lib/connections, lib/model: Track last connection duration (ref #7223) (#7242)

This adds a statistic to track the last connection duration per device.
It isn't used for much in this PR, but it's available for #7223 to use
in deciding how to order device connection attempts (deprioritizing
devices that just dropped our connection the last time).

lib/stats/device.go

@@ -10,26 +10,34 @@ import (
 	"time"
 
 	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/db/backend"
+	"github.com/syncthing/syncthing/lib/protocol"
+)
+
+const (
+	lastSeenKey     = "lastSeen"
+	connDurationKey = "lastConnDuration"
 )
 
 type DeviceStatistics struct {
-	LastSeen time.Time `json:"lastSeen"`
+	LastSeen                time.Time `json:"lastSeen"`
+	LastConnectionDurationS float64   `json:"lastConnectionDurationS"`
 }
 
 type DeviceStatisticsReference struct {
 	ns     *db.NamespacedKV
-	device string
+	device protocol.DeviceID
 }
 
-func NewDeviceStatisticsReference(ldb *db.Lowlevel, device string) *DeviceStatisticsReference {
+func NewDeviceStatisticsReference(dba backend.Backend, device protocol.DeviceID) *DeviceStatisticsReference {
 	return &DeviceStatisticsReference{
-		ns:     db.NewDeviceStatisticsNamespace(ldb, device),
+		ns:     db.NewDeviceStatisticsNamespace(dba, device.String()),
 		device: device,
 	}
 }
 
 func (s *DeviceStatisticsReference) GetLastSeen() (time.Time, error) {
-	t, ok, err := s.ns.Time("lastSeen")
+	t, ok, err := s.ns.Time(lastSeenKey)
 	if err != nil {
 		return time.Time{}, err
 	} else if !ok {
@@ -41,9 +49,25 @@ func (s *DeviceStatisticsReference) GetLastSeen() (time.Time, error) {
 	return t, nil
 }
 
+func (s *DeviceStatisticsReference) GetLastConnectionDuration() (time.Duration, error) {
+	d, ok, err := s.ns.Int64(connDurationKey)
+	if err != nil {
+		return 0, err
+	} else if !ok {
+		return 0, nil
+	}
+	l.Debugln("stats.DeviceStatisticsReference.GetLastConnectionDuration:", s.device, d)
+	return time.Duration(d), nil
+}
+
 func (s *DeviceStatisticsReference) WasSeen() error {
 	l.Debugln("stats.DeviceStatisticsReference.WasSeen:", s.device)
-	return s.ns.PutTime("lastSeen", time.Now())
+	return s.ns.PutTime(lastSeenKey, time.Now())
+}
+
+func (s *DeviceStatisticsReference) LastConnectionDuration(d time.Duration) error {
+	l.Debugln("stats.DeviceStatisticsReference.LastConnectionDuration:", s.device, d)
+	return s.ns.PutInt64(connDurationKey, d.Nanoseconds())
 }
 
 func (s *DeviceStatisticsReference) GetStatistics() (DeviceStatistics, error) {
@@ -51,7 +75,12 @@ func (s *DeviceStatisticsReference) GetStatistics() (DeviceStatistics, error) {
 	if err != nil {
 		return DeviceStatistics{}, err
 	}
+	lastConnDuration, err := s.GetLastConnectionDuration()
+	if err != nil {
+		return DeviceStatistics{}, err
+	}
 	return DeviceStatistics{
-		LastSeen: lastSeen,
+		LastSeen:                lastSeen,
+		LastConnectionDurationS: lastConnDuration.Seconds(),
 	}, nil
 }