cmd/go: add rudimentary tracing support.

Updates #38714

Change-Id: I14da982d405074d65ccf5521d431df1bf1734f9a
Reviewed-on: https://go-review.googlesource.com/c/go/+/230378
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/trace/trace.go

@@ -0,0 +1,139 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+package trace
+
+import (
+	"cmd/internal/traceviewer"
+	"context"
+	"encoding/json"
+	"errors"
+	"os"
+	"strings"
+	"sync/atomic"
+	"time"
+)
+
+var traceStarted int32
+
+func getTraceContext(ctx context.Context) (traceContext, bool) {
+	if atomic.LoadInt32(&traceStarted) == 0 {
+		return traceContext{}, false
+	}
+	v := ctx.Value(traceKey{})
+	if v == nil {
+		return traceContext{}, false
+	}
+	return v.(traceContext), true
+}
+
+// StartSpan starts a trace event with the given name. The Span ends when its Done method is called.
+func StartSpan(ctx context.Context, name string) (context.Context, *Span) {
+	tc, ok := getTraceContext(ctx)
+	if !ok {
+		return ctx, nil
+	}
+	childSpan := &Span{t: tc.t, name: name, start: time.Now()}
+	tc.t.writeEvent(&traceviewer.Event{
+		Name:  childSpan.name,
+		Time:  float64(childSpan.start.UnixNano()) / float64(time.Microsecond),
+		Phase: "B",
+	})
+	ctx = context.WithValue(ctx, traceKey{}, traceContext{tc.t})
+	return ctx, childSpan
+}
+
+type Span struct {
+	t *tracer
+
+	name  string
+	start time.Time
+	end   time.Time
+}
+
+func (s *Span) Done() {
+	if s == nil {
+		return
+	}
+	s.end = time.Now()
+	s.t.writeEvent(&traceviewer.Event{
+		Name:  s.name,
+		Time:  float64(s.end.UnixNano()) / float64(time.Microsecond),
+		Phase: "E",
+	})
+}
+
+type tracer struct {
+	file chan traceFile // 1-buffered
+}
+
+func (t *tracer) writeEvent(ev *traceviewer.Event) error {
+	f := <-t.file
+	defer func() { t.file <- f }()
+	var err error
+	if f.entries == 0 {
+		_, err = f.sb.WriteString("[\n")
+	} else {
+		_, err = f.sb.WriteString(",")
+	}
+	f.entries++
+	if err != nil {
+		return nil
+	}
+
+	if err := f.enc.Encode(ev); err != nil {
+		return err
+	}
+
+	// Write event string to output file.
+	_, err = f.f.WriteString(f.sb.String())
+	f.sb.Reset()
+	return err
+}
+
+func (t *tracer) Close() error {
+	f := <-t.file
+	defer func() { t.file <- f }()
+
+	_, firstErr := f.f.WriteString("]")
+	if err := f.f.Close(); firstErr == nil {
+		firstErr = err
+	}
+	return firstErr
+}
+
+// traceKey is the context key for tracing information. It is unexported to prevent collisions with context keys defined in
+// other packages.
+type traceKey struct{}
+
+type traceContext struct {
+	t *tracer
+}
+
+// Start starts a trace which writes to the given file.
+func Start(ctx context.Context, file string) (context.Context, func() error, error) {
+	atomic.StoreInt32(&traceStarted, 1)
+	if file == "" {
+		return nil, nil, errors.New("no trace file supplied")
+	}
+	f, err := os.Create(file)
+	if err != nil {
+		return nil, nil, err
+	}
+	t := &tracer{file: make(chan traceFile, 1)}
+	sb := new(strings.Builder)
+	t.file <- traceFile{
+		f:   f,
+		sb:  sb,
+		enc: json.NewEncoder(sb),
+	}
+	ctx = context.WithValue(ctx, traceKey{}, traceContext{t: t})
+	return ctx, t.Close, nil
+}
+
+type traceFile struct {
+	f       *os.File
+	sb      *strings.Builder
+	enc     *json.Encoder
+	entries int64
+}