lib/model: Stop summary sender faster (ref #6319) (#6341)

One of the causes of "panic: database is closed" is that we try to send
summaries after it's been closed. Calculating summaries can take a long
time and if we have a lot of folders it's not unreasonable to think
that we might be stopped in this loop, so prepare to bail here.

* push

lib/model/folder_summary.go

@@ -270,7 +270,12 @@ func (c *folderSummaryService) calculateSummaries(ctx context.Context) {
 		case <-pump.C:
 			t0 := time.Now()
 			for _, folder := range c.foldersToHandle() {
-				c.sendSummary(folder)
+				select {
+				case <-ctx.Done():
+					return
+				default:
+				}
+				c.sendSummary(ctx, folder)
 			}
 
 			// We don't want to spend all our time calculating summaries. Lets
@@ -280,7 +285,7 @@ func (c *folderSummaryService) calculateSummaries(ctx context.Context) {
 			pump.Reset(wait)
 
 		case folder := <-c.immediate:
-			c.sendSummary(folder)
+			c.sendSummary(ctx, folder)
 
 		case <-ctx.Done():
 			return
@@ -313,7 +318,7 @@ func (c *folderSummaryService) foldersToHandle() []string {
 }
 
 // sendSummary send the summary events for a single folder
-func (c *folderSummaryService) sendSummary(folder string) {
+func (c *folderSummaryService) sendSummary(ctx context.Context, folder string) {
 	// The folder summary contains how many bytes, files etc
 	// are in the folder and how in sync we are.
 	data, err := c.Summary(folder)
@@ -326,6 +331,12 @@ func (c *folderSummaryService) sendSummary(folder string) {
 	})
 
 	for _, devCfg := range c.cfg.Folders()[folder].Devices {
+		select {
+		case <-ctx.Done():
+			return
+		default:
+		}
+
 		if devCfg.DeviceID.Equals(c.id) {
 			// We already know about ourselves.
 			continue