Track assist bases and merge bases separately (#4181)

Do a few things to separate merge and assist bases
* use distinct sets for each base type
* make another function to return all base snapshots to pass to kopia assisted incrementals
* update function that removes a merge base to make it into an assist base

---

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

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [x] 🧹 Tech Debt/Cleanup

#### Issue(s)

* #3943
* #4178

#### Test Plan

- [ ] 💪 Manual
- [x] ⚡ Unit test
- [x] 💚 E2E

src/internal/kopia/backup_bases.go

@@ -15,7 +15,9 @@ import (
 // TODO(ashmrtn): Move this into some inject package. Here to avoid import
 // cycles.
 type BackupBases interface {
-	RemoveMergeBaseByManifestID(manifestID manifest.ID)
+	// ConvertToAssistBase converts the base with the given item data snapshot ID
+	// from a merge base to an assist base.
+	ConvertToAssistBase(manifestID manifest.ID)
 	Backups() []BackupEntry
 	UniqueAssistBackups() []BackupEntry
 	MinBackupVersion() int
@@ -28,6 +30,11 @@ type BackupBases interface {
 		other BackupBases,
 		reasonToKey func(identity.Reasoner) string,
 	) BackupBases
+	// SnapshotAssistBases returns the set of bases to use for kopia assisted
+	// incremental snapshot operations. It consists of the union of merge bases
+	// and assist bases. If DisableAssistBases has been called then it returns
+	// nil.
+	SnapshotAssistBases() []ManifestEntry
 }
 
 type backupBases struct {
@@ -46,35 +53,49 @@ type backupBases struct {
 	disableMergeBases bool
 }
 
-func (bb *backupBases) RemoveMergeBaseByManifestID(manifestID manifest.ID) {
+func (bb *backupBases) SnapshotAssistBases() []ManifestEntry {
+	if bb.disableAssistBases {
+		return nil
+	}
+
+	// Need to use the actual variables here because the functions will return nil
+	// depending on what's been marked as disabled.
+	return append(slices.Clone(bb.assistBases), bb.mergeBases...)
+}
+
+func (bb *backupBases) ConvertToAssistBase(manifestID manifest.ID) {
+	var (
+		snapshotMan ManifestEntry
+		base        BackupEntry
+		snapFound   bool
+	)
+
 	idx := slices.IndexFunc(
 		bb.mergeBases,
 		func(man ManifestEntry) bool {
 			return man.ID == manifestID
 		})
 	if idx >= 0 {
+		snapFound = true
+		snapshotMan = bb.mergeBases[idx]
 		bb.mergeBases = slices.Delete(bb.mergeBases, idx, idx+1)
 	}
 
-	// TODO(ashmrtn): This may not be strictly necessary but is at least easier to
-	// reason about.
-	idx = slices.IndexFunc(
-		bb.assistBases,
-		func(man ManifestEntry) bool {
-			return man.ID == manifestID
-		})
-	if idx >= 0 {
-		bb.assistBases = slices.Delete(bb.assistBases, idx, idx+1)
-	}
-
 	idx = slices.IndexFunc(
 		bb.backups,
 		func(bup BackupEntry) bool {
 			return bup.SnapshotID == string(manifestID)
 		})
 	if idx >= 0 {
+		base = bb.backups[idx]
 		bb.backups = slices.Delete(bb.backups, idx, idx+1)
 	}
+
+	// Account for whether we found the backup.
+	if idx >= 0 && snapFound {
+		bb.assistBackups = append(bb.assistBackups, base)
+		bb.assistBases = append(bb.assistBases, snapshotMan)
+	}
 }
 
 func (bb backupBases) Backups() []BackupEntry {
@@ -241,9 +262,6 @@ func (bb *backupBases) MergeBackupBases(
 
 		res.backups = append(res.backups, bup)
 		res.mergeBases = append(res.mergeBases, man)
-		// TODO(pandeyabs): Remove this once we remove overlap between
-		// between merge and assist bases as part of #3943.
-		res.assistBases = append(res.assistBases, man)
 	}
 
 	return res
@@ -383,16 +401,6 @@ func (bb *backupBases) fixupAndVerify(ctx context.Context) {
 		mergeToKeep = append(mergeToKeep, man)
 	}
 
-	// Every merge base is also a kopia assist base.
-	// TODO(pandeyabs): This should be removed as part of #3943.
-	for _, man := range bb.mergeBases {
-		if _, ok := toDrop[man.ID]; ok {
-			continue
-		}
-
-		assistToKeep = append(assistToKeep, man)
-	}
-
 	// Drop assist snapshots with overlapping reasons.
 	toDropAssists := findNonUniqueManifests(ctx, bb.assistBases)
 

src/internal/kopia/backup_bases_test.go

@@ -84,7 +84,7 @@ func (suite *BackupBasesUnitSuite) TestMinBackupVersion() {
 	}
 }
 
-func (suite *BackupBasesUnitSuite) TestRemoveMergeBaseByManifestID() {
+func (suite *BackupBasesUnitSuite) TestConvertToAssistBase() {
 	backups := []BackupEntry{
 		{Backup: &backup.Backup{SnapshotID: "1"}},
 		{Backup: &backup.Backup{SnapshotID: "2"}},
@@ -97,68 +97,71 @@ func (suite *BackupBasesUnitSuite) TestRemoveMergeBaseByManifestID() {
 		makeManifest("3", "", ""),
 	}
 
