Return duplicate error when creating a duplicate dataset

internal/metadata/project.go

@@ -3,10 +3,13 @@ package metadata
 import (
 	"context"
 	"database/sql"
+	"errors"
 	"fmt"
 	"sync"
 )
 
+var ErrDuplicatedDataset = errors.New("dataset is already created")
+
 type Project struct {
 	ID         string
 	datasets   []*Dataset
@@ -69,7 +72,7 @@ func (p *Project) AddDataset(ctx context.Context, tx *sql.Tx, dataset *Dataset)
 	p.mu.Lock()
 	defer p.mu.Unlock()
 	if _, exists := p.datasetMap[dataset.ID]; exists {
-		return fmt.Errorf("dataset %s is already created", dataset.ID)
+		return fmt.Errorf("dataset %s: %w", dataset.ID, ErrDuplicatedDataset)
 	}
 	if err := dataset.Insert(ctx, tx); err != nil {
 		return err

server/handler.go

@@ -679,7 +679,13 @@ func (h *datasetsInsertHandler) ServeHTTP(w http.ResponseWriter, r *http.Request
 		project: project,
 		dataset: &dataset,
 	})
+
 	if err != nil {
+		if errors.Is(err, metadata.ErrDuplicatedDataset) {
+			errorResponse(ctx, w, errDuplicate(err.Error()))
+			return
+		}
+
 		errorResponse(ctx, w, errInternalError(err.Error()))
 		return
 	}

server/server_test.go

@@ -12,6 +12,7 @@ import (
 	"net/url"
 	"path/filepath"
 	"strconv"
+	"strings"
 	"testing"
 	"time"
 
@@ -852,6 +853,46 @@ func TestDuplicateTableWithSchema(t *testing.T) {
 	}
 }
 
+func TestDuplicateDataset(t *testing.T) {
+	const (
+		projectName = "test"
+		datasetName = "dataset1"
+	)
+
+	ctx := context.Background()
+
+	bqServer, err := server.New(server.TempStorage)
+	if err != nil {
+		t.Fatal(err)
+	}
+	project := types.NewProject(projectName, types.NewDataset(datasetName))
+	if err := bqServer.Load(server.StructSource(project)); err != nil {
+		t.Fatal(err)
+	}
+
+	testServer := bqServer.TestServer()
+	defer func() {
+		testServer.Close()
+		bqServer.Stop(ctx)
+	}()
+
+	client, err := bigquery.NewClient(
+		ctx,
+		projectName,
+		option.WithEndpoint(testServer.URL),
+		option.WithoutAuthentication(),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer client.Close()
+	dataset := client.Dataset(datasetName)
+	err = dataset.Create(ctx, nil)
+	if err == nil || !strings.HasSuffix(err.Error(), "duplicate") {
+		t.Fatalf("expected duplicate error; got %s", err)
+	}
+}
+
 func TestDataFromStruct(t *testing.T) {
 	ctx := context.Background()