Integrate lazy item reader into conversations backup (#4921)

<!-- PR description-->




---

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

- [x] ✅ Yes, it's included

#### Type of change

<!--- Please check the type of change your PR introduces: --->
- [x] 🌻 Feature


#### Issue(s)

<!-- Can reference multiple issues. Use one of the following "magic words" - "closes, fixes" to auto-close the Github issue. -->
* Closes https://github.com/alcionai/corso/issues/4862

#### Test Plan

<!-- How will this be tested prior to merging.-->
- [x] 💪 Manual
- [x] 💚 E2E

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 this case, Corso will skip over the item but report this in the backup summary.
 - Guarantee Exchange email restoration when restoring multiple attachments. Some previous restores were failing with `ErrorItemNotFound`.
 - Avoid Graph SDK `Requests must contain extension changes exclusively.` errors by removing server-populated field from restored event items.
+- Improve Group mailbox(conversations) backup performance by only downloading new items or items with modified content.
 - Handle cases where Exchange backup stored invalid JSON blobs if there were special characters in the user content. These would result in errors during restore or restore errors.
 
 ### Known issues

src/internal/m365/collection/groups/backup.go

@@ -22,13 +22,7 @@ import (
 	"github.com/alcionai/corso/src/pkg/services/m365/api/graph"
 )
 
-// TODO: incremental support
-// multiple lines in this file are commented out so that
-// we can focus on v0 backups and re-integrate them later
-// for v1 incrementals.
-// since these lines represent otherwise standard boilerplate,
-// it's simpler to comment them for tracking than to delete
-// and re-discover them later.
+// TODO: incremental support for channels
 
 func CreateCollections[C graph.GetIDer, I groupsItemer](
 	ctx context.Context,
@@ -37,6 +31,7 @@ func CreateCollections[C graph.GetIDer, I groupsItemer](
 	tenantID string,
 	scope selectors.GroupsScope,
 	su support.StatusUpdater,
+	useLazyReader bool,
 	counter *count.Bus,
 	errs *fault.Bus,
 ) ([]data.BackupCollection, bool, error) {
@@ -76,6 +71,7 @@ func CreateCollections[C graph.GetIDer, I groupsItemer](
 		containers,
 		scope,
 		cdps[scope.Category().PathType()],
+		useLazyReader,
 		bpc.Options,
 		counter,
 		errs)
@@ -98,6 +94,7 @@ func populateCollections[C graph.GetIDer, I groupsItemer](
 	containers []container[C],
 	scope selectors.GroupsScope,
 	dps metadata.DeltaPaths,
+	useLazyReader bool,
 	ctrlOpts control.Options,
 	counter *count.Bus,
 	errs *fault.Bus,
@@ -212,9 +209,10 @@ func populateCollections[C graph.GetIDer, I groupsItemer](
 			addAndRem.Added,
 			removed,
 			c,
-			statusUpdater)
+			statusUpdater,
+			useLazyReader)
 
-		collections[c.storageDirFolders.String()] = &edc
+		collections[c.storageDirFolders.String()] = edc
 
 		// add the current path for the container ID to be used in the next backup
 		// as the "previous path", for reference in case of a rename or relocation.

src/internal/m365/collection/groups/backup_test.go

@@ -275,6 +275,7 @@ func (suite *BackupUnitSuite) TestPopulateCollections() {
 				test.mock.containers(),
 				selectors.NewGroupsBackup(nil).Channels(selectors.Any())[0],
 				nil,
+				false,
 				ctrlOpts,
 				count.New(),
 				fault.New(true))
@@ -435,6 +436,7 @@ func (suite *BackupUnitSuite) TestPopulateCollections_incremental() {
 				test.mock.containers(),
 				allScope,
 				test.deltaPaths,
+				false,
 				ctrlOpts,
 				count.New(),
 				fault.New(true))
@@ -559,6 +561,7 @@ func (suite *BackupIntgSuite) TestCreateCollections() {
 				suite.tenantID,
 				test.scope,
 				func(status *support.ControllerOperationStatus) {},
+				false,
 				count.New(),
 				fault.New(true))
 			require.NoError(t, err, clues.ToCore(err))

src/internal/m365/collection/groups/collection.go

@@ -23,7 +23,10 @@ import (
 	"github.com/alcionai/corso/src/pkg/services/m365/api/graph"
 )
 