-	expected := &backupBases{
-		backups:     []BackupEntry{backups[0], backups[1]},
-		mergeBases:  []ManifestEntry{merges[0], merges[1]},
-		assistBases: []ManifestEntry{merges[0], merges[1]},
-	}
-
 	delID := manifest.ID("3")
 
 	table := []struct {
 		name string
 		// Below indices specify which items to add from the defined sets above.
-		backup []int
+		backup       []int
-		merge  []int
+		merge        []int
-		assist []int
+		assist       []int
+		expectAssist []int
 	}{
 		{
-			name:   "Not In Bases",
+			name:         "Not In Bases",
-			backup: []int{0, 1},
+			backup:       []int{0, 1},
-			merge:  []int{0, 1},
+			merge:        []int{0, 1},
-			assist: []int{0, 1},
+			assist:       []int{0, 1},
+			expectAssist: []int{0, 1},
 		},
 		{
-			name:   "Different Indexes",
+			name:         "Different Indexes",
-			backup: []int{2, 0, 1},
+			backup:       []int{2, 0, 1},
-			merge:  []int{0, 2, 1},
+			merge:        []int{0, 2, 1},
-			assist: []int{0, 1, 2},
+			assist:       []int{0, 1},
+			expectAssist: []int{0, 1, 2},
 		},
 		{
-			name:   "First Item",
+			name:         "First Item",
-			backup: []int{2, 0, 1},
+			backup:       []int{2, 0, 1},
-			merge:  []int{2, 0, 1},
+			merge:        []int{2, 0, 1},
-			assist: []int{2, 0, 1},
+			assist:       []int{0, 1},
+			expectAssist: []int{0, 1, 2},
 		},
 		{
-			name:   "Middle Item",
+			name:         "Middle Item",
-			backup: []int{0, 2, 1},
+			backup:       []int{0, 2, 1},
-			merge:  []int{0, 2, 1},
+			merge:        []int{0, 2, 1},
-			assist: []int{0, 2, 1},
+			assist:       []int{0, 1},
+			expectAssist: []int{0, 1, 2},
 		},
 		{
-			name:   "Final Item",
+			name:         "Final Item",
-			backup: []int{0, 1, 2},
+			backup:       []int{0, 1, 2},
-			merge:  []int{0, 1, 2},
+			merge:        []int{0, 1, 2},
-			assist: []int{0, 1, 2},
+			assist:       []int{0, 1},
+			expectAssist: []int{0, 1, 2},
 		},
 		{
-			name:   "Only In Backups",
+			name:         "Only In Backups",
-			backup: []int{0, 1, 2},
+			backup:       []int{0, 1, 2},
-			merge:  []int{0, 1},
+			merge:        []int{0, 1},
-			assist: []int{0, 1},
+			assist:       []int{0, 1},
+			expectAssist: []int{0, 1},
 		},
 		{
-			name:   "Only In Merges",
+			name:         "Only In Merges",
-			backup: []int{0, 1},
+			backup:       []int{0, 1},
-			merge:  []int{0, 1, 2},
+			merge:        []int{0, 1, 2},
-			assist: []int{0, 1},
+			assist:       []int{0, 1},
+			expectAssist: []int{0, 1},
 		},
 		{
-			name:   "Only In Assists",
+			name:         "Only In Assists Noops",
-			backup: []int{0, 1},
+			backup:       []int{0, 1},
-			merge:  []int{0, 1},
+			merge:        []int{0, 1},
-			assist: []int{0, 1, 2},
+			assist:       []int{0, 1, 2},
+			expectAssist: []int{0, 1, 2},
 		},
 	}
 
@@ -177,9 +180,20 @@ func (suite *BackupBasesUnitSuite) TestRemoveMergeBaseByManifestID() {
 
 			for _, i := range test.assist {
 				bb.assistBases = append(bb.assistBases, merges[i])
+				bb.assistBackups = append(bb.assistBackups, backups[i])
 			}
 
-			bb.RemoveMergeBaseByManifestID(delID)
+			expected := &backupBases{
+				backups:    []BackupEntry{backups[0], backups[1]},
+				mergeBases: []ManifestEntry{merges[0], merges[1]},
+			}
+
+			for _, i := range test.expectAssist {
+				expected.assistBases = append(expected.assistBases, merges[i])
+				expected.assistBackups = append(expected.assistBackups, backups[i])
+			}
+
+			bb.ConvertToAssistBase(delID)
 			AssertBackupBasesEqual(t, expected, bb)
 		})
 	}
@@ -201,6 +215,9 @@ func (suite *BackupBasesUnitSuite) TestDisableMergeBases() {
 	// Assist base set should be unchanged.
 	assert.Len(t, bb.UniqueAssistBackups(), 2)
 	assert.Len(t, bb.UniqueAssistBases(), 2)
+	// Merge bases should still appear in the assist base set passed in for kopia
+	// snapshots.
+	assert.Len(t, bb.SnapshotAssistBases(), 4)
 }
 
 func (suite *BackupBasesUnitSuite) TestDisableAssistBases() {
@@ -215,6 +232,7 @@ func (suite *BackupBasesUnitSuite) TestDisableAssistBases() {
 	bb.DisableAssistBases()
 	assert.Empty(t, bb.UniqueAssistBases())
 	assert.Empty(t, bb.UniqueAssistBackups())
+	assert.Empty(t, bb.SnapshotAssistBases())
 
 	// Merge base should be unchanged.
 	assert.Len(t, bb.Backups(), 2)
@@ -242,7 +260,6 @@ func (suite *BackupBasesUnitSuite) TestMergeBackupBases() {
 			}
 
 			m := makeManifest(baseID, "", "b"+baseID, reasons...)
-			res.assistBases = append(res.assistBases, m)
 
 			b := BackupEntry{
 				Backup: &backup.Backup{
@@ -588,7 +605,7 @@ func (suite *BackupBasesUnitSuite) TestFixupAndVerify() {
 			}(),
 			expect: func() *backupBases {
 				res := validMail1()
-				res.assistBases = res.mergeBases
+				res.assistBases = nil
 				res.assistBackups = nil
 
 				return res
@@ -620,7 +637,7 @@ func (suite *BackupBasesUnitSuite) TestFixupAndVerify() {
 			}(),
 			expect: func() *backupBases {
 				res := validMail1()
-				res.assistBases = res.mergeBases
+				res.assistBases = nil
 				res.assistBackups = nil
 
