lib/api: Improve folder summary event, verbose service (#9370)

This makes a couple of small improvements to the folder summary
mechanism:

- The folder summary includes the local and remote sequence numbers in
clear text, rather than some odd sum that I'm not sure what it was
intended to represent.
- The folder summary event is generated when appropriate, regardless of
whether there is an event listener. We did this before because
generating it was expensive, and we wanted to avoid doing it
unnecessarily. Nowadays, however, it's mostly just reading out
pre-calculated metadata, and anyway, it's nice if it shows up reliably
when running with -verbose.

The point of all this is to make it easier to use these events to judge
when devices are, in fact, in sync. As-is, if I'm looking at two
devices, it's very difficult to reliably determine if they are in sync
or not. The reason is that while we can ask device A if it thinks it's
in sync, we can't see if the answer is "yes" because it has processed
all changes from B, or if it just doesn't know about the changes from B
yet. With proper sequence numbers in the event we can compare the two
and determine the truth. This makes testing a lot easier.

lib/model/folder_summary.go

@@ -25,12 +25,9 @@ import (
 	"github.com/syncthing/syncthing/lib/sync"
 )
 
-const maxDurationSinceLastEventReq = time.Minute
-
 type FolderSummaryService interface {
 	suture.Service
 	Summary(folder string) (*FolderSummary, error)
-	OnEventRequest()
 }
 
 // The folderSummaryService adds summary information events (FolderSummary and
@@ -47,23 +44,18 @@ type folderSummaryService struct {
 	// For keeping track of folders to recalculate for
 	foldersMut sync.Mutex
 	folders    map[string]struct{}
-
-	// For keeping track of when the last event request on the API was
-	lastEventReq    time.Time
-	lastEventReqMut sync.Mutex
 }
 
 func NewFolderSummaryService(cfg config.Wrapper, m Model, id protocol.DeviceID, evLogger events.Logger) FolderSummaryService {
 	service := &folderSummaryService{
-		Supervisor:      suture.New("folderSummaryService", svcutil.SpecWithDebugLogger(l)),
-		cfg:             cfg,
-		model:           m,
-		id:              id,
-		evLogger:        evLogger,
-		immediate:       make(chan string),
-		folders:         make(map[string]struct{}),
-		foldersMut:      sync.NewMutex(),
-		lastEventReqMut: sync.NewMutex(),
+		Supervisor: suture.New("folderSummaryService", svcutil.SpecWithDebugLogger(l)),
+		cfg:        cfg,
+		model:      m,
+		id:         id,
+		evLogger:   evLogger,
+		immediate:  make(chan string),
+		folders:    make(map[string]struct{}),
+		foldersMut: sync.NewMutex(),
 	}
 
 	service.Add(svcutil.AsService(service.listenForUpdates, fmt.Sprintf("%s/listenForUpdates", service)))
@@ -119,8 +111,9 @@ type FolderSummary struct {
 	StateChanged time.Time `json:"stateChanged"`
 	Error        string    `json:"error"`
 
-	Version  int64 `json:"version"` // deprecated
-	Sequence int64 `json:"sequence"`
+	Version        int64                       `json:"version"` // deprecated
+	Sequence       int64                       `json:"sequence"`
+	RemoteSequence map[protocol.DeviceID]int64 `json:"remoteSequence"`
 
 	IgnorePatterns bool   `json:"ignorePatterns"`
 	WatchError     string `json:"watchError"`
@@ -130,7 +123,8 @@ func (c *folderSummaryService) Summary(folder string) (*FolderSummary, error) {
 	res := new(FolderSummary)
 
 	var local, global, need, ro db.Counts
-	var ourSeq, remoteSeq int64
+	var ourSeq int64
+	var remoteSeq map[protocol.DeviceID]int64
 	errors, err := c.model.FolderErrors(folder)
 	if err == nil {
 		var snap *db.Snapshot
@@ -140,7 +134,7 @@ func (c *folderSummaryService) Summary(folder string) (*FolderSummary, error) {
 			need = snap.NeedSize(protocol.LocalDeviceID)
 			ro = snap.ReceiveOnlyChangedSize()
 			ourSeq = snap.Sequence(protocol.LocalDeviceID)
-			remoteSeq = snap.Sequence(protocol.GlobalDeviceID)
+			remoteSeq = snap.RemoteSequences()
 			snap.Release()
 		}
 	}
@@ -192,8 +186,9 @@ func (c *folderSummaryService) Summary(folder string) (*FolderSummary, error) {
 		res.Error = err.Error()
 	}
 
-	res.Version = ourSeq + remoteSeq  // legacy
-	res.Sequence = ourSeq + remoteSeq // new name
+	res.Version = ourSeq // legacy
+	res.Sequence = ourSeq
+	res.RemoteSequence = remoteSeq
 
 	ignorePatterns, _, _ := c.model.CurrentIgnores(folder)
 	res.IgnorePatterns = false
@@ -212,12 +207,6 @@ func (c *folderSummaryService) Summary(folder string) (*FolderSummary, error) {
 	return res, nil
 }
 
-func (c *folderSummaryService) OnEventRequest() {
-	c.lastEventReqMut.Lock()
-	c.lastEventReq = time.Now()
-	c.lastEventReqMut.Unlock()
-}
-
 // listenForUpdates subscribes to the event bus and makes note of folders that
 // need their data recalculated.
 func (c *folderSummaryService) listenForUpdates(ctx context.Context) error {
@@ -357,17 +346,6 @@ func (c *folderSummaryService) calculateSummaries(ctx context.Context) error {
 // foldersToHandle returns the list of folders needing a summary update, and
 // clears the list.
 func (c *folderSummaryService) foldersToHandle() []string {
-	// We only recalculate summaries if someone is listening to events
-	// (a request to /rest/events has been made within the last
-	// pingEventInterval).
-
-	c.lastEventReqMut.Lock()
-	last := c.lastEventReq
-	c.lastEventReqMut.Unlock()
-	if time.Since(last) > maxDurationSinceLastEventReq {
-		return nil
-	}
-
 	c.foldersMut.Lock()
 	res := make([]string, 0, len(c.folders))
 	for folder := range c.folders {