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

server/api/service_test.go

@@ -22,8 +22,18 @@ type queueBackendSpy struct{ createCalls, heartbeatCalls, cancelCalls, getCalls
 type sessionBackendSpy struct{ calls int }
 
 type proposalBackendSpy struct {
-	proposal domain.Proposal
-	calls    int
+	proposal  domain.Proposal
+	calls     int
+	mutations int
+}
+
+func (b *proposalBackendSpy) Respond(_ context.Context, playerID, _ string, key string, accept bool, revision uint64, now time.Time) (domain.Proposal, error) {
+	b.mutations++
+	updated, err := b.proposal.Respond(playerID, key, accept, revision, now)
+	if err == nil {
+		b.proposal = updated
+	}
+	return updated, err
 }
 
 func (b *proposalBackendSpy) Get(_ context.Context, playerID, _ string, _ time.Time) (domain.Proposal, error) {
@@ -389,6 +399,21 @@ func TestProposalRecoveryUsesDurableBackendAndRemainsParticipantScoped(t *testin
 	if status := get(participantSession, participantToken); status != http.StatusOK {
 		t.Fatalf("participant recovery status = %d", status)
 	}
+	respond, err := http.NewRequest(http.MethodPost, server.URL+"/v1/proposals/"+proposal.ProposalID+"/accept", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	respond.Header.Set("Authorization", "Bearer "+participantSession.SessionID+":"+participantToken)
+	respond.Header.Set("Idempotency-Key", "proposal-durable-response-123456")
+	respond.Header.Set("If-Match-Revision", "0")
+	response, err := server.Client().Do(respond)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_ = response.Body.Close()
+	if response.StatusCode != http.StatusOK || backend.mutations != 1 {
+		t.Fatalf("durable response status = %d, mutations = %d", response.StatusCode, backend.mutations)
+	}
 	if status := get(outsiderSession, outsiderToken); status != http.StatusNotFound {
 		t.Fatalf("outsider recovery status = %d", status)
 	}

server/api/store_adapters.go

@@ -37,6 +37,10 @@ func (p postgresProposalBackend) Get(ctx context.Context, playerID, proposalID s
 	return store.GetProposal(ctx, p.db, playerID, proposalID, now)
 }
 
+func (p postgresProposalBackend) Respond(ctx context.Context, playerID, proposalID, idempotencyKey string, accept bool, expectedRevision uint64, now time.Time) (domain.Proposal, error) {
+	return store.RespondToProposal(ctx, p.db, playerID, proposalID, idempotencyKey, accept, expectedRevision, now)
+}
+
 func ProposalProviderFromStore(db *sql.DB) ProposalBackend {
 	return postgresProposalBackend{db: db}
 }

server/store/proposal_recovery_sql.go

@@ -1,8 +1,11 @@
 package store
 
 import (
+	"bytes"
 	"context"
+	"crypto/sha256"
 	"database/sql"
+	"encoding/json"
 	"fmt"
 	"time"
 
@@ -27,6 +30,50 @@ FROM proposal_participants
 WHERE proposal_id = $1
 ORDER BY player_id`
 
