fixes #1745

buildGraph does not check for cycles as it walks dependency chains; this change detects a member of the cycle and reports an error identifying that member

buildengine/engine.go

@@ -189,6 +189,9 @@ func (e *Engine) Graph(projects ...ProjectKey) (map[string][]string, error) {
 
 func (e *Engine) buildGraph(key string, out map[string][]string) error {
 	var deps []string
+	if _, ok := out[key]; ok {
+		return fmt.Errorf("module %q contains a cyclical dependency", key)
+	}
 	if meta, ok := e.projectMetas.Load(ProjectKey(key)); ok {
 		deps = meta.project.Config().Dependencies
 	} else if sch, ok := e.controllerSchema.Load(key); ok {

buildengine/engine_test.go

@@ -58,3 +58,17 @@ func TestEngine(t *testing.T) {
 	err = engine.Build(ctx)
 	assert.NoError(t, err)
 }
+
+func TestCycleDetection(t *testing.T) {
+	if testing.Short() {
+		t.SkipNow()
+	}
+	ctx := log.ContextWithNewDefaultLogger(context.Background())
+	engine, err := buildengine.New(ctx, nil, []string{"testdata/projects/depcycle1", "testdata/projects/depcycle2"}, nil)
+	assert.NoError(t, err)
+
+	defer engine.Close()
+
+	_, err = engine.Graph()
+	assert.Contains(t, err.Error(), "contains a cyclical dependency")
+}

buildengine/testdata/projects/depcycle1/depcycle1.go

@@ -0,0 +1,18 @@
+package depcycle1
+
+import (
+	"context"
+	"fmt"
+	"ftl/depcycle2"
+)
+
+type Request struct{}
+type Response struct {
+	Message string
+}
+
+//ftl:verb export
+func Cycle1(ctx context.Context, req Request) (Response, error) {
+	var resp depcycle2.Response
+	return Response{Message: fmt.Sprintf("cycle1 %s", resp)}, nil
+}

buildengine/testdata/projects/depcycle1/ftl.toml

@@ -0,0 +1,2 @@
+module = "depcycle1"
+language = "go"

buildengine/testdata/projects/depcycle1/go.mod

@@ -0,0 +1,5 @@
+module ftl/depcycle1
+
+go 1.22.2
+
+replace github.com/TBD54566975/ftl => ./../../../../..

buildengine/testdata/projects/depcycle2/depcycle2.go

@@ -0,0 +1,18 @@
+package depcycle2
+
+import (
+	"context"
+	"fmt"
+	"ftl/depcycle1"
+)
+
+type Request struct{}
+type Response struct {
+	Message string
+}
+
+//ftl:verb export
+func Cycle2(ctx context.Context, req Request) (Response, error) {
+	var resp depcycle1.Response
+	return Response{Message: fmt.Sprintf("cycle2 %s", resp)}, nil
+}

buildengine/testdata/projects/depcycle2/ftl.toml

@@ -0,0 +1,2 @@
+module = "depcycle2"
+language = "go"

buildengine/testdata/projects/depcycle2/go.mod

@@ -0,0 +1,5 @@
+module ftl/depcycle2
+
+go 1.22.2
+
+replace github.com/TBD54566975/ftl => ./../../../../..