-var _ data.BackupCollection = &prefetchCollection[graph.GetIDer, groupsItemer]{}
+var (
+	_ data.BackupCollection = &prefetchCollection[graph.GetIDer, groupsItemer]{}
+	_ data.BackupCollection = &lazyFetchCollection[graph.GetIDer, groupsItemer]{}
+)
 
 const (
 	collectionChannelBufferSize = 1000
@@ -91,8 +94,22 @@ func NewCollection[C graph.GetIDer, I groupsItemer](
 	removed map[string]struct{},
 	contains container[C],
 	statusUpdater support.StatusUpdater,
-) prefetchCollection[C, I] {
-	collection := prefetchCollection[C, I]{
+	useLazyReader bool,
+) data.BackupCollection {
+	if useLazyReader {
+		return &lazyFetchCollection[C, I]{
+			BaseCollection:    baseCol,
+			added:             added,
+			contains:          contains,
+			getAndAugment:     getAndAugment,
+			removed:           removed,
+			statusUpdater:     statusUpdater,
+			stream:            make(chan data.Item, collectionChannelBufferSize),
+			protectedResource: protectedResource,
+		}
+	}
+
+	return &prefetchCollection[C, I]{
 		BaseCollection:    baseCol,
 		added:             added,
 		contains:          contains,
@@ -102,8 +119,6 @@ func NewCollection[C graph.GetIDer, I groupsItemer](
 		stream:            make(chan data.Item, collectionChannelBufferSize),
 		protectedResource: protectedResource,
 	}
-
-	return collection
 }
 
 func (col *prefetchCollection[C, I]) Items(ctx context.Context, errs *fault.Bus) <-chan data.Item {

src/internal/m365/collection/groups/collection_test.go

@@ -133,11 +133,17 @@ func (suite *CollectionUnitSuite) TestNewCollection_state() {
 				nil,
 				nil,
 				container[models.Channelable]{},
-				nil)
+				nil,
+				false)
+
 			assert.Equal(t, test.expect, c.State(), "collection state")
 			assert.Equal(t, test.curr, c.FullPath(), "full path")
 			assert.Equal(t, test.prev, c.PreviousPath(), "prev path")
-			assert.Equal(t, test.loc, c.LocationPath(), "location path")
+
+			prefetch, ok := c.(*prefetchCollection[models.Channelable, models.ChatMessageable])
+			require.True(t, ok, "collection type")
+
+			assert.Equal(t, test.loc, prefetch.LocationPath(), "location path")
 		})
 	}
 }

src/internal/m365/service/groups/backup.go

@@ -281,6 +281,9 @@ func backupChannels(
 		bc.producerConfig.ProtectedResource.ID(),
 		bc.apiCli.Channels())
 
+	// Always disable lazy reader for channels until #4321 support is added
+	useLazyReader := false
+
 	colls, canUsePreviousBackup, err := groups.CreateCollections(
 		ctx,
 		bc.producerConfig,
@@ -288,6 +291,7 @@ func backupChannels(
 		bc.creds.AzureTenantID,
 		scope,
 		bc.statusUpdater,
+		useLazyReader,
 		counter,
 		errs)
 	if err != nil {
@@ -329,6 +333,8 @@ func backupConversations(
 
 	defer close(progressBar)
 
+	useLazyReader := !bc.producerConfig.Options.ToggleFeatures.DisableLazyItemReader
+
 	colls, canUsePreviousBackup, err := groups.CreateCollections(
 		ctx,
 		bc.producerConfig,
@@ -336,6 +342,7 @@ func backupConversations(
 		bc.creds.AzureTenantID,
 		scope,
 		bc.statusUpdater,
+		useLazyReader,
 		counter,
 		errs)
 	if err != nil {

src/internal/operations/test/m365/groups/groups_test.go

@@ -4,9 +4,12 @@ import (
 	"context"
 	"testing"
 
