feat: make proposal responses durable

server/api/service.go

@@ -45,6 +45,9 @@ type SessionIssuer interface {
 type ProposalBackend interface {
 	Get(context.Context, string, string, time.Time) (domain.Proposal, error)
 }
+type ProposalMutationBackend interface {
+	Respond(context.Context, string, string, string, bool, uint64, time.Time) (domain.Proposal, error)
+}
 
 type AssignmentView struct {
 	MatchID           string    `json:"match_id"`
@@ -451,15 +454,25 @@ func (s *Service) proposalMutation(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusBadRequest, "invalid_revision")
 		return
 	}
-	s.proposalMu.Lock()
-	defer s.proposalMu.Unlock()
-	proposal, exists := s.Proposals[parts[0]]
-	if !exists || proposal == nil {
-		writeError(w, http.StatusNotFound, "not_found")
-		return
-	}
 	now := s.now()
-	updated, err := proposal.Respond(playerID, key, parts[1] == "accept", revision, now)
+	var updated domain.Proposal
+	if s.ProposalBackend != nil {
+		mutator, supportsMutation := s.ProposalBackend.(ProposalMutationBackend)
+		if !supportsMutation {
+			writeError(w, http.StatusServiceUnavailable, "proposal_unavailable")
+			return
+		}
+		updated, err = mutator.Respond(r.Context(), playerID, parts[0], key, parts[1] == "accept", revision, now)
+	} else {
+		s.proposalMu.Lock()
+		defer s.proposalMu.Unlock()
+		proposal, exists := s.Proposals[parts[0]]
+		if !exists || proposal == nil {
+			writeError(w, http.StatusNotFound, "not_found")
+			return
+		}
+		updated, err = proposal.Respond(playerID, key, parts[1] == "accept", revision, now)
+	}
 	if err != nil {
 		writeDomainError(w, err)
 		return