
	return nil
}

func stampOf(path string) (fileStamp, error) {
	info, err := os.Stat(path)
	if err != nil {
		return fileStamp{}, fmt.Errorf("config: %w", err)
	}
	return fileStamp{modTime: info.ModTime(), size: info.Size(), inode: inodeOf(info)}, nil
}