+const ProposalResponseIdempotencyScope = "proposal.respond"
+
+const ProposalResponseIdempotencyInsertSQL = `INSERT INTO idempotency_keys
+    (scope, idempotency_key, payload_digest, result)
+VALUES ($1, $2, $3, $4)
+ON CONFLICT (scope, idempotency_key) DO NOTHING`
+
+const ProposalResponseIdempotencySelectSQL = `SELECT payload_digest, result
+FROM idempotency_keys
+WHERE scope = $1 AND idempotency_key = $2
+FOR UPDATE`
+
+const ProposalLockSQL = `SELECT playlist, state, revision, expires_at
+FROM proposals
+WHERE proposal_id = $1
+FOR UPDATE`
+
+const ProposalParticipantLockSQL = `SELECT response
+FROM proposal_participants
+WHERE proposal_id = $1 AND player_id = $2
+FOR UPDATE`
+
+const ProposalParticipantRespondSQL = `UPDATE proposal_participants
+SET response = $3, responded_at = $4
+WHERE proposal_id = $1 AND player_id = $2 AND response = 'PENDING'`
+
+const ProposalCountPendingSQL = `SELECT COUNT(*)
+FROM proposal_participants
+WHERE proposal_id = $1 AND response = 'PENDING'`
+
+const ProposalAcceptSQL = `UPDATE proposals
+SET state = 'ACCEPTED', revision = revision + 1
+WHERE proposal_id = $1 AND state = 'OPEN'`
+
+const ProposalDeclineSQL = `UPDATE proposals
+SET state = 'DECLINED', revision = revision + 1
+WHERE proposal_id = $1 AND state = 'OPEN'`
+
+const ProposalRevisionBumpSQL = `UPDATE proposals
+SET revision = revision + 1
+WHERE proposal_id = $1 AND state = 'OPEN'`
+
+var ErrProposalResponseConflict = fmt.Errorf("proposal response conflict")
+
 // GetProposal recovers the full proposal only after proving the caller is a
 // participant. Expiry is advanced in the same transaction as the read so a
 // missed event cannot leave a durable proposal indefinitely OPEN.
@@ -75,3 +122,121 @@ func GetProposal(ctx context.Context, db *sql.DB, playerID, proposalID string, n
 	}
 	return proposal, nil
 }
+
+// RespondToProposal is the durable mutation counterpart to GetProposal. The
+// proposal row and participant row are locked in one transaction; the result
+// is stored under the idempotency key before the transaction commits.
+func RespondToProposal(ctx context.Context, db *sql.DB, playerID, proposalID, idempotencyKey string, accept bool, expectedRevision uint64, now time.Time) (domain.Proposal, error) {
+	if db == nil || playerID == "" || proposalID == "" || len(idempotencyKey) < 16 || len(idempotencyKey) > 128 || now.IsZero() {
+		return domain.Proposal{}, fmt.Errorf("invalid proposal response arguments")
+	}
+	digest := sha256.Sum256([]byte(fmt.Sprintf("%s|%s|%t|%d", playerID, proposalID, accept, expectedRevision)))
+	var proposal domain.Proposal
+	err := RunSerializable(ctx, db, DefaultSerializableAttempts, func(ctx context.Context, tx *sql.Tx) error {
+		result, err := tx.ExecContext(ctx, ProposalResponseIdempotencyInsertSQL, ProposalResponseIdempotencyScope, idempotencyKey, digest[:], []byte("{}"))
+		if err != nil {
+			return err
+		}
+		inserted, err := result.RowsAffected()
+		if err != nil {
+			return err
+		}
+		if inserted == 0 {
+			var priorDigest, priorResult []byte
+			if err := tx.QueryRowContext(ctx, ProposalResponseIdempotencySelectSQL, ProposalResponseIdempotencyScope, idempotencyKey).Scan(&priorDigest, &priorResult); err != nil {
+				return err
+			}
+			if !bytes.Equal(priorDigest, digest[:]) {
+				return ErrProposalResponseConflict
+			}
+			if err := json.Unmarshal(priorResult, &proposal); err != nil {
+				return fmt.Errorf("invalid stored proposal response: %w", err)
+			}
+			return nil
+		}
+
+		var playlist, state string
+		var revision uint64
+		var expiresAt time.Time
+		if err := tx.QueryRowContext(ctx, ProposalLockSQL, proposalID).Scan(&playlist, &state, &revision, &expiresAt); err != nil {
+			return err
+		}
+		if state != string(domain.Open) || !now.Before(expiresAt) {
+			return domain.ErrProposalClosed
+		}
+		if revision != expectedRevision {
+			return domain.ErrStaleRevision
+		}
+		var response string
+		if err := tx.QueryRowContext(ctx, ProposalParticipantLockSQL, proposalID, playerID).Scan(&response); err != nil {
+			return domain.ErrNotParticipant
+		}
+		if response != string(domain.Pending) {
+			return domain.ErrConflict
+		}
+		response = string(domain.DeclinedResponse)
+		if accept {
+			response = string(domain.AcceptedResponse)
+		}
+		changed, err := tx.ExecContext(ctx, ProposalParticipantRespondSQL, proposalID, playerID, response, now)
+		if err != nil {
+			return err
+		}
+		if count, err := changed.RowsAffected(); err != nil || count != 1 {
+			return ErrProposalResponseConflict
+		}
+		targetState := string(domain.Declined)
+		if accept {
+			var pending int
+			if err := tx.QueryRowContext(ctx, ProposalCountPendingSQL, proposalID).Scan(&pending); err != nil {
+				return err
+			}
+			if pending == 0 {
+				targetState = string(domain.Accepted)
+			} else {
+				targetState = state
+			}
+		}
+		if targetState != state {
+			if targetState == string(domain.Accepted) {
+				_, err = tx.ExecContext(ctx, ProposalAcceptSQL, proposalID)
+			} else {
+				_, err = tx.ExecContext(ctx, ProposalDeclineSQL, proposalID)
+			}
+			if err != nil {
+				return err
+			}
+			revision++
+		} else {
+			if _, err := tx.ExecContext(ctx, ProposalRevisionBumpSQL, proposalID); err != nil {
+				return err
+			}
+			revision++
+		}
+		proposal = domain.Proposal{ProposalID: proposalID, Playlist: domain.Playlist(playlist), State: domain.State(targetState), Revision: revision, ExpiresAt: expiresAt}
+		rows, err := tx.QueryContext(ctx, ProposalParticipantsSelectSQL, proposalID)
+		if err != nil {
+			return err
+		}
+		for rows.Next() {
+			var participant domain.ProposalParticipant
+			if err := rows.Scan(&participant.PlayerID, &participant.Response); err != nil {
+				rows.Close()
+				return err
+			}
+			proposal.Participants = append(proposal.Participants, participant)
+		}
+		if err := rows.Err(); err != nil {
+			rows.Close()
+			return err
+		}
+		rows.Close()
+		stored, err := json.Marshal(proposal)
+		if err != nil {
+			return err
+		}
+		_, err = tx.ExecContext(ctx, `UPDATE idempotency_keys SET result = $3 WHERE scope = $1 AND idempotency_key = $2`, ProposalResponseIdempotencyScope, idempotencyKey, stored)
+		return err
+	})
+	return proposal, err
+}

