apache/beam · error

%v

Error message

%v

What it means

This is a recover() wrapper that converts a panic from bundle descriptor building into a normal error. If the recovered value is already an error it is used as-is; any other panic value is wrapped via fmt.Errorf("%v", r).

Source

Thrown at sdks/go/pkg/beam/runners/prism/internal/stage.go:419

// buildDescriptor constructs a ProcessBundleDescriptor for bundles of this stage.
//
// Requirements:
// * The set of inputs to the stage only include one parallel input.
// * The side input pcollections are fully qualified with global pcollection ID, ingesting transform, and local inputID.
// * The outputs are fully qualified with global PCollectionID, producing transform, and local outputID.
//
// It assumes that the side inputs are not sourced from PCollections generated by any transform in this stage.
//
// Because we need the local ids for routing the sources/sinks information.
func buildDescriptor(stg *stage, comps *pipepb.Components, wk *worker.W, em *engine.ElementManager) (err error) {
	// Catch construction time panics and produce them as errors out.
	defer func() {
		if r := recover(); r != nil {
			switch rt := r.(type) {
			case error:
				err = rt
			default:
				err = fmt.Errorf("%v", r)
			}
		}
	}()
	// Assume stage has an indicated primary input

	coders := map[string]*pipepb.Coder{}
	transforms := map[string]*pipepb.PTransform{}
	pcollections := map[string]*pipepb.PCollection{}

	clonePColToBundle := func(pid string) *pipepb.PCollection {
		col := proto.Clone(comps.GetPcollections()[pid]).(*pipepb.PCollection)
		pcollections[pid] = col
		return col
	}

	// Update coders for Stateful transforms.
	for _, tid := range stg.transforms {
		t := comps.GetTransforms()[tid]

View on GitHub (pinned to 12126d8942)

Solutions

1. Read the converted error's message to identify the panicking value
2. Check the stage's coder definitions in the pipeline proto for nils or missing references
3. Retry with a simpler pipeline to isolate the coder causing the panic
4. File a Beam issue with logs if it's a prism bug

if err := buildDescriptor(...); err != nil {
    slog.Error("descriptor build recovered panic", "err", err)
    return err
}

When it happens

Trigger: Any panic during descriptor construction — typically coder lookups or map indexing on missing/nil coders while assembling the bundle's process-bundle descriptor.

Common situations: Runner bugs handling unusual coder combinations (e.g. cross-language or windowed coders) that trigger nil map access or index panics.

Related errors

• Nested FullValues must be nested as pointers.
• KV coder with more than 2 components: %s
• WindowedValue coder with more than 2 components: %s
• ShardedKey coder must have only 1 component: %s
• Runner forgot to LP this Row Coder. %v

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/7e67cd78f88a6cdb. Report an issue: GitHub.