From 922da29f679cb32852966bb888c02c3dd64749c9 Mon Sep 17 00:00:00 2001
From: shollands <shollands@snapchat.com>
Date: Tue, 10 Mar 2026 14:38:37 -0700
Subject: [PATCH] Fix: return HTTP 409 instead of 500 for duplicate datasets
 and jobs

When creating a dataset or job that already exists, the emulator returns
HTTP 500 (InternalServerError) instead of HTTP 409 (Conflict). This
causes the BigQuery Python client to retry with exponential backoff for
up to 600 seconds, since it treats 500 as transient. The exists_ok=True
parameter also fails to suppress the error because it only checks for 409.

This fix follows the existing ErrDuplicatedTable pattern already in the
codebase: sentinel errors in the metadata package, checked with
errors.Is in ServeHTTP, mapped to errDuplicate() for the HTTP response.

Handle() method signatures are unchanged, addressing the feedback on #184.

Fixes #256
Supersedes #184

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 internal/metadata/project.go | 10 +++++--
 server/handler.go            | 12 +++++++--
 server/server_test.go        | 52 ++++++++++++++++++++++++++++++++++++
 3 files changed, 70 insertions(+), 4 deletions(-)

diff --git a/internal/metadata/project.go b/internal/metadata/project.go
index cecca2578..0213eba8a 100644
--- a/internal/metadata/project.go
+++ b/internal/metadata/project.go
@@ -3,10 +3,16 @@ package metadata
 import (
 	"context"
 	"database/sql"
+	"errors"
 	"fmt"
 	"sync"
 )
 
+var (
+	ErrDuplicatedDataset = errors.New("dataset is already created")
+	ErrDuplicatedJob     = errors.New("job is already created")
+)
+
 type Project struct {
 	ID         string
 	datasets   []*Dataset
@@ -69,7 +75,7 @@ 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
@@ -111,7 +117,7 @@ func (p *Project) AddJob(ctx context.Context, tx *sql.Tx, job *Job) error {
 	p.mu.Lock()
 	defer p.mu.Unlock()
 	if _, exists := p.jobMap[job.ID]; exists {
-		return fmt.Errorf("job %s is already created", job.ID)
+		return fmt.Errorf("job %s: %w", job.ID, ErrDuplicatedJob)
 	}
 	if err := job.Insert(ctx, tx); err != nil {
 		return err
diff --git a/server/handler.go b/server/handler.go
index 7a5d2c0b7..8e6f53272 100644
--- a/server/handler.go
+++ b/server/handler.go
@@ -681,7 +681,11 @@ func (h *datasetsInsertHandler) ServeHTTP(w http.ResponseWriter, r *http.Request
 		dataset: &dataset,
 	})
 	if err != nil {
-		errorResponse(ctx, w, errInternalError(err.Error()))
+		if errors.Is(err, metadata.ErrDuplicatedDataset) {
+			errorResponse(ctx, w, errDuplicate(err.Error()))
+		} else {
+			errorResponse(ctx, w, errInternalError(err.Error()))
+		}
 		return
 	}
 	encodeResponse(ctx, w, res)
@@ -1016,7 +1020,11 @@ func (h *jobsInsertHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		job:     &job,
 	})
 	if err != nil {
-		errorResponse(ctx, w, errJobInternalError(err.Error()))
+		if errors.Is(err, metadata.ErrDuplicatedJob) {
+			errorResponse(ctx, w, errDuplicate(err.Error()))
+		} else {
+			errorResponse(ctx, w, errJobInternalError(err.Error()))
+		}
 		return
 	}
 	encodeResponse(ctx, w, res)
diff --git a/server/server_test.go b/server/server_test.go
index a27f49208..e1bb89443 100644
--- a/server/server_test.go
+++ b/server/server_test.go
@@ -867,6 +867,58 @@ func TestDuplicateTable(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)
+	}
+	if err := bqServer.SetProject(projectName); 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)
+	if err := dataset.Create(ctx, &bigquery.DatasetMetadata{
+		Name: datasetName,
+	}); err != nil {
+		t.Fatalf("%+v", err)
+	}
+
+	if err := dataset.Create(ctx, &bigquery.DatasetMetadata{
+		Name: datasetName,
+	}); err != nil {
+		ge := err.(*googleapi.Error)
+		if ge.Code != 409 {
+			t.Fatalf("expected 409 Conflict, got %d: %+v", ge.Code, ge)
+		}
+	} else {
+		t.Fatal("expected error when dataset name duplicates")
+	}
+}
+
 func TestDuplicateTableWithSchema(t *testing.T) {
 	const (
 		projectName = "test"