fix: support legacy factorPackIds format and tests

closes #1185

pkg/crashlog/ips.go

@@ -329,6 +329,31 @@ type BinaryImage struct {
 	Slide  uint64 `json:"slide,omitempty"`
 }
 
+// FactorPackIDs accepts both the current string-list rollout encoding and the legacy object encoding.
+type FactorPackIDs []string
+
+func (ids *FactorPackIDs) UnmarshalJSON(data []byte) error {
+	if bytes.Equal(data, []byte("null")) {
+		*ids = nil
+		return nil
+	}
+
+	var values []string
+	arrayErr := json.Unmarshal(data, &values)
+	if arrayErr == nil {
+		*ids = values
+		return nil
+	}
+
+	var legacyObject struct{}
+	if err := json.Unmarshal(data, &legacyObject); err == nil {
+		*ids = nil
+		return nil
+	}
+
+	return arrayErr
+}
+
 func (bi *BinaryImage) UnmarshalJSON(b []byte) error {
 	var raw []json.RawMessage
 	if err := json.Unmarshal(b, &raw); err != nil {
@@ -696,10 +721,9 @@ type IPSPayload struct {
 	} `json:"legacyInfo"`
 	TrialInfo struct {
 		Rollouts []struct {
-			RolloutID     string `json:"rolloutId,omitempty"`
-			FactorPackIds struct {
-			} `json:"factorPackIds"`
-			DeploymentID int `json:"deploymentId,omitempty"`
+			RolloutID     string        `json:"rolloutId,omitempty"`
+			FactorPackIds FactorPackIDs `json:"factorPackIds"`
+			DeploymentID  int           `json:"deploymentId,omitempty"`
 		} `json:"rollouts,omitempty"`
 		Experiments []struct {
 			TreatmentID  string `json:"treatmentId,omitempty"`

pkg/crashlog/ips_test.go

@@ -0,0 +1,105 @@
+package crashlog
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"slices"
+	"testing"
+)
+
+func TestIPSPayloadTrialInfoFactorPackIdsArray(t *testing.T) {
+	var payload IPSPayload
+	data := []byte(`{
+		"trialInfo": {
+			"rollouts": [
+				{
+					"rolloutId": "66d35d7fe4d6bf7664f40ddf",
+					"factorPackIds": ["68c1a34bd359577bbe8f2182"],
+					"deploymentId": 240000067
+				}
+			]
+		}
+	}`)
+
+	if err := json.Unmarshal(data, &payload); err != nil {
+		t.Fatalf("json.Unmarshal: %v", err)
+	}
+
+	rollouts := payload.TrialInfo.Rollouts
+	if len(rollouts) != 1 {
+		t.Fatalf("expected 1 rollout, got %d", len(rollouts))
+	}
+	assertFactorPackIDs(t, rollouts[0].FactorPackIds, "68c1a34bd359577bbe8f2182")
+}
+
+func TestOpenIPSAcceptsTrialInfoFactorPackIdsArray(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "ios26.ips")
+	data := []byte(`{"bug_type":"309","os_version":"iPhone OS 26.0 (23A000)"}
+{
+	"modelCode": "iPhone17,1",
+	"trialInfo": {
+		"rollouts": [
+			{
+				"rolloutId": "66d35d7fe4d6bf7664f40ddf",
+				"factorPackIds": ["68c1a34bd359577bbe8f2182"],
+				"deploymentId": 240000067
+			}
+		]
+	}
+}`)
+	if err := os.WriteFile(path, data, 0o600); err != nil {
+		t.Fatalf("os.WriteFile: %v", err)
+	}
+
+	ips, err := OpenIPS(path, &Config{})
+	if err != nil {
+		t.Fatalf("OpenIPS: %v", err)
+	}
+
+	rollouts := ips.Payload.TrialInfo.Rollouts
+	if len(rollouts) != 1 {
+		t.Fatalf("expected 1 rollout, got %d", len(rollouts))
+	}
+	assertFactorPackIDs(t, rollouts[0].FactorPackIds, "68c1a34bd359577bbe8f2182")
+}
+
+func TestIPSPayloadTrialInfoFactorPackIdsLegacyObject(t *testing.T) {
+	var payload IPSPayload
+	data := []byte(`{
+		"trialInfo": {
+			"rollouts": [
+				{
+					"rolloutId": "66d35d7fe4d6bf7664f40ddf",
+					"factorPackIds": {},
+					"deploymentId": 240000067
+				}
+			]
+		}
+	}`)
+
+	if err := json.Unmarshal(data, &payload); err != nil {
+		t.Fatalf("json.Unmarshal: %v", err)
+	}
+
+	rollouts := payload.TrialInfo.Rollouts
+	if len(rollouts) != 1 {
+		t.Fatalf("expected 1 rollout, got %d", len(rollouts))
+	}
+	assertFactorPackIDs(t, rollouts[0].FactorPackIds)
+}
+
+func TestFactorPackIdsRejectsInvalidShape(t *testing.T) {
+	var factorPackIDs FactorPackIDs
+	if err := json.Unmarshal([]byte(`"68c1a34bd359577bbe8f2182"`), &factorPackIDs); err == nil {
+		t.Fatal("expected invalid scalar factorPackIds to fail")
+	}
+}
+
+func assertFactorPackIDs(t *testing.T, got FactorPackIDs, want ...string) {
+	t.Helper()
+
+	if !slices.Equal(got, want) {
+		t.Fatalf("unexpected factor pack IDs: got %#v, want %#v", got, want)
+	}
+}