sweepbatcher: log the reason of acceptance failure

Add Info log message in cases when addSweep returns accept=false and err=nil.

sweepbatcher/sweep_batch.go

@@ -438,6 +438,8 @@ func (b *batch) addSweep(ctx context.Context, sweep *sweep) (bool, error) {
 	// If the provided sweep is nil, we can't proceed with any checks, so
 	// we just return early.
 	if sweep == nil {
+		b.log.Infof("the sweep is nil")
+
 		return false, nil
 	}
 
@@ -471,21 +473,36 @@ func (b *batch) addSweep(ctx context.Context, sweep *sweep) (bool, error) {
 	// the batch, do not add another sweep to prevent the tx from becoming
 	// non-standard.
 	if len(b.sweeps) >= MaxSweepsPerBatch {
+		b.log.Infof("the batch has already too many sweeps (%d >= %d)",
+			len(b.sweeps), MaxSweepsPerBatch)
+
 		return false, nil
 	}
 
 	// Since all the actions of the batch happen sequentially, we could
 	// arrive here after the batch got closed because of a spend. In this
 	// case we cannot add the sweep to this batch.
 	if b.state != Open {
+		b.log.Infof("the batch state (%v) is not open", b.state)
+
 		return false, nil
 	}
 
 	// If this batch contains a single sweep that spends to a non-wallet
 	// address, or the incoming sweep is spending to non-wallet address,
 	// we cannot add this sweep to the batch.
 	for _, s := range b.sweeps {
-		if s.isExternalAddr || sweep.isExternalAddr {
+		if s.isExternalAddr {
+			b.log.Infof("the batch already has a sweep (%x) with "+
+				"an external address", s.swapHash[:6])
+
+			return false, nil
+		}
+
+		if sweep.isExternalAddr {
+			b.log.Infof("the batch is not empty and new sweep (%x)"+
+				" has an external address", sweep.swapHash[:6])
+
 			return false, nil
 		}
 	}
@@ -498,6 +515,11 @@ func (b *batch) addSweep(ctx context.Context, sweep *sweep) (bool, error) {
 			int32(math.Abs(float64(sweep.timeout - s.timeout)))
 
 		if timeoutDistance > b.cfg.maxTimeoutDistance {
+			b.log.Infof("too long timeout distance between the "+
+				"batch and sweep %x: %d > %d",
+				sweep.swapHash[:6], timeoutDistance,
+				b.cfg.maxTimeoutDistance)
+
 			return false, nil
 		}
 	}