cmd/internal/goobj: add test case for object file reader

Add test in which a input Go object file contains a very large number
of relocations (more than 1<<20).

Updates #41621.

Change-Id: If1ebf3c4fefbf55ddec4e05c5299e7c48fc697d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/278493
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: Than McIntosh <thanm@google.com>

src/cmd/internal/goobj/objfile_test.go

@@ -9,6 +9,11 @@ import (
 	"bytes"
 	"cmd/internal/bio"
 	"cmd/internal/objabi"
+	"fmt"
+	"internal/testenv"
+	"io/ioutil"
+	"os"
+	"os/exec"
 	"testing"
 )
 
@@ -69,3 +74,60 @@ func TestReadWrite(t *testing.T) {
 		t.Errorf("read Aux2 mismatch: got %v %v", a2.Type(), a2.Sym())
 	}
 }
+
+var issue41621prolog = `
+package main
+var lines = []string{
+`
+
+var issue41621epilog = `
+}
+func getLines() []string {
+	return lines
+}
+func main() {
+	println(getLines())
+}
+`
+
+func TestIssue41621LargeNumberOfRelocations(t *testing.T) {
+	if testing.Short() || (objabi.GOARCH != "amd64") {
+		t.Skipf("Skipping large number of relocations test in short mode or on %s", objabi.GOARCH)
+	}
+	testenv.MustHaveGoBuild(t)
+
+	tmpdir, err := ioutil.TempDir("", "lotsofrelocs")
+	if err != nil {
+		t.Fatalf("can't create temp directory: %v\n", err)
+	}
+	defer os.RemoveAll(tmpdir)
+
+	// Emit testcase.
+	var w bytes.Buffer
+	fmt.Fprintf(&w, issue41621prolog)
+	for i := 0; i < 1048576+13; i++ {
+		fmt.Fprintf(&w, "\t\"%d\",\n", i)
+	}
+	fmt.Fprintf(&w, issue41621epilog)
+	err = ioutil.WriteFile(tmpdir+"/large.go", w.Bytes(), 0666)
+	if err != nil {
+		t.Fatalf("can't write output: %v\n", err)
+	}
+
+	// Emit go.mod
+	w.Reset()
+	fmt.Fprintf(&w, "module issue41621\n\ngo 1.12\n")
+	err = ioutil.WriteFile(tmpdir+"/go.mod", w.Bytes(), 0666)
+	if err != nil {
+		t.Fatalf("can't write output: %v\n", err)
+	}
+	w.Reset()
+
+	// Build.
+	cmd := exec.Command(testenv.GoToolPath(t), "build", "-o", "large")
+	cmd.Dir = tmpdir
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Build failed: %v, output: %s", err, out)
+	}
+}