{"record":{"id":"1816bd9557d37c06","repo":"apache/beam","slug":"failed-to-construct-userfn","errorCode":null,"errorMessage":"failed to construct userfn","messagePattern":"failed to construct userfn","errorType":"exception","errorClass":null,"httpStatus":null,"severity":"error","filePath":"sdks/go/pkg/beam/core/runtime/graphx/serialize.go","lineNumber":297,"sourceCode":"\t\t\twrapped := errors.Wrap(err, \"bad type\")\n\t\t\treturn nil, errors.WithContextf(wrapped, \"failed to decode dynamic DoFn %v\", u)\n\t\t}\n\t\treturn graph.NewFn(&graph.DynFn{\n\t\t\tName: u.Dynfn.Name,\n\t\t\tT:    t,\n\t\t\tData: u.Dynfn.Data,\n\t\t\tGen:  gen.(func(string, reflect.Type, []byte) reflectx.Func),\n\t\t})\n\t}\n\tif u.Fn != nil {\n\t\tfn, err := decodeUserFn(u.Fn)\n\t\tif err != nil {\n\t\t\twrapped := errors.Wrap(err, \"failed to decode userfn\")\n\t\t\treturn nil, errors.WithContextf(wrapped, \"decoding DoFn %v\", u)\n\t\t}\n\t\tfx, err := funcx.New(reflectx.MakeFunc(fn))\n\t\tif err != nil {\n\t\t\twrapped := errors.Wrap(err, \"failed to construct userfn\")\n\t\t\treturn nil, errors.WithContextf(wrapped, \"decoding DoFn %v\", u)\n\t\t}\n\t\treturn &graph.Fn{Fn: fx}, nil\n\t}\n\n\tt, err := decodeType(u.Type)\n\tif err != nil {\n\t\twrapped := errors.Wrap(err, \"bad type\")\n\t\treturn nil, errors.WithContextf(wrapped, \"decoding structural DoFn %v\", u)\n\t}\n\telem := reflect.New(t)\n\tif err := jsonx.UnmarshalFrom(elem.Interface(), strings.NewReader(u.Opt)); err != nil {\n\t\twrapped := errors.Wrap(err, \"bad struct encoding\")\n\t\treturn nil, errors.WithContextf(wrapped, \"decoding structural DoFn %v\", u)\n\t}\n\tfn := elem.Elem().Interface()\n\treturn graph.NewFn(fn)\n}","sourceCodeStart":279,"sourceCodeEnd":315,"githubUrl":"https://github.com/apache/beam/blob/12126d8942aaf848030c478b4c6a28c6af861c66/sdks/go/pkg/beam/core/runtime/graphx/serialize.go#L279-L315","documentation":"After successfully decoding the userfn, decodeFn calls funcx.New(reflectx.MakeFunc(fn)) to validate and wrap the function's signature; failure means the reconstructed function has a signature funcx cannot analyze (not a valid func, unsupported parameter/return kinds).","triggerScenarios":"DecodeMultiEdge decoding a Fn whose decoded userfn value is not a valid function or has a signature outside funcx's supported DoFn/emit shapes.","commonSituations":"DoFn registered as a non-func value; signature shapes from custom/dynamic fn generation that violate funcx expectations; version skew producing mismatched expectations.","solutions":["Check the wrapped funcx error for the exact signature complaint and fix the DoFn's method signature.","Ensure the registered fn is actually a func with supported parameter/return types (context, iterators, emitters).","Use standard DoFn signatures matching current Beam Go docs.","Align Beam versions between producer and consumer of the pipeline proto."],"exampleFix":"// before\nfunc (f *myFn) Process() int { return 1 } // unsupported signature\n\n// after\nfunc (f *myFn) ProcessElement(ctx context.Context, elm int, emit func(int)) { emit(elm) }","handlingStrategy":"validation","validationCode":"if reflect.ValueOf(fn).Kind() != reflect.Func {\n    return fmt.Errorf(\"userfn must be a func, got %T\", fn)\n}\nif _, err := funcx.New(reflectx.MakeFunc(fn)); err != nil {\n    return fmt.Errorf(\"unsupported fn signature: %w\", err)\n}","typeGuard":"func isFunc(v interface{}) bool { return v != nil && reflect.ValueOf(v).Kind() == reflect.Func }","tryCatchPattern":"fx, err := funcx.New(reflectx.MakeFunc(fn))\nif err != nil {\n    return fmt.Errorf(\"DoFn signature not supported by funcx: %w\", err)\n}","preventionTips":["Stick to documented DoFn signatures (ProcessElement with ctx, inputs, emitters).","Compile-time check with var _ = funcx.New(reflectx.MakeFunc(myFn.ProcessElement))."],"tags":["go","reflection","function-signature","apache-beam"],"backgroundTag":"invalid-function-signature","analyzedSha":"12126d8942aaf848030c478b4c6a28c6af861c66","analyzedAt":"2026-09-13T01:50:10.254Z","contentChangedAt":"2026-09-13T01:50:10.254Z","schemaVersion":2},"datasetVersion":"2026-09-14T11:17:12.474Z"}