full backup if prev snapshots have no details (#2049)

## Description

In the event that a backup is completed but the details, somehow, isn't persisted, we want the next backup to do a full, instead of an incremental, backup.  If we don't have this protection the following backups could end up in a bad state.  Future changes will add better resilience so that the fallback isn't needed.

## Does this PR need a docs update or release note?

- [x] ⛔ No 

## Type of change

- [x] 🌻 Feature

## Issue(s)

* #1878

## Test Plan

- [x] 💚 E2E

src/internal/kopia/snapshot_manager.go

@@ -322,11 +322,9 @@ func fetchPrevSnapshotManifests(
 				// If the manifest already exists and it's incomplete then we should
 				// merge the reasons for consistency. This will become easier to handle
 				// once we update how checkpoint manifests are tagged.
-				if len(found.IncompleteReason) == 0 {
-					continue
+				if len(found.IncompleteReason) > 0 {
+					found.Reasons = append(found.Reasons, m.Reasons...)
 				}
-
-				found.Reasons = append(found.Reasons, m.Reasons...)
 			}
 		}
 	}

src/internal/operations/backup.go

@@ -283,6 +283,31 @@ func produceManifestsAndMetadata(
 			continue
 		}
 
+		tk, _ := kopia.MakeTagKV(kopia.TagBackupID)
+
+		bID := man.Tags[tk]
+		if len(bID) == 0 {
+			return nil, nil, errors.New("missing backup id in prior manifest")
+		}
+
+		dID, _, err := sw.GetDetailsIDFromBackupID(ctx, model.StableID(bID))
+		if err != nil {
+			return nil, nil, errors.Wrap(err, "retrieving prior backup data")
+		}
+
+		// if no detailsID exists for any of the complete manifests, we want
+		// to fall back to a complete backup.  This is a temporary prevention
+		// mechanism to keep backups from falling into a perpetually bad state.
+		// This makes an assumption that the ID points to a populated set of
+		// details; we aren't doing the work to look them up.
+		if len(dID) == 0 {
+			logger.Ctx(ctx).Infow(
+				"backup missing details ID, falling back to full backup",
+				"backup_id", bID)
+
+			return ms, nil, nil
+		}
+
 		colls, err := collectMetadata(ctx, kw, man, metadataFiles, tenantID)
 		if err != nil && !errors.Is(err, kopia.ErrNotFound) {
 			// prior metadata isn't guaranteed to exist.