 				return res
@@ -651,14 +668,9 @@ func (suite *BackupBasesUnitSuite) TestFixupAndVerify() {
 			}(),
 		},
 		{
-			name: "Single Valid Entry",
+			name:   "Single Valid Entry",
-			bb:   validMail1(),
+			bb:     validMail1(),
-			expect: func() *backupBases {
+			expect: validMail1(),
-				res := validMail1()
-				res.assistBases = append(res.mergeBases, res.assistBases...)
-
-				return res
-			}(),
 		},
 		{
 			name: "Single Valid Entry With Incomplete Assist With Same Reason",
@@ -670,12 +682,7 @@ func (suite *BackupBasesUnitSuite) TestFixupAndVerify() {
 
 				return res
 			}(),
-			expect: func() *backupBases {
+			expect: validMail1(),
-				res := validMail1()
-
-				res.assistBases = append(res.mergeBases, res.assistBases...)
-				return res
-			}(),
 		},
 		{
 			name: "Single Valid Entry With Backup With Old Deets ID",
@@ -697,8 +704,6 @@ func (suite *BackupBasesUnitSuite) TestFixupAndVerify() {
 				res.assistBackups[0].DetailsID = res.assistBackups[0].StreamStoreID
 				res.assistBackups[0].StreamStoreID = ""
 
-				res.assistBases = append(res.mergeBases, res.assistBases...)
-
 				return res
 			}(),
 		},
@@ -726,8 +731,6 @@ func (suite *BackupBasesUnitSuite) TestFixupAndVerify() {
 					res.assistBases[0].Reasons,
 					NewReason("", ro, path.ExchangeService, path.ContactsCategory))
 
-				res.assistBases = append(res.mergeBases, res.assistBases...)
-
 				return res
 			}(),
 		},
@@ -791,7 +794,6 @@ func (suite *BackupBasesUnitSuite) TestFixupAndVerify() {
 				res.mergeBases = append(
 					res.mergeBases,
 					makeMan(path.EventsCategory, "id4", "", "bid4"))
-				res.assistBases = append(res.mergeBases, res.assistBases...)
 
 				return res
 			}(),
@@ -845,8 +847,6 @@ func (suite *BackupBasesUnitSuite) TestFixupAndVerify() {
 					res.assistBases,
 					makeMan(path.EventsCategory, "id4", "", "bid4"))
 
-				res.assistBases = append(res.mergeBases, res.assistBases...)
-
 				return res
 			}(),
 		},

src/internal/kopia/base_finder_test.go

@@ -300,6 +300,7 @@ func (suite *BaseFinderUnitSuite) TestNoResult_NoBackupsOrSnapshots() {
 	bb := bf.FindBases(ctx, reasons, nil)
 	assert.Empty(t, bb.MergeBases())
 	assert.Empty(t, bb.UniqueAssistBases())
+	assert.Empty(t, bb.SnapshotAssistBases())
 }
 
 func (suite *BaseFinderUnitSuite) TestNoResult_ErrorListingSnapshots() {
@@ -319,6 +320,7 @@ func (suite *BaseFinderUnitSuite) TestNoResult_ErrorListingSnapshots() {
 	bb := bf.FindBases(ctx, reasons, nil)
 	assert.Empty(t, bb.MergeBases())
 	assert.Empty(t, bb.UniqueAssistBases())
+	assert.Empty(t, bb.SnapshotAssistBases())
 }
 
