Try to catch concurrent serialization issues (#3479)

Run a bunch of serialization goroutines at the same time to try to catch any buffer reuse errors there may be. Fails on the old version of corso code that didn't make defensive copies.

---

#### 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
- [x] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Test Plan

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

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

@@ -0,0 +1,135 @@
+package api_test
+
+import (
+	"context"
+	"fmt"
+	"testing"
+
+	"github.com/alcionai/clues"
+	"github.com/microsoftgraph/msgraph-sdk-go/models"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"github.com/stretchr/testify/suite"
+
+	"github.com/alcionai/corso/src/internal/common/ptr"
+	"github.com/alcionai/corso/src/internal/connector/support"
+	"github.com/alcionai/corso/src/internal/tester"
+	"github.com/alcionai/corso/src/pkg/services/m365/api"
+)
+
+type ItemSerializationUnitSuite struct {
+	tester.Suite
+}
+
+func TestItemSerializationUnitSuite(t *testing.T) {
+	suite.Run(t, &ItemSerializationUnitSuite{Suite: tester.NewUnitSuite(t)})
+}
+
+func (suite *ItemSerializationUnitSuite) TestConcurrentItemSerialization() {
+	var (
+		user      = "a-user"
+		instances = 100
+	)
+
+	table := []struct {
+		name                   string
+		serializer             func(t *testing.T, ctx context.Context, idx int) []byte
+		deserializeAndGetField func(t *testing.T, bs []byte) string
+	}{
+		{
+			name: "Exchange Mail",
+			serializer: func(t *testing.T, ctx context.Context, idx int) []byte {
+				subject := fmt.Sprintf("%d", idx)
+
+				item := models.NewMessage()
+				item.SetSubject(&subject)
+
+				bs, err := api.Mail{}.Serialize(ctx, item, user, subject)
+				require.NoError(t, err, clues.ToCore(err))
+
+				return bs
+			},
+			deserializeAndGetField: func(t *testing.T, bs []byte) string {
+				item, err := support.CreateMessageFromBytes(bs)
+				require.NoError(
+					t,
+					err,
+					"deserializing message of %q: %v",
+					string(bs),
+					clues.ToCore(err))
+
+				return ptr.Val(item.GetSubject())
+			},
+		},
+		{
+			name: "Exchange Event",
+			serializer: func(t *testing.T, ctx context.Context, idx int) []byte {
+				subject := fmt.Sprintf("%d", idx)
+
+				item := models.NewEvent()
+				item.SetSubject(&subject)
+
+				bs, err := api.Events{}.Serialize(ctx, item, user, subject)
+				require.NoError(t, err, clues.ToCore(err))
+
+				return bs
+			},
+			deserializeAndGetField: func(t *testing.T, bs []byte) string {
+				item, err := support.CreateEventFromBytes(bs)
+				require.NoError(
+					t,
+					err,
+					"deserializing event of %q: %v",
+					string(bs),
+					clues.ToCore(err))
+
+				return ptr.Val(item.GetSubject())
+			},
+		},
+		{
+			name: "Exchange Contact",
+			serializer: func(t *testing.T, ctx context.Context, idx int) []byte {
+				name := fmt.Sprintf("%d", idx)
+
+				item := models.NewContact()
+				item.SetGivenName(&name)
+
+				bs, err := api.Contacts{}.Serialize(ctx, item, user, name)
+				require.NoError(t, err, clues.ToCore(err))
+
+				return bs
+			},
+			deserializeAndGetField: func(t *testing.T, bs []byte) string {
+				item, err := support.CreateContactFromBytes(bs)
+				require.NoError(
+					t,
+					err,
+					"deserializing contact of %q: %v",
+					string(bs),
+					clues.ToCore(err))
+
+				return ptr.Val(item.GetGivenName())
+			},
+		},
+	}
+
+	for _, test := range table {
+		suite.Run(test.name, func() {
+			ctx, flusher := tester.NewContext()
+			defer flusher()
+
+			t := suite.T()
+			output := make([][]byte, instances)
+
+			for i := 0; i < instances; i++ {
+				output[i] = test.serializer(t, ctx, i)
+			}
+
+			for i := 0; i < instances; i++ {
+				got := test.deserializeAndGetField(t, output[i])
+				// I'm lazy and don't want to deal with the error from atoi functions.
+				assert.Equal(t, fmt.Sprintf("%d", i), got, "item output")
+			}
+		})
+	}
+}