Split tombstone handling for groups services (#5170)

<!-- PR description-->

* Prerequisite to the currently failing PR https://github.com/alcionai/corso/pull/5154 
* Conversations tombstone IDs are currently 2 part ( `convID/threadID`).
* However, tombstones are looked up by `convID` part only, see [code](9a603d1f21/src/internal/m365/collection/groups/backup.go (L128)).
* Adding a change so that tombstone IDs are reduced to `convID`. Otherwise, we'd never delete tombstones while processing collections, and we'll run into `conflict: tombstone exists for a live collection` errors.
* This is safe to do as there is always a 1:1 relationship between `convID` and `threadID`. For e.g. attempting to create another thread inside a conversation creates a new conversation.

---

#### 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

<!--- Please check the type of change your PR introduces: --->
- [ ] 🌻 Feature
- [x] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Issue(s)

<!-- Can reference multiple issues. Use one of the following "magic words" - "closes, fixes" to auto-close the Github issue. -->
* #<issue>

#### Test Plan

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

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

@@ -105,13 +105,17 @@ func populateCollections[C graph.GetIDer, I groupsItemer](
 		// channel ID -> delta url or folder path lookups
 		deltaURLs = map[string]string{}
 		currPaths = map[string]string{}
-		// copy of previousPaths.  every channel present in the slice param
-		// gets removed from this map; the remaining channels at the end of
-		// the process have been deleted.
-		tombstones = makeTombstones(dps)
-		el         = errs.Local()
+		el        = errs.Local()
 	)
 
+	// Copy of previousPaths. Every container present in the slice param
+	// gets removed from this map; the remaining containers at the end of
+	// the process have been deleted.
+	tombstones, err := bh.makeTombstones(dps)
+	if err != nil {
+		return nil, clues.StackWC(ctx, err)
+	}
+
 	logger.Ctx(ctx).Infow("filling collections", "len_deltapaths", len(dps))
 
 	for _, c := range containers {

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

@@ -149,6 +149,13 @@ func (bh mockBackupHandler) getItemMetadata(
 	return nil, 0, errMetadataFilesNotSupported
 }
 
+//lint:ignore U1000 false linter issue due to generics
+func (bh mockBackupHandler) makeTombstones(
+	dps metadata.DeltaPaths,
+) (map[string]string, error) {
+	return makeTombstones(dps), nil
+}
+
 // ---------------------------------------------------------------------------
 // Unit Suite
 // ---------------------------------------------------------------------------

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

@@ -9,6 +9,7 @@ import (
 
 	"github.com/alcionai/corso/src/internal/common/ptr"
 	"github.com/alcionai/corso/src/pkg/backup/details"
+	"github.com/alcionai/corso/src/pkg/backup/metadata"
 	"github.com/alcionai/corso/src/pkg/path"
 	"github.com/alcionai/corso/src/pkg/selectors"
 	"github.com/alcionai/corso/src/pkg/services/m365/api"
@@ -131,6 +132,12 @@ func (bh channelsBackupHandler) supportsItemMetadata() bool {
 	return false
 }
 
+func (bh channelsBackupHandler) makeTombstones(
+	dps metadata.DeltaPaths,
+) (map[string]string, error) {
+	return makeTombstones(dps), nil
+}
+
 func channelContainer(ch models.Channelable) container[models.Channelable] {
 	return container[models.Channelable]{
 		storageDirFolders:   path.Elements{ptr.Val(ch.GetId())},

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

@@ -12,6 +12,7 @@ import (
 	"github.com/alcionai/corso/src/internal/common/ptr"
 	"github.com/alcionai/corso/src/internal/m365/collection/groups/metadata"
 	"github.com/alcionai/corso/src/pkg/backup/details"
+	deltaPath "github.com/alcionai/corso/src/pkg/backup/metadata"
 	"github.com/alcionai/corso/src/pkg/path"
 	"github.com/alcionai/corso/src/pkg/selectors"
 	"github.com/alcionai/corso/src/pkg/services/m365/api"
@@ -175,6 +176,36 @@ func (bh conversationsBackupHandler) supportsItemMetadata() bool {
 	return true
 }
 