 func (suite *BaseFinderUnitSuite) TestGetBases() {
@@ -331,11 +333,8 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 		expectedBaseReasons map[int][]identity.Reasoner
 		// Use this to denote the Reasons a kopia assised incrementals manifest is
 		// selected. The int maps to the index of the manifest in data.
-		// TODO(pandeyabs): Remove this once we have 1:1 mapping between snapshots
+		expectedAssistReasons map[int][]identity.Reasoner
-		// and backup models.
+		backupData            []backupInfo
-		expectedAssistManifestReasons map[int][]identity.Reasoner
-		expectedAssistReasons         map[int][]identity.Reasoner
-		backupData                    []backupInfo
 	}{
 		{
 			name:  "Return Older Merge Base If Fail To Get Manifest",
@@ -361,10 +360,6 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 			expectedBaseReasons: map[int][]identity.Reasoner{
 				1: testUser1Mail,
 			},
-			expectedAssistManifestReasons: map[int][]identity.Reasoner{
-				1: testUser1Mail,
-			},
-			expectedAssistReasons: map[int][]identity.Reasoner{},
 			backupData: []backupInfo{
 				newBackupModel(testBackup2, true, true, false, nil, nil),
 				newBackupModel(testBackup1, true, true, false, nil, nil),
@@ -391,10 +386,6 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 					testMail,
 					testUser1),
 			},
-			expectedBaseReasons: map[int][]identity.Reasoner{},
-			expectedAssistManifestReasons: map[int][]identity.Reasoner{
-				1: testUser1Mail,
-			},
 			expectedAssistReasons: map[int][]identity.Reasoner{
 				1: testUser1Mail,
 			},
@@ -433,9 +424,6 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 			expectedBaseReasons: map[int][]identity.Reasoner{
 				1: testUser1Mail,
 			},
-			expectedAssistManifestReasons: map[int][]identity.Reasoner{
-				1: testUser1Mail,
-			},
 			backupData: []backupInfo{
 				newBackupModel(testBackup2, false, false, false, nil, assert.AnError),
 				newBackupModel(testBackup1, true, true, false, nil, nil),
@@ -465,10 +453,6 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 			expectedBaseReasons: map[int][]identity.Reasoner{
 				1: testUser1Mail,
 			},
-			expectedAssistManifestReasons: map[int][]identity.Reasoner{
-				1: testUser1Mail,
-			},
-			expectedAssistReasons: map[int][]identity.Reasoner{},
 			backupData: []backupInfo{
 				newBackupModel(testBackup2, true, false, false, nil, nil),
 				newBackupModel(testBackup1, true, true, false, nil, nil),
@@ -493,10 +477,6 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 			expectedBaseReasons: map[int][]identity.Reasoner{
 				0: testUser1Mail,
 			},
-			expectedAssistManifestReasons: map[int][]identity.Reasoner{
-				0: testUser1Mail,
-			},
-			expectedAssistReasons: map[int][]identity.Reasoner{},
 			backupData: []backupInfo{
 				newBackupModel(testBackup1, true, true, true, nil, nil),
 			},
@@ -520,10 +500,6 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 			expectedBaseReasons: map[int][]identity.Reasoner{
 				0: testAllUsersAllCats,
 			},
-			expectedAssistManifestReasons: map[int][]identity.Reasoner{
-				0: testAllUsersAllCats,
-			},
-			expectedAssistReasons: map[int][]identity.Reasoner{},
 			backupData: []backupInfo{
 				newBackupModel(testBackup1, true, true, false, nil, nil),
 			},
@@ -544,10 +520,6 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 					testUser2,
 					testUser3),
 			},
-			expectedBaseReasons: map[int][]identity.Reasoner{},
-			expectedAssistManifestReasons: map[int][]identity.Reasoner{
-				0: testAllUsersAllCats,
-			},
 			expectedAssistReasons: map[int][]identity.Reasoner{
 				0: testAllUsersAllCats,
 			},
@@ -599,18 +571,6 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 					NewReason("", testUser3, path.ExchangeService, path.EventsCategory),
 				},
 			},
-			expectedAssistManifestReasons: map[int][]identity.Reasoner{
-				0: {
-					NewReason("", testUser1, path.ExchangeService, path.EmailCategory),
-					NewReason("", testUser2, path.ExchangeService, path.EmailCategory),
-					NewReason("", testUser3, path.ExchangeService, path.EmailCategory),
-				},
-				1: {
-					NewReason("", testUser1, path.ExchangeService, path.EventsCategory),
-					NewReason("", testUser2, path.ExchangeService, path.EventsCategory),
-					NewReason("", testUser3, path.ExchangeService, path.EventsCategory),
-				},
-			},
 			backupData: []backupInfo{
 				newBackupModel(testBackup1, true, true, false, nil, nil),
 				newBackupModel(testBackup2, true, true, false, nil, nil),
@@ -657,22 +617,6 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 					NewReason("", testUser2, path.ExchangeService, path.EventsCategory),
 				},
 			},
-			expectedAssistManifestReasons: map[int][]identity.Reasoner{
-				0: {
-					NewReason("", testUser1, path.ExchangeService, path.EventsCategory),
-					NewReason("", testUser2, path.ExchangeService, path.EventsCategory),
-				},
-				1: {
-					NewReason("", testUser1, path.ExchangeService, path.EmailCategory),
-					NewReason("", testUser2, path.ExchangeService, path.EmailCategory),
-				},
-				2: {
-					NewReason("", testUser1, path.ExchangeService, path.EmailCategory),
-					NewReason("", testUser2, path.ExchangeService, path.EmailCategory),
-					NewReason("", testUser1, path.ExchangeService, path.EventsCategory),
-					NewReason("", testUser2, path.ExchangeService, path.EventsCategory),
-				},
-			},
 			expectedAssistReasons: map[int][]identity.Reasoner{
 				0: {
 					NewReason("", testUser1, path.ExchangeService, path.EventsCategory),
@@ -725,9 +669,6 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 			expectedBaseReasons: map[int][]identity.Reasoner{
 				0: testUser1Mail,
 			},
-			expectedAssistManifestReasons: map[int][]identity.Reasoner{
-				0: testUser1Mail,
-			},
 			backupData: []backupInfo{
 				newBackupModel(testBackup1, true, true, false, nil, nil),
 				// Shouldn't be returned but have here just so we can see.
@@ -758,9 +699,6 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 			expectedBaseReasons: map[int][]identity.Reasoner{
 				1: testUser1Mail,
 			},
-			expectedAssistManifestReasons: map[int][]identity.Reasoner{
-				1: testUser1Mail,
-			},
 			backupData: []backupInfo{
 				// Shouldn't be returned but have here just so we can see.
 				newBackupModel(testBackup1, true, true, false, nil, nil),
@@ -788,8 +726,6 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 					testMail,
 					testUser1),
 			},
-			expectedBaseReasons:           map[int][]identity.Reasoner{},
-			expectedAssistManifestReasons: map[int][]identity.Reasoner{},
 			backupData: []backupInfo{
 				// Shouldn't be returned but have here just so we can see.
 				newBackupModel(testBackup1, true, true, false, nil, nil),
@@ -812,9 +748,6 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 			expectedBaseReasons: map[int][]identity.Reasoner{
 				0: testUser1Mail,
 			},
-			expectedAssistManifestReasons: map[int][]identity.Reasoner{
-				0: testUser1Mail,
-			},
 			backupData: []backupInfo{
 				newBackupModel(testBackup1, true, true, false, nil, nil),
 			},
@@ -845,9 +778,6 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 			expectedBaseReasons: map[int][]identity.Reasoner{
 				0: testUser1Mail,
 			},
-			expectedAssistManifestReasons: map[int][]identity.Reasoner{
-				0: testUser1Mail,
-			},
 			backupData: []backupInfo{
 				newBackupModel(testBackup2, true, true, false, nil, nil),
 				// Shouldn't be returned but here just so we can check.
@@ -894,10 +824,6 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 			expectedBaseReasons: map[int][]identity.Reasoner{
 				2: testUser1Mail,
 			},
-			expectedAssistManifestReasons: map[int][]identity.Reasoner{
-				0: testUser1Mail,
-				2: testUser1Mail,
-			},
 			expectedAssistReasons: map[int][]identity.Reasoner{
 				0: testUser1Mail,
 			},
@@ -944,10 +870,6 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 			expectedBaseReasons: map[int][]identity.Reasoner{
 				0: testUser1Mail,
 			},
-			expectedAssistManifestReasons: map[int][]identity.Reasoner{
-				0: testUser1Mail,
-			},
-			expectedAssistReasons: map[int][]identity.Reasoner{},
 			backupData: []backupInfo{
 				newBackupModel(testBackup2, true, true, false, nil, nil),
 				newBackupModel(
@@ -980,10 +902,6 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 					testMail,
 					testUser1),
 			},
-			expectedBaseReasons: map[int][]identity.Reasoner{},
-			expectedAssistManifestReasons: map[int][]identity.Reasoner{
-				0: testUser1Mail,
-			},
 			expectedAssistReasons: map[int][]identity.Reasoner{
 				0: testUser1Mail,
 			},
@@ -1020,10 +938,6 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 			expectedBaseReasons: map[int][]identity.Reasoner{
 				0: testUser1Mail,
 			},
-			expectedAssistManifestReasons: map[int][]identity.Reasoner{
-				0: testUser1Mail,
-			},
-			expectedAssistReasons: map[int][]identity.Reasoner{},
 			backupData: []backupInfo{
 				newBackupModel(testBackup2, true, true, false, nil, nil),
 				newBackupModel(
@@ -1074,7 +988,7 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 				t,
 				bb.UniqueAssistBases(),
 				test.manifestData,
-				test.expectedAssistManifestReasons)
+				test.expectedAssistReasons)
 		})
 	}
 }

