Retry events item enumeration with smaller page size for some cases (#5194)

Update the events item enumerator to switch to a smaller page size for
the delta pager if we fail enumeration with a 503 error and no content.
We've found that that situation is indicative of the Graph server
being slow and a smaller page size allows us to make progress still

---

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

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

#### Type of change

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

#### Issue(s)

#### Test Plan

- [ ] 💪 Manual
- [x] ⚡ Unit test (in another PR)
- [ ] 💚 E2E

src/pkg/services/m365/api/config.go

@@ -10,6 +10,7 @@ import (
 const (
 	maxNonDeltaPageSize    = int32(999)
 	maxDeltaPageSize       = int32(500)
+	minEventsDeltaPageSize = int32(10)
 )
 
 // selectable values, case insensitive

src/pkg/services/m365/api/events_pager.go

@@ -2,6 +2,7 @@ package api
 
 import (
 	"context"
+	"errors"
 	"fmt"
 
 	"github.com/alcionai/clues"
@@ -9,6 +10,7 @@ import (
 	"github.com/microsoftgraph/msgraph-sdk-go/users"
 
 	"github.com/alcionai/corso/src/internal/common/ptr"
+	"github.com/alcionai/corso/src/pkg/logger"
 	"github.com/alcionai/corso/src/pkg/path"
 	"github.com/alcionai/corso/src/pkg/services/m365/api/graph"
 	"github.com/alcionai/corso/src/pkg/services/m365/api/pagers"
@@ -200,12 +202,29 @@ func (c Events) NewEventsDeltaPager(
 	ctx context.Context,
 	userID, containerID, prevDeltaLink string,
 	selectProps ...string,
+) pagers.DeltaHandler[models.Eventable] {
+	return c.newEventsDeltaPagerWithPageSize(
+		ctx,
+		userID,
+		containerID,
+		prevDeltaLink,
+		c.options.DeltaPageSize,
+		selectProps...)
+}
+
+func (c Events) newEventsDeltaPagerWithPageSize(
+	ctx context.Context,
+	userID string,
+	containerID string,
+	prevDeltaLink string,
+	pageSize int32,
+	selectProps ...string,
 ) pagers.DeltaHandler[models.Eventable] {
 	options := &users.ItemCalendarsItemEventsDeltaRequestBuilderGetRequestConfiguration{
 		// do NOT set Top.  It limits the total items received.
 		QueryParameters: &users.ItemCalendarsItemEventsDeltaRequestBuilderGetQueryParameters{},
 		Headers: newPreferHeaders(
-			preferPageSize(c.options.DeltaPageSize),
+			preferPageSize(pageSize),
 			preferImmutableIDs(c.options.ToggleFeatures.ExchangeImmutableIDs)),
 	}
 
@@ -259,11 +278,45 @@ func (c Events) GetAddedAndRemovedItemIDs(
 		containerID,
 		prevDeltaLink,
 		idAnd()...)
+
+	// Experiments showed that non-delta endpoint didn't have the same performance
+	// degradation that delta endpoint did so a larger page size should be ok.
 	pager := c.NewEventsPager(
 		userID,
 		containerID,
 		idAnd(lastModifiedDateTime)...)
 
+	// Try running the query with the given limits. If we fail to do a delta
+	// enumeration with a 5xx error try rerunning the query with a smaller page
+	// size. We've seen some resources where this consistently happens but a small
+	// page size allows us to make progress.
+	addedRemoved, err := pagers.GetAddedAndRemovedItemIDs[models.Eventable](
+		ctx,
+		pager,
+		deltaPager,
+		prevDeltaLink,
+		config.CanMakeDeltaQueries,
+		config.LimitResults,
+		pagers.AddedAndRemovedByAddtlData[models.Eventable])
+	if err == nil || !errors.Is(err, graph.ErrServiceUnavailableEmptyResp) {
+		return addedRemoved, clues.Stack(err).OrNil()
+	}
+
+	effectivePageSize := minEventsDeltaPageSize
+
+	logger.Ctx(ctx).Infow(
+		"retrying event item query with reduced page size",
+		"delta_pager_effective_page_size", effectivePageSize,
+		"delta_pager_default_page_size", c.options.DeltaPageSize)
+
+	deltaPager = c.newEventsDeltaPagerWithPageSize(
+		ctx,
+		userID,
+		containerID,
+		prevDeltaLink,
+		effectivePageSize,
+		idAnd()...)
+
 	return pagers.GetAddedAndRemovedItemIDs[models.Eventable](
 		ctx,
 		pager,

