fix(forwarder): preserve Cursor fork context

Store checkpoint turns as validated blobs and restore prefetched parent turns so forked conversations retain their replay history and rewind prefix.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/backend/forwarder/imported_blobs.go

@@ -0,0 +1,167 @@
+package forwarder
+
+import (
+	"crypto/sha256"
+	"fmt"
+
+	"google.golang.org/protobuf/proto"
+
+	"cursor/gen/agentv1"
+	modeladapter "cursor/internal/backend/agent/model"
+	promptengine "cursor/internal/backend/agent/prompt"
+)
+
+type importedBlobStore map[string][]byte
+
+func newImportedBlobStore(items []*agentv1.PreFetchedBlob) (importedBlobStore, error) {
+	if len(items) == 0 {
+		return nil, nil
+	}
+	store := make(importedBlobStore, len(items))
+	for _, item := range items {
+		if item == nil || len(item.GetId()) == 0 {
+			continue
+		}
+		if len(item.GetId()) != sha256.Size {
+			return nil, fmt.Errorf("prefetched blob id length %d, want %d", len(item.GetId()), sha256.Size)
+		}
+		digest := sha256.Sum256(item.GetValue())
+		if string(digest[:]) != string(item.GetId()) {
+			return nil, fmt.Errorf("prefetched blob %x failed SHA-256 validation", item.GetId())
+		}
+		store[string(item.GetId())] = append([]byte(nil), item.GetValue()...)
+	}
+	return store, nil
+}
+
+func (store importedBlobStore) resolve(id []byte) ([]byte, bool) {
+	if len(id) == 0 || len(store) == 0 {
+		return nil, false
+	}
+	value, ok := store[string(id)]
+	return append([]byte(nil), value...), ok
+}
+
+func decodeImportedTurn(raw []byte, blobs importedBlobStore) (*agentv1.ConversationTurnStructure, []byte, error) {
+	if data, ok := blobs.resolve(raw); ok {
+		turn := &agentv1.ConversationTurnStructure{}
+		if err := proto.Unmarshal(data, turn); err != nil || turn.GetTurn() == nil {
+			return nil, nil, fmt.Errorf("decode imported turn blob %x: %w", raw, firstNonNilError(err, fmt.Errorf("turn payload is empty")))
+		}
+		return turn, append([]byte(nil), raw...), nil
+	}
+	turn := &agentv1.ConversationTurnStructure{}
+	if err := proto.Unmarshal(raw, turn); err == nil && turn.GetTurn() != nil {
+		return turn, nil, nil
+	}
+	if len(raw) == sha256.Size {
+		return nil, append([]byte(nil), raw...), nil
+	}
+	return nil, nil, fmt.Errorf("decode imported inline turn")
+}
+
+func decodeImportedUserMessage(raw []byte, blobs importedBlobStore) (*agentv1.UserMessage, error) {
+	data := raw
+	if resolved, ok := blobs.resolve(raw); ok {
+		data = resolved
+	} else if len(raw) == sha256.Size {
+		candidate := &agentv1.UserMessage{}
+		if err := proto.Unmarshal(raw, candidate); err != nil || !hasKnownUserMessageContent(candidate) {
+			return nil, fmt.Errorf("missing prefetched user message blob %x", raw)
+		}
+		return candidate, nil
+	}
+	message := &agentv1.UserMessage{}
+	if err := proto.Unmarshal(data, message); err != nil {
+		return nil, fmt.Errorf("decode imported turn user_message: %w", err)
+	}
+	return message, nil
+}
+
+func decodeImportedStep(raw []byte, blobs importedBlobStore) (*agentv1.ConversationStep, error) {
+	data := raw
+	if resolved, ok := blobs.resolve(raw); ok {
+		data = resolved
+	} else if len(raw) == sha256.Size {
+		candidate := &agentv1.ConversationStep{}
+		if err := proto.Unmarshal(raw, candidate); err != nil || candidate.GetMessage() == nil {
+			return nil, fmt.Errorf("missing prefetched conversation step blob %x", raw)
+		}
+		return candidate, nil
+	}
+	step := &agentv1.ConversationStep{}
+	if err := proto.Unmarshal(data, step); err != nil {
+		return nil, fmt.Errorf("decode imported turn step: %w", err)
+	}
+	if step.GetMessage() == nil {
+		return nil, fmt.Errorf("decode imported turn step: payload is empty")
+	}
+	return step, nil
+}
+
+func importedBlobTurnMessages(turn *agentv1.ConversationTurnStructure, blobs importedBlobStore) ([]modeladapter.Message, error) {
+	if turn == nil || turn.GetAgentConversationTurn() == nil {
+		return nil, nil
+	}
+	agentTurn := turn.GetAgentConversationTurn()
+	messages := make([]modeladapter.Message, 0, 1+len(agentTurn.GetSteps()))
+	if len(agentTurn.GetUserMessage()) > 0 {
+		userMessage, err := decodeImportedUserMessage(agentTurn.GetUserMessage(), blobs)
+		if err != nil {
+			return nil, err
+		}
+		if replay, ok := promptengine.BuildUserMessageReplayMessage(userMessage); ok {
+			messages = append(messages, toModelMessage(replay))
+		}
+	}
+	for _, rawStep := range agentTurn.GetSteps() {
+		if len(rawStep) == 0 {
+			continue
+		}
+		step, err := decodeImportedStep(rawStep, blobs)
+		if err != nil {
+			return nil, err
+		}
+		for _, replay := range promptengine.BuildLegacyMessagesFromConversationStep(step) {
+			messages = append(messages, toModelMessage(replay))
+		}
+	}
+	return messages, nil
+}
+
+func importedTurnIDs(turns [][]byte, blobs importedBlobStore) ([][]byte, error) {
+	ids := make([][]byte, 0, len(turns))
+	for _, raw := range turns {
+		if len(raw) == 0 {
+			continue
+		}
+		_, id, err := decodeImportedTurn(raw, blobs)
+		if err != nil {
+			return nil, err
+		}
+		if len(id) > 0 {
+			ids = append(ids, id)
+		}
+	}
+	return ids, nil
+}
+
+func hasKnownUserMessageContent(message *agentv1.UserMessage) bool {
+	if message == nil {
+		return false
+	}
+	return message.GetText() != "" ||
+		message.GetMessageId() != "" ||
+		message.GetSelectedContext() != nil ||
+		message.GetRichText() != "" ||
+		len(message.GetConversationStateBlobId()) > 0 ||
+		len(message.GetTextBlobId()) > 0 ||
+		len(message.GetRichTextBlobId()) > 0
+}
+
+func firstNonNilError(err error, fallback error) error {
+	if err != nil {
+		return err
+	}
+	return fallback
+}