fix: fence concurrent queue mutations

server/domain/queue_test.go

@@ -3,6 +3,7 @@ package domain
 import (
 	"errors"
 	"reflect"
+	"sync"
 	"testing"
 	"time"
 )
@@ -77,3 +78,32 @@ func TestQueueCreateIdempotencyIncludesCandidatePayload(t *testing.T) {
 		t.Fatalf("changed create payload error = %v", err)
 	}
 }
+
+func TestQueueConcurrentCreateKeepsOneActiveTicketPerPlayer(t *testing.T) {
+	q := NewQueue()
+	now := time.Unix(1000, 0)
+	var wg sync.WaitGroup
+	results := make(chan error, 2)
+	for i := 0; i < 2; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			id := string(rune('a' + i))
+			_, err := q.Create("same-player", "ticket-"+id, "create-"+id+"-123456", Candidate{TicketID: "ticket-" + id, PlayerID: "same-player", EnqueuedAt: now}, now)
+			results <- err
+		}(i)
+	}
+	wg.Wait()
+	close(results)
+	succeeded := 0
+	for err := range results {
+		if err == nil {
+			succeeded++
+		} else if !errors.Is(err, ErrPlayerQueued) {
+			t.Fatalf("unexpected concurrent create error: %v", err)
+		}
+	}
+	if succeeded != 1 {
+		t.Fatalf("concurrent creates succeeded %d times", succeeded)
+	}
+}