go-runtime/ftl/ftltest/testdata/go/pubsub/go.mod

@@ -21,11 +21,11 @@ require (
 	github.com/alecthomas/repr v0.4.0 // indirect
 	github.com/alecthomas/types v0.16.0 // indirect
 	github.com/alessio/shellescape v1.4.2 // indirect
-	github.com/amacneil/dbmate/v2 v2.16.0 // indirect
-	github.com/aws/aws-sdk-go-v2 v1.27.0 // indirect
-	github.com/aws/aws-sdk-go-v2/internal/configsources v1.3.7 // indirect
-	github.com/aws/aws-sdk-go-v2/internal/endpoints/v2 v2.6.7 // indirect
-	github.com/aws/aws-sdk-go-v2/service/secretsmanager v1.29.1 // indirect
+	github.com/amacneil/dbmate/v2 v2.17.0 // indirect
+	github.com/aws/aws-sdk-go-v2 v1.27.2 // indirect
+	github.com/aws/aws-sdk-go-v2/internal/configsources v1.3.9 // indirect
+	github.com/aws/aws-sdk-go-v2/internal/endpoints/v2 v2.6.9 // indirect
+	github.com/aws/aws-sdk-go-v2/service/secretsmanager v1.29.3 // indirect
 	github.com/aws/smithy-go v1.20.2 // indirect
 	github.com/danieljoos/wincred v1.2.0 // indirect
 	github.com/deckarep/golang-set/v2 v2.6.0 // indirect
@@ -47,18 +47,18 @@ require (
 	github.com/puzpuzpuz/xsync/v3 v3.1.0 // indirect
 	github.com/swaggest/jsonschema-go v0.3.70 // indirect
 	github.com/swaggest/refl v1.3.0 // indirect
-	github.com/zalando/go-keyring v0.2.4 // indirect
+	github.com/zalando/go-keyring v0.2.5 // indirect
 	go.opentelemetry.io/otel v1.27.0 // indirect
 	go.opentelemetry.io/otel/metric v1.27.0 // indirect
 	go.opentelemetry.io/otel/trace v1.27.0 // indirect
 	golang.org/x/crypto v0.24.0 // indirect
-	golang.org/x/exp v0.0.0-20240506185415-9bf2ced13842 // indirect
+	golang.org/x/exp v0.0.0-20240604190554-fc45aab8b7f8 // indirect
 	golang.org/x/mod v0.18.0 // indirect
 	golang.org/x/net v0.26.0 // indirect
 	golang.org/x/sync v0.7.0 // indirect
 	golang.org/x/sys v0.21.0 // indirect
 	golang.org/x/text v0.16.0 // indirect
-	google.golang.org/protobuf v1.34.1 // indirect
+	google.golang.org/protobuf v1.34.2 // indirect
 )
 
 replace github.com/TBD54566975/ftl => ./../../../../../..

go-runtime/ftl/ftltest/testdata/go/subscriber/go.mod

@@ -20,11 +20,11 @@ require (
 	github.com/alecthomas/repr v0.4.0 // indirect
 	github.com/alecthomas/types v0.16.0 // indirect
 	github.com/alessio/shellescape v1.4.2 // indirect
-	github.com/amacneil/dbmate/v2 v2.16.0 // indirect
-	github.com/aws/aws-sdk-go-v2 v1.27.0 // indirect
-	github.com/aws/aws-sdk-go-v2/internal/configsources v1.3.7 // indirect
-	github.com/aws/aws-sdk-go-v2/internal/endpoints/v2 v2.6.7 // indirect
-	github.com/aws/aws-sdk-go-v2/service/secretsmanager v1.29.1 // indirect
+	github.com/amacneil/dbmate/v2 v2.17.0 // indirect
+	github.com/aws/aws-sdk-go-v2 v1.27.2 // indirect
+	github.com/aws/aws-sdk-go-v2/internal/configsources v1.3.9 // indirect
+	github.com/aws/aws-sdk-go-v2/internal/endpoints/v2 v2.6.9 // indirect
+	github.com/aws/aws-sdk-go-v2/service/secretsmanager v1.29.3 // indirect
 	github.com/aws/smithy-go v1.20.2 // indirect
 	github.com/danieljoos/wincred v1.2.0 // indirect
 	github.com/deckarep/golang-set/v2 v2.6.0 // indirect
@@ -46,18 +46,18 @@ require (
 	github.com/puzpuzpuz/xsync/v3 v3.1.0 // indirect
 	github.com/swaggest/jsonschema-go v0.3.70 // indirect
 	github.com/swaggest/refl v1.3.0 // indirect
-	github.com/zalando/go-keyring v0.2.4 // indirect
+	github.com/zalando/go-keyring v0.2.5 // indirect
 	go.opentelemetry.io/otel v1.27.0 // indirect
 	go.opentelemetry.io/otel/metric v1.27.0 // indirect
 	go.opentelemetry.io/otel/trace v1.27.0 // indirect
 	golang.org/x/crypto v0.24.0 // indirect
-	golang.org/x/exp v0.0.0-20240506185415-9bf2ced13842 // indirect
+	golang.org/x/exp v0.0.0-20240604190554-fc45aab8b7f8 // indirect
 	golang.org/x/mod v0.18.0 // indirect
 	golang.org/x/net v0.26.0 // indirect
 	golang.org/x/sync v0.7.0 // indirect
 	golang.org/x/sys v0.21.0 // indirect
 	golang.org/x/text v0.16.0 // indirect
-	google.golang.org/protobuf v1.34.1 // indirect
+	google.golang.org/protobuf v1.34.2 // indirect
 )
 
 replace github.com/TBD54566975/ftl => ./../../../../../..