src/internal/kopia/mock_backup_base.go

@@ -16,6 +16,7 @@ func AssertBackupBasesEqual(t *testing.T, expect, got BackupBases) {
 		assert.Empty(t, got.MergeBases(), "merge bases")
 		assert.Empty(t, got.UniqueAssistBackups(), "assist backups")
 		assert.Empty(t, got.UniqueAssistBases(), "assist bases")
+		assert.Empty(t, got.SnapshotAssistBases(), "snapshot assist bases")
 
 		return
 	}
@@ -24,7 +25,8 @@ func AssertBackupBasesEqual(t *testing.T, expect, got BackupBases) {
 		if len(expect.Backups()) > 0 ||
 			len(expect.MergeBases()) > 0 ||
 			len(expect.UniqueAssistBackups()) > 0 ||
-			len(expect.UniqueAssistBases()) > 0 {
+			len(expect.UniqueAssistBases()) > 0 ||
+			len(expect.SnapshotAssistBases()) > 0 {
 			assert.Fail(t, "got was nil but expected non-nil result %v", expect)
 		}
 
@@ -35,6 +37,7 @@ func AssertBackupBasesEqual(t *testing.T, expect, got BackupBases) {
 	assert.ElementsMatch(t, expect.MergeBases(), got.MergeBases(), "merge bases")
 	assert.ElementsMatch(t, expect.UniqueAssistBackups(), got.UniqueAssistBackups(), "assist backups")
 	assert.ElementsMatch(t, expect.UniqueAssistBases(), got.UniqueAssistBases(), "assist bases")
+	assert.ElementsMatch(t, expect.SnapshotAssistBases(), got.SnapshotAssistBases(), "snapshot assist bases")
 }
 
 func NewMockBackupBases() *MockBackupBases {
@@ -52,8 +55,6 @@ func (bb *MockBackupBases) WithBackups(b ...BackupEntry) *MockBackupBases {
 
 func (bb *MockBackupBases) WithMergeBases(m ...ManifestEntry) *MockBackupBases {
 	bb.backupBases.mergeBases = append(bb.MergeBases(), m...)
-	bb.backupBases.assistBases = append(bb.UniqueAssistBases(), m...)
-
 	return bb
 }
 
@@ -67,7 +68,12 @@ func (bb *MockBackupBases) WithAssistBases(m ...ManifestEntry) *MockBackupBases
 	return bb
 }
 
-func (bb *MockBackupBases) ClearMockAssistBases() *MockBackupBases {
+func (bb *MockBackupBases) MockDisableAssistBases() *MockBackupBases {
-	bb.backupBases.DisableAssistBases()
+	bb.DisableAssistBases()
+	return bb
+}
+
+func (bb *MockBackupBases) MockDisableMergeBases() *MockBackupBases {
+	bb.DisableMergeBases()
 	return bb
 }

src/internal/kopia/wrapper.go

@@ -178,7 +178,7 @@ func (w Wrapper) ConsumeBackupCollections(
 			mergeBase = bases.MergeBases()
 		}
 
-		assistBase = bases.UniqueAssistBases()
+		assistBase = bases.SnapshotAssistBases()
 	}
 
 	dirTree, err := inflateDirTree(

src/internal/kopia/wrapper_test.go

@@ -1002,7 +1002,7 @@ func (suite *KopiaIntegrationSuite) TestBackupCollections() {
 		{
 			name: "Merge Only",
 			baseBackups: func(base ManifestEntry) BackupBases {
-				return NewMockBackupBases().WithMergeBases(base).ClearMockAssistBases()
+				return NewMockBackupBases().WithMergeBases(base).MockDisableAssistBases()
 			},
 			// Pass in empty collections to force a backup. Otherwise we'll skip
 			// actually trying to do anything because we'll see there's nothing that

src/internal/operations/manifests_test.go

@@ -249,6 +249,19 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata() {
 		}
 	}
 
