Enhance Proof Delivery Resilience with ParSliceErrCollect in ChainPorter

fn/concurrency.go

@@ -2,7 +2,9 @@ package fn
 
 import (
 	"context"
+	"fmt"
 	"runtime"
+	"sync"
 
 	"golang.org/x/sync/errgroup"
 )
@@ -32,3 +34,48 @@ func ParSlice[V any](ctx context.Context, s []V, f ErrFunc[V]) error {
 
 	return errGroup.Wait()
 }
+
+// ParSliceErrCollect can be used to execute a function on each element of a
+// slice in parallel. This function is fully blocking and will wait for all
+// goroutines to finish (subject to context cancellation/timeout). Any errors
+// will be collected and returned as a map of slice element index to error.
+// Active goroutines limited with number of CPU.
+func ParSliceErrCollect[V any](ctx context.Context, s []V,
+	f ErrFunc[V]) (map[int]error, error) {
+
+	errGroup, ctx := errgroup.WithContext(ctx)
+	errGroup.SetLimit(runtime.NumCPU())
+
+	var instanceErrorsMutex sync.Mutex
+	instanceErrors := make(map[int]error, len(s))
+
+	for idx := range s {
+		errGroup.Go(func() error {
+			err := f(ctx, s[idx])
+			if err != nil {
+				instanceErrorsMutex.Lock()
+				instanceErrors[idx] = err
+				instanceErrorsMutex.Unlock()
+			}
+
+			// Avoid returning an error here, as that would cancel
+			// the errGroup and terminate all slice element
+			// processing instances. Instead, collect the error and
+			// return it later.
+			return nil
+		})
+	}
+
+	// Now we will wait/block for all goroutines to finish.
+	//
+	// The goroutines that are executing in parallel should not return an
+	// error, but the Wait call may return an error if the context is
+	// canceled or timed out.
+	err := errGroup.Wait()
+	if err != nil {
+		return nil, fmt.Errorf("failed to wait on error group in "+
+			"ParSliceErrorCollect: %w", err)
+	}
+
+	return instanceErrors, nil
+}

fn/func.go

@@ -1,6 +1,10 @@
 package fn
 
-import "fmt"
+import (
+	"fmt"
+
+	"github.com/lightningnetwork/lnd/fn"
+)
 
 // Reducer represents a function that takes an accumulator and the value, then
 // returns a new accumulator.
@@ -263,3 +267,19 @@ func Last[T any](xs []*T, pred func(*T) bool) (*T, error) {
 
 	return matches[len(matches)-1], nil
 }
+
+// KV is a generic struct that holds a key-value pair.
+type KV[K any, V any] struct {
+	Key   K
+	Value V
+}
+
+// PeekMap non-deterministically selects and returns a single key-value pair
+// from the given map.
+func PeekMap[K comparable, V any](m map[K]V) fn.Option[KV[K, V]] {
+	for k, v := range m {
+		return fn.Some(KV[K, V]{Key: k, Value: v})
+	}
+
+	return fn.None[KV[K, V]]()
+}

tapfreighter/chain_porter.go

@@ -870,11 +870,43 @@ func (p *ChainPorter) transferReceiverProof(pkg *sendPackage) error {
 
 	// If we have a non-interactive proof, then we'll launch several
 	// goroutines to deliver the proof(s) to the receiver(s).
-	err := fn.ParSlice(ctx, pkg.OutboundPkg.Outputs, deliver)
+	instanceErrors, err := fn.ParSliceErrCollect(
+		ctx, pkg.OutboundPkg.Outputs, deliver,
+	)
 	if err != nil {
 		return fmt.Errorf("error delivering proof(s): %w", err)
 	}
 
+	// If there were any errors during the proof delivery process, we'll
+	// log them all here.
+	for idx := range instanceErrors {
+		output := pkg.OutboundPkg.Outputs[idx]
+		instanceErr := instanceErrors[idx]
+
+		scriptPubKey := output.ScriptKey.PubKey.SerializeCompressed()
+		anchorOutpoint := output.Anchor.OutPoint.String()
+		courierAddr := string(output.ProofCourierAddr)
+
+		log.Errorf("Error delivering transfer output proof "+
+			"(anchor_outpoint=%s, script_pub_key=%v, "+
+			"position=%d, proof_courier_addr=%s, "+
+			"proof_delivery_status=%v): %v",
+			anchorOutpoint, scriptPubKey, output.Position,
+			courierAddr, output.ProofDeliveryComplete,
+			instanceErr)
+	}
+
+	// Return the first error encountered during the proof delivery process,
+	// if any.
+	var firstErr error
+	fn.PeekMap(instanceErrors).WhenSome(func(kv fn.KV[int, error]) {
+		firstErr = err
+	})
+
+	if firstErr != nil {
+		return firstErr
+	}
+
 	// At this point, the transfer is fully finalised and successful:
 	// - The anchoring transaction has been confirmed on-chain.
 	// - The proof(s) have been delivered to the receiver(s).