+	"github.com/alcionai/clues"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 	"github.com/stretchr/testify/suite"
 
+	"github.com/alcionai/corso/src/internal/events"
 	evmock "github.com/alcionai/corso/src/internal/events/mock"
 	"github.com/alcionai/corso/src/internal/m365/collection/drive"
 	. "github.com/alcionai/corso/src/internal/operations/test/m365"
@@ -312,21 +315,27 @@ func (suite *GroupsBackupIntgSuite) TestBackup_Run_groupsBasic() {
 	sel.Include(
 		selTD.GroupsBackupLibraryFolderScope(sel),
 		selTD.GroupsBackupChannelScope(sel),
-		sel.Conversation(selectors.Any()))
+		selTD.GroupsBackupConversationScope(sel))
 
 	bo, bod := PrepNewTestBackupOp(t, ctx, mb, sel.Selector, opts, version.Backup, counter)
 	defer bod.Close(t, ctx)
 
+	reasons, err := bod.Sel.Reasons(bod.Acct.ID(), false)
+	require.NoError(t, err, clues.ToCore(err))
+
 	RunAndCheckBackup(t, ctx, &bo, mb, false)
-	CheckBackupIsInManifests(
-		t,
-		ctx,
-		bod.KW,
-		bod.SW,
-		&bo,
-		bod.Sel,
-		bod.Sel.ID(),
-		path.ChannelMessagesCategory)
+
+	for _, reason := range reasons {
+		CheckBackupIsInManifests(
+			t,
+			ctx,
+			bod.KW,
+			bod.SW,
+			&bo,
+			bod.Sel,
+			bod.Sel.ID(),
+			reason.Category())
+	}
 
 	_, expectDeets := deeTD.GetDeetsInBackup(
 		t,
@@ -347,6 +356,66 @@ func (suite *GroupsBackupIntgSuite) TestBackup_Run_groupsBasic() {
 		bod.SSS,
 		expectDeets,
 		false)
+
+	// Basic, happy path incremental test.  No changes are dictated or expected.
+	// This only tests that an incremental backup is runnable at all, and that it
+	// produces fewer results than the last backup.
+	//
+	// Incremental testing for conversations is limited because of API restrictions.
+	// Since graph doesn't provide us a way to programmatically delete conversations,
+	// or create new conversations without a delegated token, we can't do incremental
+	// testing with newly added items.
+	incMB := evmock.NewBus()
+	incBO := NewTestBackupOp(
+		t,
+		ctx,
+		bod,
+		incMB,
+		opts,
+		count.New())
+
+	RunAndCheckBackup(t, ctx, &incBO, incMB, true)
+
+	for _, reason := range reasons {
+		CheckBackupIsInManifests(
+			t,
+			ctx,
+			bod.KW,
+			bod.SW,
+			&incBO,
+			bod.Sel,
+			bod.Sel.ID(),
+			reason.Category())
+	}
+
+	_, expectDeets = deeTD.GetDeetsInBackup(
+		t,
+		ctx,
+		incBO.Results.BackupID,
+		bod.Acct.ID(),
+		bod.Sel.ID(),
+		bod.Sel.PathService(),
+		whatSet,
+		bod.KMS,
+		bod.SSS)
+	deeTD.CheckBackupDetails(
+		t,
+		ctx,
+		incBO.Results.BackupID,
+		whatSet,
+		bod.KMS,
+		bod.SSS,
+		expectDeets,
+		false)
+
+	assert.NotZero(
+		t,
+		incBO.Results.Counts[string(count.PersistedCachedFiles)],
+		"cached items")
+	assert.Greater(t, bo.Results.ItemsWritten, incBO.Results.ItemsWritten, "incremental items written")
+	assert.Greater(t, bo.Results.BytesRead, incBO.Results.BytesRead, "incremental bytes read")
+	assert.Greater(t, bo.Results.BytesUploaded, incBO.Results.BytesUploaded, "incremental bytes uploaded")
+	assert.Equal(t, 1, incMB.TimesCalled[events.BackupEnd], "incremental backup-end events")
 }
 
 func (suite *GroupsBackupIntgSuite) TestBackup_Run_groupsExtensions() {