+	makeBackup := func(snapID string, cats ...path.CategoryType) kopia.BackupEntry {
+		return kopia.BackupEntry{
+			Backup: &backup.Backup{
+				BaseModel: model.BaseModel{
+					ID: model.StableID(snapID + "bup"),
+				},
+				SnapshotID:    snapID,
+				StreamStoreID: snapID + "store",
+			},
+			Reasons: buildReasons(ro, path.ExchangeService, cats...),
+		}
+	}
+
 	table := []struct {
 		name        string
 		bf          *mockBackupFinder
@@ -276,8 +289,9 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata() {
 			name: "don't get metadata",
 			bf: &mockBackupFinder{
 				data: map[string]kopia.BackupBases{
-					ro: kopia.NewMockBackupBases().WithMergeBases(
+					ro: kopia.NewMockBackupBases().
-						makeMan("id1", "", path.EmailCategory)),
+						WithMergeBases(makeMan("id1", "", path.EmailCategory)).
+						WithBackups(makeBackup("id1", path.EmailCategory)),
 				},
 			},
 			rp: mockRestoreProducer{},
@@ -288,8 +302,10 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata() {
 			assertErr: assert.NoError,
 			assertB:   assert.False,
 			expectDCS: nil,
-			expectMans: kopia.NewMockBackupBases().WithAssistBases(
+			expectMans: kopia.NewMockBackupBases().
-				makeMan("id1", "", path.EmailCategory)),
+				WithMergeBases(makeMan("id1", "", path.EmailCategory)).
+				WithBackups(makeBackup("id1", path.EmailCategory)).
+				MockDisableMergeBases(),
 		},
 		{
 			name: "don't get metadata, incomplete manifest",
@@ -359,9 +375,9 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata() {
 			name: "one valid man, extra incomplete man",
 			bf: &mockBackupFinder{
 				data: map[string]kopia.BackupBases{
-					ro: kopia.NewMockBackupBases().WithMergeBases(
+					ro: kopia.NewMockBackupBases().
-						makeMan("id1", "", path.EmailCategory)).WithAssistBases(
+						WithMergeBases(makeMan("id1", "", path.EmailCategory)).
-						makeMan("id2", "checkpoint", path.EmailCategory)),
+						WithAssistBases(makeMan("id2", "checkpoint", path.EmailCategory)),
 				},
 			},
 			rp: mockRestoreProducer{
@@ -377,17 +393,17 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata() {
 			assertErr: assert.NoError,
 			assertB:   assert.True,
 			expectDCS: []mockColl{{id: "id1"}},
-			expectMans: kopia.NewMockBackupBases().WithMergeBases(
+			expectMans: kopia.NewMockBackupBases().
-				makeMan("id1", "", path.EmailCategory)).WithAssistBases(
+				WithMergeBases(makeMan("id1", "", path.EmailCategory)).
-				makeMan("id2", "checkpoint", path.EmailCategory)),
+				WithAssistBases(makeMan("id2", "checkpoint", path.EmailCategory)),
 		},
 		{
-			name: "one valid man, extra incomplete man, no assist bases",
+			name: "one valid man, extra incomplete man, drop assist bases",
 			bf: &mockBackupFinder{
 				data: map[string]kopia.BackupBases{
-					ro: kopia.NewMockBackupBases().WithMergeBases(
+					ro: kopia.NewMockBackupBases().
-						makeMan("id1", "", path.EmailCategory)).WithAssistBases(
+						WithMergeBases(makeMan("id1", "", path.EmailCategory)).
-						makeMan("id2", "checkpoint", path.EmailCategory)),
+						WithAssistBases(makeMan("id2", "checkpoint", path.EmailCategory)),
 				},
 			},
 			rp: mockRestoreProducer{
@@ -404,9 +420,9 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata() {
 			assertErr:  assert.NoError,
 			assertB:    assert.True,
 			expectDCS:  []mockColl{{id: "id1"}},
-			expectMans: kopia.NewMockBackupBases().WithMergeBases(
+			expectMans: kopia.NewMockBackupBases().
-				makeMan("id1", "", path.EmailCategory)).
+				WithMergeBases(makeMan("id1", "", path.EmailCategory)).
-				ClearMockAssistBases(),
+				MockDisableAssistBases(),
 		},
 		{
 			name: "multiple valid mans",
@@ -438,8 +454,8 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata() {
 			name: "error collecting metadata",
 			bf: &mockBackupFinder{
 				data: map[string]kopia.BackupBases{
-					ro: kopia.NewMockBackupBases().WithMergeBases(
+					ro: kopia.NewMockBackupBases().
-						makeMan("id1", "", path.EmailCategory)),
+						WithMergeBases(makeMan("id1", "", path.EmailCategory)),
 				},
 			},
 			rp: mockRestoreProducer{err: assert.AnError},
@@ -575,9 +591,9 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata_Fallb
 			name: "don't get metadata, only fallbacks",
 			bf: &mockBackupFinder{
 				data: map[string]kopia.BackupBases{
-					fbro: kopia.NewMockBackupBases().WithMergeBases(
+					fbro: kopia.NewMockBackupBases().
-						makeMan(fbro, "fb_id1", "", path.EmailCategory)).WithBackups(
+						WithMergeBases(makeMan(fbro, "fb_id1", "", path.EmailCategory)).
-						makeBackup(fbro, "fb_id1", path.EmailCategory)),
+						WithBackups(makeBackup(fbro, "fb_id1", path.EmailCategory)),
 				},
 			},
 			rp:              mockRestoreProducer{},
@@ -586,8 +602,10 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata_Fallb
 			assertErr:       assert.NoError,
 			assertB:         assert.False,
 			expectDCS:       nil,
-			expectMans: kopia.NewMockBackupBases().WithAssistBases(
+			expectMans: kopia.NewMockBackupBases().
-				makeMan(fbro, "fb_id1", "", path.EmailCategory)),
+				WithMergeBases(makeMan(fbro, "fb_id1", "", path.EmailCategory)).
+				WithBackups(makeBackup(fbro, "fb_id1", path.EmailCategory)).
+				MockDisableMergeBases(),
 		},
 		{
 			name: "only fallbacks",
