ddtrace/tracer: encode span IDs in execution traces efficiently (#2268)

We were previously encoding span IDs into execution traces as base-10
strings. This is wasteful, in terms of CPU time to encode the ID and
more importantly in terms of how much of the limited execution trace
data the profiler collects will be taken up by span IDs. They can be
encoded directly as 8-byte unsigned integers.

ddtrace/tracer/exec_tracer_test.go

@@ -0,0 +1,95 @@
+// Unless explicitly stated otherwise all files in this repository are licensed
+// under the Apache License Version 2.0.
+// This product includes software developed at Datadog (https://www.datadoghq.com/).
+// Copyright 2016 Datadog, Inc.
+
+// Tests in this file rely on parsing execution tracer data, which can change
+// formats across Go releases. This guard should be updated as the Go trace
+// parser library is upgraded to support new versions.
+//go:build !go1.21
+
+package tracer
+
+import (
+	"bytes"
+	"context"
+	"encoding/binary"
+	rt "runtime/trace"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	gotraceui "honnef.co/go/gotraceui/trace"
+)
+
+func TestExecutionTraceSpans(t *testing.T) {
+	if rt.IsEnabled() {
+		t.Skip("runtime execution tracing is already enabled")
+	}
+
+	buf := new(bytes.Buffer)
+	if err := rt.Start(buf); err != nil {
+		t.Fatal(err)
+	}
+	// Ensure we unconditionally stop tracing. It's safe to call this
+	// multiple times.
+	defer rt.Stop()
+
+	_, _, _, stop := startTestTracer(t)
+	defer stop()
+
+	root, ctx := StartSpanFromContext(context.Background(), "root")
+	child, _ := StartSpanFromContext(ctx, "child")
+	root.Finish()
+	child.Finish()
+
+	rt.Stop()
+
+	execTrace, err := gotraceui.Parse(buf, nil)
+	if err != nil {
+		t.Fatalf("parsing trace: %s", err)
+	}
+
+	type traceSpan struct {
+		name   string
+		parent string
+		spanID uint64
+	}
+
+	spans := make(map[int]*traceSpan)
+	for _, ev := range execTrace.Events {
+		switch ev.Type {
+		case gotraceui.EvUserTaskCreate:
+			id := int(ev.Args[0])
+			name := execTrace.Strings[ev.Args[2]]
+			var parent string
+			if p, ok := spans[int(ev.Args[1])]; ok {
+				parent = p.name
+			}
+			spans[id] = &traceSpan{
+				name:   name,
+				parent: parent,
+			}
+		case gotraceui.EvUserLog:
+			id := int(ev.Args[0])
+			span, ok := spans[id]
+			if !ok {
+				continue
+			}
+			key := execTrace.Strings[ev.Args[1]]
+			if key == "datadog.uint64_span_id" {
+				span.spanID = binary.LittleEndian.Uint64([]byte(execTrace.Strings[ev.Args[3]]))
+			}
+		}
+	}
+
+	want := []traceSpan{
+		{name: "root", spanID: root.Context().SpanID()},
+		{name: "child", parent: "root", spanID: child.Context().SpanID()},
+	}
+	var got []traceSpan
+	for _, v := range spans {
+		got = append(got, *v)
+	}
+
+	assert.ElementsMatch(t, want, got)
+}

ddtrace/tracer/tracer.go

@@ -7,6 +7,7 @@ package tracer
 
 import (
 	gocontext "context"
+	"encoding/binary"
 	"os"
 	"runtime/pprof"
 	rt "runtime/trace"
@@ -702,7 +703,12 @@ func startExecutionTracerTask(ctx gocontext.Context, span *span) (gocontext.Cont
 		// skipped.
 		ctx = globalinternal.WithExecutionNotTraced(ctx)
 	}
-	rt.Log(ctx, "span id", strconv.FormatUint(span.SpanID, 10))
+	var b [8]byte
+	binary.LittleEndian.PutUint64(b[:], span.SpanID)
+	// TODO: can we make string(b[:]) not allocate? e.g. with unsafe
+	// shenanigans? rt.Log won't retain the message string, though perhaps
+	// we can't assume that will always be the case.
+	rt.Log(ctx, "datadog.uint64_span_id", string(b[:]))
 	return ctx, end
 }