Fetch calendar by name on 409 ErrorFolderExist (#3318)

Handle 409, `ErrorFolderExists` error in restore path while creating destination calendar. We need to fix this for all m365 services. This PR is focused on calendar only.

Context:
`CreateCalendar()` may fail with ErrorFolderExists under certain error conditions. For e.g. consider below scenario.

1. `CreateCalendar()` does a POST to graph to create restore destination calendar but this fails with 5xx.
2. It's possible that step 1 may have left some dirty state in graph. For e.g. it's possible that the destination folder in step 1 was actually created, but 5xx was returned due to other reasons.
3. So when we reattempt POST in such a scenario, we sometimes observe ErrorFolderExists error .
4. Corso should be resilient to such errors. To fix this, when we encounter such an error, we will do a GET to fetch the restore destination folder and add it to folder cache.


---

#### 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.-->
Integration tests and mock tests will be added in a follow up PR. 
- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

src/internal/connector/exchange/api/events.go

@@ -99,6 +99,45 @@ func (c Events) GetContainerByID(
 	return graph.CalendarDisplayable{Calendarable: cal}, nil
 }
 
+// GetContainerByName fetches a calendar by name
+func (c Events) GetContainerByName(
+	ctx context.Context,
+	userID, name string,
+) (models.Calendarable, error) {
+	filter := fmt.Sprintf("name eq '%s'", name)
+	options := &users.ItemCalendarsRequestBuilderGetRequestConfiguration{
+		QueryParameters: &users.ItemCalendarsRequestBuilderGetQueryParameters{
+			Filter: &filter,
+		},
+	}
+
+	ctx = clues.Add(ctx, "calendar_name", name)
+
+	resp, err := c.Stable.Client().UsersById(userID).Calendars().Get(ctx, options)
+	if err != nil {
+		return nil, graph.Stack(ctx, err).WithClues(ctx)
+	}
+
+	// We only allow the api to match one calendar with provided name.
+	// Return an error if multiple calendars exist (unlikely) or if no calendar
+	// is found.
+	if len(resp.GetValue()) != 1 {
+		err = clues.New("unexpected number of calendars returned").
+			With("returned_calendar_count", len(resp.GetValue()))
+		return nil, err
+	}
+
+	// Sanity check ID and name
+	cal := resp.GetValue()[0]
+	cd := CalendarDisplayable{Calendarable: cal}
+
+	if err := checkIDAndName(cd); err != nil {
+		return nil, err
+	}
+
+	return cal, nil
+}
+
 // GetItem retrieves an Eventable item.
 func (c Events) GetItem(
 	ctx context.Context,

src/internal/connector/exchange/service_restore.go

@@ -689,10 +689,20 @@ func establishEventsRestoreLocation(
 	ctx = clues.Add(ctx, "is_new_cache", isNewCache)
 
 	temp, err := ac.Events().CreateCalendar(ctx, user, folders[0])
-	if err != nil {
+	if err != nil && !graph.IsErrFolderExists(err) {
 		return "", err
 	}
 
+	// 409 handling: Fetch folder if it exists and add to cache.
+	// This is rare, but may happen if CreateCalendar() POST fails with 5xx,
+	// potentially leaving dirty state in graph.
+	if graph.IsErrFolderExists(err) {
+		temp, err = ac.Events().GetContainerByName(ctx, user, folders[0])
+		if err != nil {
+			return "", err
+		}
+	}
+
 	folderID := ptr.Val(temp.GetId())
 
 	if isNewCache {

src/internal/connector/graph/errors.go

@@ -41,6 +41,10 @@ const (
 	syncFolderNotFound          errorCode = "ErrorSyncFolderNotFound"
 	syncStateInvalid            errorCode = "SyncStateInvalid"
 	syncStateNotFound           errorCode = "SyncStateNotFound"
+	// This error occurs when an attempt is made to create a folder that has
+	// the same name as another folder in the same parent. Such duplicate folder
+	// names are not allowed by graph.
+	folderExists errorCode = "ErrorFolderExists"
 )
 
 type errorMessage string
@@ -178,6 +182,10 @@ func IsMalwareResp(ctx context.Context, resp *http.Response) bool {
 	return false
 }
 
+func IsErrFolderExists(err error) bool {
+	return hasErrorCode(err, folderExists)
+}
+
 // ---------------------------------------------------------------------------
 // error parsers
 // ---------------------------------------------------------------------------

src/internal/connector/graph/errors_test.go

@@ -300,3 +300,48 @@ func (suite *GraphErrorsUnitSuite) TestMalwareInfo() {
 
 	assert.Equal(suite.T(), expect, ItemInfo(&i))
 }
+
+func (suite *GraphErrorsUnitSuite) TestIsErrFolderExists() {
+	table := []struct {
+		name   string
+		err    error
+		expect assert.BoolAssertionFunc
+	}{
+		{
+			name:   "nil",
+			err:    nil,
+			expect: assert.False,
+		},
+		{
+			name:   "non-matching",
+			err:    assert.AnError,
+			expect: assert.False,
+		},
+		{
+			name:   "non-matching oDataErr",
+			err:    odErr("folder doesn't exist"),
+			expect: assert.False,
+		},
+		{
+			name:   "matching oDataErr",
+			err:    odErr(string(folderExists)),
+			expect: assert.True,
+		},
+		// next two tests are to make sure the checks are case insensitive
+		{
+			name:   "oDataErr camelcase",
+			err:    odErr("ErrorFolderExists"),
+			expect: assert.True,
+		},
+		{
+			name:   "oDataErr lowercase",
+			err:    odErr("errorfolderexists"),
+			expect: assert.True,
+		},
+	}
+	for _, test := range table {
+		suite.Run(test.name, func() {
+			test.expect(suite.T(), IsErrFolderExists(test.err))
+		})
+	}
+}