src/pkg/services/m365/api/events_pager_test.go

@@ -1,9 +1,14 @@
 package api
 
 import (
+	"fmt"
+	"net/http"
+	stdpath "path"
+	"strings"
 	"testing"
 
 	"github.com/alcionai/clues"
+	"github.com/h2non/gock"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"github.com/stretchr/testify/suite"
@@ -12,8 +17,140 @@ import (
 	"github.com/alcionai/corso/src/internal/common/ptr"
 	"github.com/alcionai/corso/src/internal/tester"
 	"github.com/alcionai/corso/src/internal/tester/tconfig"
+	"github.com/alcionai/corso/src/pkg/count"
 )
 
+// ---------------------------------------------------------------------------
+// Unit tests
+// ---------------------------------------------------------------------------
+
+type EventsPagerUnitSuite struct {
+	tester.Suite
+}
+
+func TestEventsPagerUnitSuite(t *testing.T) {
+	suite.Run(t, &EventsPagerUnitSuite{
+		Suite: tester.NewUnitSuite(t),
+	})
+}
+
+func (suite *EventsPagerUnitSuite) TestGetAddedAndRemovedItemIDs_SendsCorrectDeltaPageSize() {
+	const (
+		validEmptyResponse = `{
+  "@odata.context": "https://graph.microsoft.com/beta/$metadata#Collection(event)",
+  "value": [],
+  "@odata.deltaLink": "link"
+}`
+
+		// deltaPath helps make gock matching a little easier since it splits out
+		// the hostname from the remainder of the URL. Graph SDK uses the URL
+		// directly though.
+		deltaPath = "/prev-delta"
+		prevDelta = graphAPIHostURL + deltaPath
+
+		userID      = "user-id"
+		containerID = "container-id"
+	)
+
+	deltaTests := []struct {
+		name       string
+		reqPath    string
+		inputDelta string
+	}{
+		{
+			name: "NoPrevDelta",
+			reqPath: stdpath.Join(
+				"/beta",
+				"users",
+				userID,
+				"calendars",
+				containerID,
+				"events",
+				"delta"),
+		},
+		{
+			name:       "HasPrevDelta",
+			reqPath:    deltaPath,
+			inputDelta: prevDelta,
+		},
+	}
+
+	for _, deltaTest := range deltaTests {
+		suite.Run(deltaTest.name, func() {
+			t := suite.T()
+
+			ctx, flush := tester.NewContext(t)
+			t.Cleanup(flush)
+
+			a := tconfig.NewFakeM365Account(t)
+			creds, err := a.M365Config()
+			require.NoError(t, err, clues.ToCore(err))
+
+			client, err := gockClient(creds, count.New())
+			require.NoError(t, err, clues.ToCore(err))
+
+			t.Cleanup(gock.Off)
+
+			// Number of retries and delay between retries is handled by a kiota
+			// middleware. We can change the default config parameters when setting up
+			// the mock in a later PR.
+			gock.New(graphAPIHostURL).
+				Get(deltaTest.reqPath).
+				Times(4).
+				Reply(http.StatusServiceUnavailable).
+				BodyString("").
+				Type("text/plain")
+
+			gock.New(graphAPIHostURL).
+				Get(deltaTest.reqPath).
+				SetMatcher(gock.NewMatcher()).
+				// Need a custom Matcher since the prefer header is also used for
+				// immutable ID behavior.
+				AddMatcher(func(got *http.Request, want *gock.Request) (bool, error) {
+					var (
+						found         bool
+						preferHeaders = got.Header.Values("Prefer")
+						expected      = fmt.Sprintf(
+							"odata.maxpagesize=%d",
+							minEventsDeltaPageSize)
+					)
+
+					for _, header := range preferHeaders {
+						if strings.Contains(header, expected) {
+							found = true
+							break
+						}
+					}
+
+					assert.Truef(
+						t,
+						found,
+						"header %s not found in set %v",
+						expected,
+						preferHeaders)
+
+					return true, nil
+				}).
+				Reply(http.StatusOK).
+				JSON(validEmptyResponse)
+
+			_, err = client.Events().GetAddedAndRemovedItemIDs(
+				ctx,
+				userID,
+				containerID,
+				deltaTest.inputDelta,
+				CallConfig{
+					CanMakeDeltaQueries: true,
+				})
+			require.NoError(t, err, clues.ToCore(err))
+		})
+	}
+}
+
+// ---------------------------------------------------------------------------
+// Integration tests
+// ---------------------------------------------------------------------------
+
 type EventsPagerIntgSuite struct {
 	tester.Suite
 	its intgTesterSetup

