Log part-merged nodes dropped to tracing

So we have more idea what happened, in case of issue.

app/api_topologies.go

@@ -479,7 +479,7 @@ func (r *Registry) makeTopologyList(rep Reporter) CtxHandlerFunc {
 			respondWith(ctx, w, http.StatusInternalServerError, err)
 			return
 		}
-		report.UnsafeRemovePartMergedNodes()
+		report.UnsafeRemovePartMergedNodes(ctx)
 		respondWith(ctx, w, http.StatusOK, r.renderTopologies(ctx, report, req))
 	}
 }
@@ -580,7 +580,7 @@ func (r *Registry) captureRenderer(rep Reporter, f rendererHandler) CtxHandlerFu
 			respondWith(ctx, w, http.StatusInternalServerError, err)
 			return
 		}
-		rpt.UnsafeRemovePartMergedNodes()
+		rpt.UnsafeRemovePartMergedNodes(ctx)
 		req.ParseForm()
 		renderer, filter, err := r.RendererForTopology(topologyID, req.Form, rpt)
 		if err != nil {

app/api_topology.go

@@ -182,7 +182,7 @@ func (wc *websocketState) update(ctx context.Context) error {
 	if err != nil {
 		return errors.Wrap(err, "Error generating report")
 	}
-	re.UnsafeRemovePartMergedNodes()
+	re.UnsafeRemovePartMergedNodes(ctx)
 	renderer, filter, err := topologyRegistry.RendererForTopology(wc.topologyID, wc.values, re)
 	if err != nil {
 		return errors.Wrap(err, "Error generating report")

app/benchmark_internal_test.go

@@ -87,7 +87,7 @@ func getReport(b *testing.B) report.Report {
 
 func benchmarkRender(b *testing.B, f func(report.Report)) {
 	r := getReport(b)
-	r.UnsafeRemovePartMergedNodes()
+	r.UnsafeRemovePartMergedNodes(context.Background())
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
 		b.StopTimer()

app/collector_test.go

@@ -52,7 +52,7 @@ func TestCollector(t *testing.T) {
 	merged := report.MakeReport()
 	merged.UnsafeMerge(r1)
 	merged.UnsafeMerge(r2)
-	merged.UnsafeRemovePartMergedNodes()
+	merged.UnsafeRemovePartMergedNodes(context.Background())
 	have, err = c.Report(ctx, mtime.Now())
 	if err != nil {
 		t.Error(err)

report/report.go

@@ -1,11 +1,14 @@
 package report
 
 import (
+	"context"
 	"fmt"
 	"math/rand"
 	"strings"
 	"time"
 
+	opentracing "github.com/opentracing/opentracing-go"
+
 	"github.com/weaveworks/scope/common/xfer"
 )
 
@@ -367,14 +370,23 @@ func (r *Report) UnsafeUnMerge(other Report) {
 // E.g. if a node is removed from source between two full reports, then we
 // might only have a delta of its last state. Remove that from the set.
 // The original is modified.
-func (r *Report) UnsafeRemovePartMergedNodes() {
-	r.WalkTopologies(func(t *Topology) {
+func (r *Report) UnsafeRemovePartMergedNodes(ctx context.Context) {
+	dropped := map[string]int{}
+	r.WalkNamedTopologies(func(name string, t *Topology) {
 		for k, v := range t.Nodes {
 			if v.isPartMerged() {
 				delete(t.Nodes, k)
+				dropped[name]++
 			}
 		}
 	})
+	if span := opentracing.SpanFromContext(ctx); span != nil && len(dropped) > 0 {
+		msg := ""
+		for name, count := range dropped {
+			msg += fmt.Sprintf("%s: %d, ", name, count)
+		}
+		span.LogKV("dropped-part-merged", msg)
+	}
 }
 
 // WalkTopologies iterates through the Topologies of the report,