lib/model, gui: Correct completion percentages when there are lots of deletes (fixes #3496)

We used to consider deleted files & directories 128 bytes large. After
the delta indexes change a bug slipped in where deleted files would be
weighted according to their old non-deleted size. Both ways are
incorrect (but the latest change made it worse), as if there are more
files deleted than remaining data in the repo the needSize can be
greater than the globalSize, resulting in a negative completion
percentage.

This change makes it so that deleted items are zero bytes large, which
makes more sense. Instead we expose the number of files that we need to
delete as a separate field in the Completion() result, and hack the
percentage down to 95% complete if it was 100% complete but we need to
delete files. This latter part is sort of ugly, but necessary to give
the user some sort of feedback.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3556

lib/model/model.go

@@ -463,6 +463,7 @@ type FolderCompletion struct {
 	CompletionPct float64
 	NeedBytes     int64
 	GlobalBytes   int64
+	NeedDeletes   int64
 }
 
 // Completion returns the completion status, in percent, for the given device
@@ -487,14 +488,20 @@ func (m *Model) Completion(device protocol.DeviceID, folder string) FolderComple
 	counts := m.deviceDownloads[device].GetBlockCounts(folder)
 	m.pmut.RUnlock()
 
-	var need, fileNeed, downloaded int64
+	var need, fileNeed, downloaded, deletes int64
 	rf.WithNeedTruncated(device, func(f db.FileIntf) bool {
 		ft := f.(db.FileInfoTruncated)
 
+		// If the file is deleted, we account it only in the deleted column.
+		if ft.Deleted {
+			deletes++
+			return true
+		}
+
 		// This might might be more than it really is, because some blocks can be of a smaller size.
 		downloaded = int64(counts[ft.Name] * protocol.BlockSize)
 
-		fileNeed = ft.Size - downloaded
+		fileNeed = ft.FileSize() - downloaded
 		if fileNeed < 0 {
 			fileNeed = 0
 		}
@@ -505,12 +512,22 @@ func (m *Model) Completion(device protocol.DeviceID, folder string) FolderComple
 
 	needRatio := float64(need) / float64(tot)
 	completionPct := 100 * (1 - needRatio)
+
+	// If the completion is 100% but there are deletes we need to handle,
+	// drop it down a notch. Hack for consumers that look only at the
+	// percentage (our own GUI does the same calculation as here on it's own
+	// and needs the same fixup).
+	if need == 0 && deletes > 0 {
+		completionPct = 95 // chosen by fair dice roll
+	}
+
 	l.Debugf("%v Completion(%s, %q): %f (%d / %d = %f)", m, device, folder, completionPct, need, tot, needRatio)
 
 	return FolderCompletion{
 		CompletionPct: completionPct,
 		NeedBytes:     need,
 		GlobalBytes:   tot,
+		NeedDeletes:   deletes,
 	}
 }
 
@@ -1762,7 +1779,7 @@ func (m *Model) internalScanFolderSubdirs(folder string, subDirs []string) error
 					nf := protocol.FileInfo{
 						Name:       f.Name,
 						Type:       f.Type,
-						Size:       f.Size,
+						Size:       0,
 						ModifiedS:  f.ModifiedS,
 						ModifiedNs: f.ModifiedNs,
 						Deleted:    true,
@@ -1927,6 +1944,7 @@ func (m *Model) Override(folder string) {
 			need.Deleted = true
 			need.Blocks = nil
 			need.Version = need.Version.Update(m.shortID)
+			need.Size = 0
 		} else {
 			// We have the file, replace with our version
 			have.Version = have.Version.Merge(need.Version).Update(m.shortID)