cmd/go: process -debug-trace flag for cmd/test and cmd/vet

These commands are build-like commands that do their own flag
processing, so the value of debug-trace isn't available until
the command starts running. Start tracing in the cmd's run
function.

Updates #38714

Change-Id: I4d633e6ee907bf09feac52c2aff3daceb9b20e12
Reviewed-on: https://go-review.googlesource.com/c/go/+/248324
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/test/test.go

@@ -31,6 +31,7 @@ import (
 	"cmd/go/internal/lockedfile"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/str"
+	"cmd/go/internal/trace"
 	"cmd/go/internal/work"
 	"cmd/internal/test2json"
 )
@@ -571,6 +572,23 @@ func runTest(ctx context.Context, cmd *base.Command, args []string) {
 
 	pkgArgs, testArgs = testFlags(args)
 
+	if cfg.DebugTrace != "" {
+		var close func() error
+		var err error
+		ctx, close, err = trace.Start(ctx, cfg.DebugTrace)
+		if err != nil {
+			base.Fatalf("failed to start trace: %v", err)
+		}
+		defer func() {
+			if err := close(); err != nil {
+				base.Fatalf("failed to stop trace: %v", err)
+			}
+		}()
+	}
+
+	ctx, span := trace.StartSpan(ctx, fmt.Sprint("Running ", cmd.Name(), " command"))
+	defer span.Done()
+
 	work.FindExecCmd() // initialize cached result
 
 	work.BuildInit()

src/cmd/go/internal/vet/vet.go

@@ -6,12 +6,16 @@
 package vet
 
 import (
+	"context"
+	"fmt"
+	"path/filepath"
+
 	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
 	"cmd/go/internal/load"
 	"cmd/go/internal/modload"
+	"cmd/go/internal/trace"
 	"cmd/go/internal/work"
-	"context"
-	"path/filepath"
 )
 
 // Break init loop.
@@ -54,6 +58,23 @@ func runVet(ctx context.Context, cmd *base.Command, args []string) {
 
 	vetFlags, pkgArgs := vetFlags(args)
 
+	if cfg.DebugTrace != "" {
+		var close func() error
+		var err error
+		ctx, close, err = trace.Start(ctx, cfg.DebugTrace)
+		if err != nil {
+			base.Fatalf("failed to start trace: %v", err)
+		}
+		defer func() {
+			if err := close(); err != nil {
+				base.Fatalf("failed to stop trace: %v", err)
+			}
+		}()
+	}
+
+	ctx, span := trace.StartSpan(ctx, fmt.Sprint("Running ", cmd.Name(), " command"))
+	defer span.Done()
+
 	work.BuildInit()
 	work.VetFlags = vetFlags
 	if len(vetFlags) > 0 {