server/store/proposal_recovery_sql_test.go

@@ -7,10 +7,15 @@ import (
 
 func TestProposalRecoverySQLBindsParticipantAndExpiresAtReadBoundary(t *testing.T) {
 	for query, fragments := range map[string][]string{
-		ProposalExpireSQL:             {"state = 'OPEN'", "expires_at <= $2", "revision = revision + 1"},
-		ProposalParticipantExpireSQL:  {"response = 'PENDING'", "response = 'TIMED_OUT'"},
-		ProposalRecoverySelectSQL:     {"proposal_id = $1", "player_id = $2", "EXISTS"},
-		ProposalParticipantsSelectSQL: {"proposal_id = $1", "ORDER BY player_id"},
+		ProposalExpireSQL:                    {"state = 'OPEN'", "expires_at <= $2", "revision = revision + 1"},
+		ProposalParticipantExpireSQL:         {"response = 'PENDING'", "response = 'TIMED_OUT'"},
+		ProposalRecoverySelectSQL:            {"proposal_id = $1", "player_id = $2", "EXISTS"},
+		ProposalParticipantsSelectSQL:        {"proposal_id = $1", "ORDER BY player_id"},
+		ProposalResponseIdempotencyInsertSQL: {"ON CONFLICT (scope, idempotency_key) DO NOTHING", "payload_digest"},
+		ProposalLockSQL:                      {"proposal_id = $1", "FOR UPDATE"},
+		ProposalParticipantLockSQL:           {"proposal_id = $1", "player_id = $2", "FOR UPDATE"},
+		ProposalParticipantRespondSQL:        {"response = 'PENDING'", "responded_at"},
+		ProposalRevisionBumpSQL:              {"revision = revision + 1", "state = 'OPEN'"},
 	} {
 		for _, fragment := range fragments {
 			if !contains(query, fragment) {