[dev.link] cmd/link: invoke oldlink if old object format is chosen

Now we can choose the old object file format by setting
-gcflags=all=-go115newobj=false -asmflags=all=-go115newobj=false -ldflags=all=-go115newobj=false

Tested that setting all three to default false and it still works.

Change-Id: I9514b62a676916cc383b8afa389489fe7b8fa2bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/224625
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/link/internal/ld/main.go

@@ -39,6 +39,7 @@ import (
 	"flag"
 	"log"
 	"os"
+	"os/exec"
 	"runtime"
 	"runtime/pprof"
 	"strings"
@@ -98,6 +99,8 @@ var (
 
 	benchmarkFlag     = flag.String("benchmark", "", "set to 'mem' or 'cpu' to enable phase benchmarking")
 	benchmarkFileFlag = flag.String("benchmarkprofile", "", "emit phase profiles to `base`_phase.{cpu,mem}prof")
+
+	flagGo115Newobj = flag.Bool("go115newobj", true, "use new object file format")
 )
 
 // Main is the main entry point for the linker code.
@@ -137,6 +140,10 @@ func Main(arch *sys.Arch, theArch Arch) {
 
 	objabi.Flagparse(usage)
 
+	if !*flagGo115Newobj {
+		oldlink()
+	}
+
 	switch *flagHeadType {
 	case "":
 	case "windowsgui":
@@ -401,3 +408,48 @@ func startProfile() {
 		})
 	}
 }
+
+// Invoke the old linker and exit.
+func oldlink() {
+	linker := os.Args[0]
+	if strings.HasSuffix(linker, "link") {
+		linker = linker[:len(linker)-4] + "oldlink"
+	} else if strings.HasSuffix(linker, "link.exe") {
+		linker = linker[:len(linker)-8] + "oldlink.exe"
+	} else {
+		log.Fatal("cannot find oldlink. arg0=", linker)
+	}
+
+	// Copy args, filter out -go115newobj flag
+	args := make([]string, 0, len(os.Args)-1)
+	skipNext := false
+	for i, a := range os.Args {
+		if i == 0 {
+			continue // skip arg0
+		}
+		if skipNext {
+			skipNext = false
+			continue
+		}
+		if a == "-go115newobj" {
+			skipNext = true
+			continue
+		}
+		if strings.HasPrefix(a, "-go115newobj=") {
+			continue
+		}
+		args = append(args, a)
+	}
+
+	cmd := exec.Command(linker, args...)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	err := cmd.Run()
+	if err == nil {
+		os.Exit(0)
+	}
+	if _, ok := err.(*exec.ExitError); ok {
+		os.Exit(2) // would be nice to use ExitError.ExitCode(), but that is too new
+	}
+	log.Fatal("invoke oldlink failed:", err)
+}

src/cmd/link/link_test.go

@@ -447,3 +447,27 @@ func TestStrictDup(t *testing.T) {
 		t.Errorf("unexpected output:\n%s", out)
 	}
 }
+
+func TestOldLink(t *testing.T) {
+	// Test that old object file format still works.
+	// TODO(go115newobj): delete.
+
+	testenv.MustHaveGoBuild(t)
+
+	tmpdir, err := ioutil.TempDir("", "TestOldLink")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpdir)
+
+	src := filepath.Join(tmpdir, "main.go")
+	err = ioutil.WriteFile(src, []byte("package main; func main(){}\n"), 0666)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cmd := exec.Command(testenv.GoToolPath(t), "run", "-gcflags=all=-go115newobj=false", "-asmflags=all=-go115newobj=false", "-ldflags=-go115newobj=false", src)
+	if out, err := cmd.CombinedOutput(); err != nil {
+		t.Errorf("%v: %v:\n%s", cmd.Args, err, out)
+	}
+}