cmd/go: add a -debug-runtime-trace flag

The runtime/trace package proved useful for investigating go command
performance, and it makes sense (to me) to make this available for
development behind an undocumented flag, at the cost of ~25KB of binary
size. We could of course futher hide this functionality behind an
experiment or build tag, if necessary.

Updates #59157

Change-Id: I612320920ca935f1ee10bb6a803b7952f36c939b
Reviewed-on: https://go-review.googlesource.com/c/go/+/477896
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>

src/cmd/go/internal/cfg/cfg.go

@@ -97,8 +97,9 @@ var (
 
 	CmdName string // "build", "install", "list", "mod tidy", etc.
 
-	DebugActiongraph string // -debug-actiongraph flag (undocumented, unstable)
-	DebugTrace       string // -debug-trace flag
+	DebugActiongraph  string // -debug-actiongraph flag (undocumented, unstable)
+	DebugTrace        string // -debug-trace flag
+	DebugRuntimeTrace string // -debug-runtime-trace flag (undocumented, unstable)
 
 	// GoPathError is set when GOPATH is not set. it contains an
 	// explanation why GOPATH is unset.

src/cmd/go/internal/work/build.go

@@ -332,6 +332,7 @@ func AddBuildFlags(cmd *base.Command, mask BuildFlagMask) {
 	// Undocumented, unstable debugging flags.
 	cmd.Flag.StringVar(&cfg.DebugActiongraph, "debug-actiongraph", "", "")
 	cmd.Flag.StringVar(&cfg.DebugTrace, "debug-trace", "", "")
+	cmd.Flag.StringVar(&cfg.DebugRuntimeTrace, "debug-runtime-trace", "", "")
 }
 
 // AddCoverFlags adds coverage-related flags to "cmd". If the

src/cmd/go/main.go

@@ -16,6 +16,7 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	rtrace "runtime/trace"
 	"strings"
 
 	"cmd/go/internal/base"
@@ -220,6 +221,20 @@ func invoke(cmd *base.Command, args []string) {
 		cmd.Flag.Parse(args[1:])
 		args = cmd.Flag.Args()
 	}
+
+	if cfg.DebugRuntimeTrace != "" {
+		f, err := os.Create(cfg.DebugRuntimeTrace)
+		if err != nil {
+			base.Fatalf("creating trace file: %v", err)
+		}
+		if err := rtrace.Start(f); err != nil {
+			base.Fatalf("starting event trace: %v", err)
+		}
+		defer func() {
+			rtrace.Stop()
+		}()
+	}
+
 	ctx := maybeStartTrace(context.Background())
 	ctx, span := trace.StartSpan(ctx, fmt.Sprint("Running ", cmd.Name(), " command"))
 	cmd.Run(ctx, cmd, args)