@@ -608,17 +626,17 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata_Fallb
 			assertErr:       assert.NoError,
 			assertB:         assert.True,
 			expectDCS:       []mockColl{{id: "fb_id1"}},
-			expectMans: kopia.NewMockBackupBases().WithMergeBases(
+			expectMans: kopia.NewMockBackupBases().
-				makeMan(fbro, "fb_id1", "", path.EmailCategory)).WithBackups(
+				WithMergeBases(makeMan(fbro, "fb_id1", "", path.EmailCategory)).
-				makeBackup(fbro, "fb_id1", path.EmailCategory)),
+				WithBackups(makeBackup(fbro, "fb_id1", path.EmailCategory)),
 		},
 		{
-			name: "only fallbacks, no assist",
+			name: "only fallbacks, drop assist",
 			bf: &mockBackupFinder{
 				data: map[string]kopia.BackupBases{
-					fbro: kopia.NewMockBackupBases().WithMergeBases(
+					fbro: kopia.NewMockBackupBases().
-						makeMan(fbro, "fb_id1", "", path.EmailCategory)).WithBackups(
+						WithMergeBases(makeMan(fbro, "fb_id1", "", path.EmailCategory)).
-						makeBackup(fbro, "fb_id1", path.EmailCategory)),
+						WithBackups(makeBackup(fbro, "fb_id1", path.EmailCategory)),
 				},
 			},
 			rp: mockRestoreProducer{
@@ -632,20 +650,20 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata_Fallb
 			assertErr:       assert.NoError,
 			assertB:         assert.True,
 			expectDCS:       []mockColl{{id: "fb_id1"}},
-			expectMans: kopia.NewMockBackupBases().WithMergeBases(
+			expectMans: kopia.NewMockBackupBases().
-				makeMan(fbro, "fb_id1", "", path.EmailCategory)).WithBackups(
+				WithMergeBases(makeMan(fbro, "fb_id1", "", path.EmailCategory)).
-				makeBackup(fbro, "fb_id1", path.EmailCategory)).
+				WithBackups(makeBackup(fbro, "fb_id1", path.EmailCategory)).
-				ClearMockAssistBases(),
+				MockDisableAssistBases(),
 		},
 		{
 			name: "complete mans and fallbacks",
 			bf: &mockBackupFinder{
 				data: map[string]kopia.BackupBases{
-					ro: kopia.NewMockBackupBases().WithMergeBases(
+					ro: kopia.NewMockBackupBases().
-						makeMan(ro, "id1", "", path.EmailCategory)),
+						WithMergeBases(makeMan(ro, "id1", "", path.EmailCategory)),
-					fbro: kopia.NewMockBackupBases().WithMergeBases(
+					fbro: kopia.NewMockBackupBases().
-						makeMan(fbro, "fb_id1", "", path.EmailCategory)).WithBackups(
+						WithMergeBases(makeMan(fbro, "fb_id1", "", path.EmailCategory)).
-						makeBackup(fbro, "fb_id1", path.EmailCategory)),
+						WithBackups(makeBackup(fbro, "fb_id1", path.EmailCategory)),
 				},
 			},
 			rp: mockRestoreProducer{
@@ -692,13 +710,13 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata_Fallb
 			name: "complete and incomplete mans and fallbacks",
 			bf: &mockBackupFinder{
 				data: map[string]kopia.BackupBases{
-					ro: kopia.NewMockBackupBases().WithMergeBases(
+					ro: kopia.NewMockBackupBases().
-						makeMan(ro, "id1", "", path.EmailCategory)).WithAssistBases(
+						WithMergeBases(makeMan(ro, "id1", "", path.EmailCategory)).
-						makeMan(ro, "id2", "checkpoint", path.EmailCategory)),
+						WithAssistBases(makeMan(ro, "id2", "checkpoint", path.EmailCategory)),
-					fbro: kopia.NewMockBackupBases().WithMergeBases(
+					fbro: kopia.NewMockBackupBases().
-						makeMan(fbro, "fb_id1", "", path.EmailCategory)).WithBackups(
+						WithMergeBases(makeMan(fbro, "fb_id1", "", path.EmailCategory)).
-						makeBackup(fbro, "fb_id1", path.EmailCategory)).WithAssistBases(
+						WithBackups(makeBackup(fbro, "fb_id1", path.EmailCategory)).
-						makeMan(fbro, "fb_id2", "checkpoint", path.EmailCategory)),
+						WithAssistBases(makeMan(fbro, "fb_id2", "checkpoint", path.EmailCategory)),
 				},
 			},
 			rp: mockRestoreProducer{
@@ -715,9 +733,9 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata_Fallb
 			assertErr:       assert.NoError,
 			assertB:         assert.True,
 			expectDCS:       []mockColl{{id: "id1"}},
-			expectMans: kopia.NewMockBackupBases().WithMergeBases(
+			expectMans: kopia.NewMockBackupBases().
-				makeMan(ro, "id1", "", path.EmailCategory)).WithAssistBases(
+				WithMergeBases(makeMan(ro, "id1", "", path.EmailCategory)).
-				makeMan(ro, "id2", "checkpoint", path.EmailCategory)),
+				WithAssistBases(makeMan(ro, "id2", "checkpoint", path.EmailCategory)),
 		},
 		{
 			name: "incomplete mans and complete fallbacks",
@@ -725,9 +743,9 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata_Fallb
 				data: map[string]kopia.BackupBases{
 					ro: kopia.NewMockBackupBases().WithAssistBases(
 						makeMan(ro, "id2", "checkpoint", path.EmailCategory)),
-					fbro: kopia.NewMockBackupBases().WithMergeBases(
+					fbro: kopia.NewMockBackupBases().
-						makeMan(fbro, "fb_id1", "", path.EmailCategory)).WithBackups(
+						WithMergeBases(makeMan(fbro, "fb_id1", "", path.EmailCategory)).
-						makeBackup(fbro, "fb_id1", path.EmailCategory)),
+						WithBackups(makeBackup(fbro, "fb_id1", path.EmailCategory)),
 				},
 			},
 			rp: mockRestoreProducer{
@@ -742,10 +760,10 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata_Fallb
 			assertErr:       assert.NoError,
 			assertB:         assert.True,
 			expectDCS:       []mockColl{{id: "fb_id1"}},
-			expectMans: kopia.NewMockBackupBases().WithMergeBases(
+			expectMans: kopia.NewMockBackupBases().
-				makeMan(fbro, "fb_id1", "", path.EmailCategory)).WithBackups(
+				WithMergeBases(makeMan(fbro, "fb_id1", "", path.EmailCategory)).