src/pkg/services/m365/api/graph/errors.go

@@ -111,6 +111,11 @@ const (
 	LabelsSkippable = "skippable_errors"
 )
 
+// ErrServiceUnavailableEmptyResp indicates the remote service returned a 503
+// with an empty response body. This can sometimes happen if a request times out
+// during processing.
+var ErrServiceUnavailableEmptyResp = clues.New("service unavailable and no returned content")
+
 // ---------------------------------------------------------------------------
 // error categorization
 // ---------------------------------------------------------------------------
@@ -395,6 +400,21 @@ func stackReq(
 	resp *http.Response,
 	e error,
 ) *clues.Err {
+	// For service unavailable stack an error we can check against. We can't check
+	// this at higher layers because we won't have the content length or server
+	// response code thanks to graph SDK. We do need to check this before checking
+	// if there was no error though because the status code itself doesn't count
+	// as an error.
+	//
+	// Returning an error here is non-standard, but if we don't return something
+	// then all we get from graph SDK is an error saying "content is empty" which
+	// isn't particularly useful.
+	if resp != nil &&
+		resp.ContentLength == 0 &&
+		resp.StatusCode == http.StatusServiceUnavailable {
+		e = clues.Stack(ErrServiceUnavailableEmptyResp, e)
+	}
+
 	if e == nil {
 		return nil
 	}

src/pkg/services/m365/api/graph/middleware.go

@@ -164,7 +164,15 @@ func (mw RetryMiddleware) Intercept(
 		mw.isRetriableRespCode(ctx, resp)
 
 	if !retriable {
-		return resp, stackReq(ctx, req, resp, err).OrNil()
+		// Returning a response and an error causes output from either some part of
+		// the middleware/graph/golang or the mocking library used for testing.
+		// Return one or the other to avoid this.
+		err := stackReq(ctx, req, resp, err).OrNil()
+		if err != nil {
+			return nil, err
+		}
+
+		return resp, nil
 	}
 
 	exponentialBackOff := backoff.NewExponentialBackOff()

src/pkg/services/m365/api/graph/middleware_test.go

@@ -43,6 +43,7 @@ func newMWReturns(code int, body []byte, err error) mwReturns {
 	}
 
 	resp := &http.Response{
+		ContentLength: int64(len(body)),
 		StatusCode:    code,
 		Body:          brc,
 	}

src/pkg/services/m365/api/graph/service_mock.go

@@ -6,6 +6,7 @@ import (
 	"github.com/alcionai/clues"
 	"github.com/h2non/gock"
 	abstractions "github.com/microsoft/kiota-abstractions-go"
+	khttp "github.com/microsoft/kiota-http-go"
 	msgraphsdkgo "github.com/microsoftgraph/msgraph-sdk-go"
 