+func (bh conversationsBackupHandler) makeTombstones(
+	dps deltaPath.DeltaPaths,
+) (map[string]string, error) {
+	r := make(map[string]string, len(dps))
+
+	for id, v := range dps {
+		// ID is of format conversationID/threadID. Tombstones are looked up
+		// by conversationID only, so remove the threadID part. This is safe
+		// because every conversation has only one thread.
+		elems := path.Split(id)
+		if len(elems) != 2 {
+			return nil, clues.New("invalid prev path")
+		}
+
+		r[elems[0]] = v.Path
+	}
+
+	// We are assuming a 1:1 mapping between conversations and threads. While
+	// this is true today, graph behavior may change in future. Throw an error
+	// if the assumption is violated.
+	//
+	// We cannot catch this error with tests because creating conversations
+	// requires delegated access.
+	if len(dps) != len(r) {
+		return nil, clues.New("multiple threads exist for a conversation")
+	}
+
+	return r, nil
+}
+
 func conversationThreadContainer(
 	c models.Conversationable,
 	t models.ConversationThreadable,

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

@@ -14,6 +14,7 @@ import (
 	"github.com/alcionai/corso/src/internal/common/ptr"
 	"github.com/alcionai/corso/src/internal/m365/collection/groups/metadata"
 	"github.com/alcionai/corso/src/internal/tester"
+	deltaPath "github.com/alcionai/corso/src/pkg/backup/metadata"
 )
 
 const (
@@ -64,3 +65,72 @@ func (suite *ConversationHandlerUnitSuite) TestGetItemMetadata() {
 	assert.Equal(t, []string{resourceEmail}, meta.Recipients, "incorrect recipients")
 	assert.Equal(t, ptr.Val(conv.GetTopic()), meta.Topic, "incorrect topic")
 }
+
+func (suite *ConversationHandlerUnitSuite) TestMakeTombstones() {
+	table := []struct {
+		name        string
+		dps         deltaPath.DeltaPaths
+		expected    map[string]string
+		expectedErr require.ErrorAssertionFunc
+	}{
+		{
+			name: "valid",
+			dps: deltaPath.DeltaPaths{
+				"c1/t1": deltaPath.DeltaPath{
+					Path: "p1",
+				},
+				"c2/t2": deltaPath.DeltaPath{
+					Path: "p2",
+				},
+			},
+			expected: map[string]string{
+				"c1": "p1",
+				"c2": "p2",
+			},
+			expectedErr: require.NoError,
+		},
+		{
+			name: "invalid prev path",
+			dps: deltaPath.DeltaPaths{
+				"c1": deltaPath.DeltaPath{
+					Path: "p1",
+				},
+			},
+			expected:    nil,
+			expectedErr: require.Error,
+		},
+		{
+			name: "invalid prev path 2",
+			dps: deltaPath.DeltaPaths{
+				"c1/t1/a1": deltaPath.DeltaPath{
+					Path: "p1",
+				},
+			},
+			expected:    nil,
+			expectedErr: require.Error,
+		},
+		{
+			name: "multiple threads exist for a conversation",
+			dps: deltaPath.DeltaPaths{
+				"c1/t1": deltaPath.DeltaPath{
+					Path: "p1",
+				},
+				"c1/t2": deltaPath.DeltaPath{
+					Path: "p2",
+				},
+			},
+			expected:    nil,
+			expectedErr: require.Error,
+		},
+	}
+	for _, test := range table {
+		suite.Run(test.name, func() {
+			t := suite.T()
+			bh := conversationsBackupHandler{}
+
+			result, err := bh.makeTombstones(test.dps)
+			test.expectedErr(t, err)
+			assert.Equal(t, test.expected, result)
+		})
+	}
+}

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

@@ -7,6 +7,7 @@ import (
 	"github.com/microsoft/kiota-abstractions-go/serialization"
 
 	"github.com/alcionai/corso/src/pkg/backup/details"
+	"github.com/alcionai/corso/src/pkg/backup/metadata"
 	"github.com/alcionai/corso/src/pkg/path"
 	"github.com/alcionai/corso/src/pkg/selectors"
 	"github.com/alcionai/corso/src/pkg/services/m365/api"
@@ -30,6 +31,7 @@ type backupHandler[C graph.GetIDer, I groupsItemer] interface {
 	includeContainerer[C]
 	canonicalPather
 	canMakeDeltaQuerieser
+	makeTombstoneser
 }
 
 type getItemAndAugmentInfoer[C graph.GetIDer, I groupsItemer] interface {
@@ -107,6 +109,13 @@ type canMakeDeltaQuerieser interface {
 	canMakeDeltaQueries() bool
 }
 
+// produces a set of id:path pairs from the deltapaths map.
+// Each entry in the set will, if not removed, produce a collection
+// that will delete the tombstone by path.
+type makeTombstoneser interface {
+	makeTombstones(dps metadata.DeltaPaths) (map[string]string, error)
+}
+
 // ---------------------------------------------------------------------------
 // Container management
 // ---------------------------------------------------------------------------