cmd/link: fix export data truncation bug

Similar fix as in CL 60773 for fixing cmd/pack.

Fixes #21703.

Change-Id: I457ed8a3be828fd458abc5c8c1cc766a9f7aab13
Reviewed-on: https://go-review.googlesource.com/79135
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/link/link_test.go

@@ -1,6 +1,13 @@
 package main
 
-import "testing"
+import (
+	"internal/testenv"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"testing"
+)
 
 var AuthorPaidByTheColumnInch struct {
 	fog int `
@@ -28,3 +35,38 @@ func TestLargeSymName(t *testing.T) {
 	// the bufio buffer. Issue #15104.
 	_ = AuthorPaidByTheColumnInch
 }
+
+func TestIssue21703(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	const source = `
+package main
+const X = "\n!\n"
+func main() {}
+`
+
+	tmpdir, err := ioutil.TempDir("", "issue21703")
+	if err != nil {
+		t.Fatalf("failed to create temp dir: %v\n", err)
+	}
+	defer os.RemoveAll(tmpdir)
+
+	err = ioutil.WriteFile(filepath.Join(tmpdir, "main.go"), []byte(source), 0666)
+	if err != nil {
+		t.Fatalf("failed to write main.go: %v\n", err)
+	}
+
+	cmd := exec.Command(testenv.GoToolPath(t), "tool", "compile", "main.go")
+	cmd.Dir = tmpdir
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("failed to compile main.go: %v, output: %s\n", err, out)
+	}
+
+	cmd = exec.Command(testenv.GoToolPath(t), "tool", "link", "main.o")
+	cmd.Dir = tmpdir
+	out, err = cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("failed to link main.o: %v, output: %s\n", err, out)
+	}
+}