 	"github.com/alcionai/corso/src/pkg/account"
@@ -34,6 +35,9 @@ func NewGockService(
 	counter *count.Bus,
 	opts ...Option,
 ) (*Service, error) {
+	// Need to initialize the concurrency limiter else we'll get an error.
+	InitializeConcurrencyLimiter(context.Background(), true, 4)
+
 	a, err := CreateGockAdapter(
 		creds.AzureTenantID,
 		creds.AzureClientID,
@@ -54,11 +58,27 @@ func CreateGockAdapter(
 	counter *count.Bus,
 	opts ...Option,
 ) (abstractions.RequestAdapter, error) {
-	httpClient, cc := KiotaHTTPClient(counter, opts...)
+	cc := populateConfig(opts...)
+	// We need to manufacture our own graph client since gock.InterceptClient
+	// replaces the transport which replaces all the middleware we add. Start with
+	// a client without any middleware and then replace the transport completely
+	// with the mocked one since there's no real in between that we could use.
+	clientOptions := msgraphsdkgo.GetDefaultClientOptions()
+	middlewares := kiotaMiddlewares(&clientOptions, cc, counter)
 
-	// This makes sure that we are able to intercept any requests via
-	// gock. Only necessary for testing.
-	gock.InterceptClient(httpClient)
+	//nolint:lll
+	// This is lifted from
+	// https://github.com/microsoft/kiota-http-go/blob/e32eb086c8d28002dcba922f3271d56327ba8b03/pipeline.go#L75
+	// which was found by following
+	// https://github.com/microsoftgraph/msgraph-sdk-go-core/blob/93a2c8acb7dfff7f3e2791670f51ccb001d7b127/graph_client_factory.go#L26
+	httpClient := khttp.GetDefaultClient()
+	httpClient.Transport = khttp.NewCustomTransportWithParentTransport(
+		// gock's default transport isn't quite the same as the default graph uses
+		// but since we're mocking things it's probably ok.
+		gock.NewTransport(),
+		middlewares...)
+
+	cc.apply(httpClient)
 
 	ng, err := msgraphsdkgo.NewGraphRequestAdapterWithParseNodeFactoryAndSerializationWriterFactoryAndHttpClient(
 		// Use our own mock auth instance. This allows us to completely avoid having

src/pkg/services/m365/api/helper_test.go

@@ -38,6 +38,7 @@ func gockClient(creds account.M365Config, counter *count.Bus) (Client, error) {
 		Credentials: creds,
 		Stable:      s,
 		LargeItem:   li,
+		options:     control.DefaultOptions(),
 	}, nil
 }
 

src/pkg/services/m365/api/mail_test.go

@@ -313,9 +313,12 @@ func (suite *MailAPIIntgSuite) TestMail_attachmentListDownload() {
 					Reply(200).
 					JSON(graphTD.ParseableToMap(suite.T(), atts))
 
-				interceptV1Path("users", "user", "messages", mid, "attachments", aid).
-					Reply(200).
-					JSON(graphTD.ParseableToMap(suite.T(), attch))
+				// TODO(pandeyabs): This does not test getAttachmentsIterated code path.
+				// We need a separate test for that.
+				// For that we'd need to fail /attachments call(above) with an error.
+				// This would trigger a /attachments?$select=id,size call, followed by
+				// an $expand=microsoft.graph.itemAttachment/item call for each attachment.
+				// These paths need to be added to the gock intercepts.
 			},
 			attachmentCount: 1,
 			size:            200,
@@ -349,11 +352,8 @@ func (suite *MailAPIIntgSuite) TestMail_attachmentListDownload() {
 					Reply(200).
 					JSON(graphTD.ParseableToMap(suite.T(), atts))
 
-				for i := 0; i < 5; i++ {
-					interceptV1Path("users", "user", "messages", mid, "attachments", aid).
-						Reply(200).
-						JSON(graphTD.ParseableToMap(suite.T(), attch))
-				}
+				// TODO(pandeyabs): This does not test getAttachmentsIterated code path.
+				// We need a separate test for that.
 			},
 			attachmentCount: 5,
 			size:            1000,