-				makeBackup(fbro, "fb_id1", path.EmailCategory)).WithAssistBases(
+				WithBackups(makeBackup(fbro, "fb_id1", path.EmailCategory)).
-				makeMan(ro, "id2", "checkpoint", path.EmailCategory)),
+				WithAssistBases(makeMan(ro, "id2", "checkpoint", path.EmailCategory)),
 		},
 		{
 			name: "incomplete mans and complete fallbacks, no assist bases",
@@ -753,9 +771,9 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata_Fallb
 				data: map[string]kopia.BackupBases{
 					ro: kopia.NewMockBackupBases().WithAssistBases(
 						makeMan(ro, "id2", "checkpoint", path.EmailCategory)),
-					fbro: kopia.NewMockBackupBases().WithMergeBases(
+					fbro: kopia.NewMockBackupBases().
-						makeMan(fbro, "fb_id1", "", path.EmailCategory)).WithBackups(
+						WithMergeBases(makeMan(fbro, "fb_id1", "", path.EmailCategory)).
-						makeBackup(fbro, "fb_id1", path.EmailCategory)),
+						WithBackups(makeBackup(fbro, "fb_id1", path.EmailCategory)),
 				},
 			},
 			rp: mockRestoreProducer{
@@ -771,10 +789,10 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata_Fallb
 			assertErr:       assert.NoError,
 			assertB:         assert.True,
 			expectDCS:       []mockColl{{id: "fb_id1"}},
-			expectMans: kopia.NewMockBackupBases().WithMergeBases(
+			expectMans: kopia.NewMockBackupBases().
-				makeMan(fbro, "fb_id1", "", path.EmailCategory)).WithBackups(
+				WithMergeBases(makeMan(fbro, "fb_id1", "", path.EmailCategory)).
-				makeBackup(fbro, "fb_id1", path.EmailCategory)).
+				WithBackups(makeBackup(fbro, "fb_id1", path.EmailCategory)).
-				ClearMockAssistBases(),
+				MockDisableAssistBases(),
 		},
 		{
 			name: "complete mans and incomplete fallbacks",
@@ -807,9 +825,9 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata_Fallb
 				data: map[string]kopia.BackupBases{
 					ro: kopia.NewMockBackupBases().WithMergeBases(
 						makeMan(ro, "id1", "", path.EmailCategory, path.ContactsCategory)),
-					fbro: kopia.NewMockBackupBases().WithMergeBases(
+					fbro: kopia.NewMockBackupBases().
-						makeMan(fbro, "fb_id1", "", path.EmailCategory, path.ContactsCategory)).WithBackups(
+						WithMergeBases(makeMan(fbro, "fb_id1", "", path.EmailCategory, path.ContactsCategory)).
-						makeBackup(fbro, "fb_id1", path.EmailCategory, path.ContactsCategory)),
+						WithBackups(makeBackup(fbro, "fb_id1", path.EmailCategory, path.ContactsCategory)),
 				},
 			},
 			rp: mockRestoreProducer{
@@ -839,9 +857,9 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata_Fallb
 				data: map[string]kopia.BackupBases{
 					ro: kopia.NewMockBackupBases().WithMergeBases(
 						makeMan(ro, "id1", "", path.EmailCategory)),
-					fbro: kopia.NewMockBackupBases().WithMergeBases(
+					fbro: kopia.NewMockBackupBases().
-						makeMan(fbro, "fb_id1", "", path.ContactsCategory)).WithBackups(
+						WithMergeBases(makeMan(fbro, "fb_id1", "", path.ContactsCategory)).
-						makeBackup(fbro, "fb_id1", path.ContactsCategory)),
+						WithBackups(makeBackup(fbro, "fb_id1", path.ContactsCategory)),
 				},
 			},
 			rp: mockRestoreProducer{
@@ -858,10 +876,11 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata_Fallb
 			assertErr: assert.NoError,
 			assertB:   assert.True,
 			expectDCS: []mockColl{{id: "id1"}, {id: "fb_id1"}},
-			expectMans: kopia.NewMockBackupBases().WithMergeBases(
+			expectMans: kopia.NewMockBackupBases().
-				makeMan(ro, "id1", "", path.EmailCategory),
+				WithMergeBases(
-				makeMan(fbro, "fb_id1", "", path.ContactsCategory)).WithBackups(
+					makeMan(ro, "id1", "", path.EmailCategory),
-				makeBackup(fbro, "fb_id1", path.ContactsCategory)),
+					makeMan(fbro, "fb_id1", "", path.ContactsCategory)).
+				WithBackups(makeBackup(fbro, "fb_id1", path.ContactsCategory)),
 		},
 		{
 			name: "complete mans and complete fallbacks, fallback has superset of reasons",
@@ -869,9 +888,11 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata_Fallb
 				data: map[string]kopia.BackupBases{
 					ro: kopia.NewMockBackupBases().WithMergeBases(
 						makeMan(ro, "id1", "", path.EmailCategory)),
-					fbro: kopia.NewMockBackupBases().WithMergeBases(
+					fbro: kopia.NewMockBackupBases().
-						makeMan(fbro, "fb_id1", "", path.EmailCategory, path.ContactsCategory)).WithBackups(
+						WithMergeBases(
-						makeBackup(fbro, "fb_id1", path.EmailCategory, path.ContactsCategory)),
+							makeMan(fbro, "fb_id1", "", path.EmailCategory, path.ContactsCategory)).
+						WithBackups(
+							makeBackup(fbro, "fb_id1", path.EmailCategory, path.ContactsCategory)),
 				},
 			},
 			rp: mockRestoreProducer{
@@ -892,10 +913,12 @@ func (suite *OperationsManifestsUnitSuite) TestProduceManifestsAndMetadata_Fallb
 			assertErr: assert.NoError,
 			assertB:   assert.True,
 			expectDCS: []mockColl{{id: "id1"}, {id: "fb_id1"}},
-			expectMans: kopia.NewMockBackupBases().WithMergeBases(
+			expectMans: kopia.NewMockBackupBases().
-				makeMan(ro, "id1", "", path.EmailCategory),
+				WithMergeBases(
-				makeMan(fbro, "fb_id1", "", path.ContactsCategory)).WithBackups(
+					makeMan(ro, "id1", "", path.EmailCategory),
-				makeBackup(fbro, "fb_id1", path.ContactsCategory)),
+					makeMan(fbro, "fb_id1", "", path.ContactsCategory)).
+				WithBackups(
+					makeBackup(fbro, "fb_id1", path.ContactsCategory)),
 		},
 	}