cmd/internal/objfile: emit better error for Go object of a different version

The Go object file format can change from version to version.
Tools like cmd/objdump and cmd/nm only onderstand the current
version of the object file. Currently, when it encounters an
object built with a different version of the toolchain, it emits
a generic error "unrecognized object file", which is not very
helpful for users. This CL makes it emit a clearer error. Now it
emits

objdump: open go116.o: go object of a different version: go116ld

Change-Id: I063c6078ed1da78f97cea65796779ae093a1a8cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/315609
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/internal/archive/archive.go

@@ -106,6 +106,12 @@ var (
 	errNotObject        = errors.New("unrecognized object file format")
 )
 
+type ErrGoObjOtherVersion struct{ magic []byte }
+
+func (e ErrGoObjOtherVersion) Error() string {
+	return fmt.Sprintf("go object of a different version: %s", e.magic)
+}
+
 // An objReader is an object file reader.
 type objReader struct {
 	a      *Archive
@@ -389,7 +395,7 @@ func (r *objReader) parseArchive(verbose bool) error {
 // The object file consists of a textual header ending in "\n!\n"
 // and then the part we want to parse begins.
 // The format of that part is defined in a comment at the top
-// of src/liblink/objfile.c.
+// of cmd/internal/goobj/objfile.go.
 func (r *objReader) parseObject(o *GoObj, size int64) error {
 	h := make([]byte, 0, 256)
 	var c1, c2, c3 byte
@@ -418,6 +424,9 @@ func (r *objReader) parseObject(o *GoObj, size int64) error {
 		return err
 	}
 	if !bytes.Equal(p, []byte(goobj.Magic)) {
+		if bytes.HasPrefix(p, []byte("\x00go1")) && bytes.HasSuffix(p, []byte("ld")) {
+			return r.error(ErrGoObjOtherVersion{p})
+		}
 		return r.error(errCorruptObject)
 	}
 	r.skip(o.Size)

src/cmd/internal/objfile/objfile.go

@@ -6,6 +6,7 @@
 package objfile
 
 import (
+	"cmd/internal/archive"
 	"debug/dwarf"
 	"debug/gosym"
 	"fmt"
@@ -73,6 +74,8 @@ func Open(name string) (*File, error) {
 	}
 	if f, err := openGoFile(r); err == nil {
 		return f, nil
+	} else if _, ok := err.(archive.ErrGoObjOtherVersion); ok {
+		return nil, fmt.Errorf("open %s: %v", name, err)
 	}
 	for _, try := range openers {
 		if raw, err := try(r); err == nil {

src/cmd/objdump/objdump_test.go

@@ -345,3 +345,18 @@ func TestGoobjFileNumber(t *testing.T) {
 		t.Logf("output:\n%s", text)
 	}
 }
+
+func TestGoObjOtherVersion(t *testing.T) {
+	testenv.MustHaveExec(t)
+	t.Parallel()
+
+	obj := filepath.Join("testdata", "go116.o")
+	cmd := exec.Command(exe, obj)
+	out, err := cmd.CombinedOutput()
+	if err == nil {
+		t.Fatalf("objdump go116.o succeeded unexpectly")
+	}
+	if !strings.Contains(string(out), "go object of a different version") {
+		t.Errorf("unexpected error message:\n%s", out)
+	}
+}