fix: turns out ON CONFLICT DO NOTHING doesn't work with RETURNING

The workaround is to do a dummy update and return the updated id. Also
added a test.

backend/controller/dal/dal_test.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"io"
 	"reflect"
+	"sync"
 	"testing"
 	"time"
 
@@ -369,6 +370,38 @@ func TestDAL(t *testing.T) {
 	})
 }
 
+func TestCreateArtefactConflict(t *testing.T) {
+	ctx := log.ContextWithNewDefaultLogger(context.Background())
+	conn := sqltest.OpenForTesting(ctx, t)
+	dal, err := New(ctx, conn, optional.None[string]())
+	assert.NoError(t, err)
+
+	ids := make(chan sha256.SHA256, 2)
+
+	wg := sync.WaitGroup{}
+	wg.Add(2)
+	createContent := func() {
+		defer wg.Done()
+		tx1, err := dal.Begin(ctx)
+		assert.NoError(t, err)
+		digest, err := tx1.CreateArtefact(ctx, []byte("content"))
+		assert.NoError(t, err)
+		time.Sleep(time.Second * 2)
+		err = tx1.Commit(ctx)
+		assert.NoError(t, err)
+		ids <- digest
+	}
+
+	go createContent()
+	go createContent()
+
+	wg.Wait()
+
+	id1 := <-ids
+	id2 := <-ids
+	assert.Equal(t, id1, id2)
+}
+
 func artefactContent(t testing.TB, artefacts []*model.Artefact) [][]byte {
 	t.Helper()
 	var result [][]byte

backend/controller/sql/queries.sql

@@ -35,7 +35,8 @@ WHERE deployment_id = $1;
 -- Create a new artefact and return the artefact ID.
 INSERT INTO artefacts (digest, content)
 VALUES ($1, $2)
-ON CONFLICT (digest) DO NOTHING
+ON CONFLICT (digest)
+DO UPDATE SET digest = EXCLUDED.digest
 RETURNING id;
 
 -- name: AssociateArtefactWithDeployment :exec

cmd/ftl/cmd_serve.go

@@ -45,7 +45,7 @@ type serveCmd struct {
 	Stop                bool                 `help:"Stop the running FTL instance. Can be used with --background to restart the server" default:"false"`
 	StartupTimeout      time.Duration        `help:"Timeout for the server to start up." default:"1m"`
 	ObservabilityConfig observability.Config `embed:"" prefix:"o11y-"`
-	DatabaseImage       string               `help:"The container image to start for the database" default:"postgres:15.4" env:"FTL_DATABASE_IMAGE" hidden:""`
+	DatabaseImage       string               `help:"The container image to start for the database" default:"postgres:15.8" env:"FTL_DATABASE_IMAGE" hidden:""`
 	controller.CommonConfig
 }