cmd/go: simplify ELF note reading and enable during bootstrap

The bootstrap restriction is to avoid needing cgo for package net.
There's no problem with building debug/elf and debug/dwarf,
so do that.

An upcoming CL is going to add more note processing code,
and it simplifies things not to have to think about the code being
missing half the time.

Change-Id: I0e2f120ac23f14db6ecfcec7bfe254a69abcf7b6
Reviewed-on: https://go-review.googlesource.com/10703Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/build.go

@@ -891,6 +891,8 @@ var buildorder = []string{
 	"hash",
 	"crypto",
 	"crypto/sha1",
+	"debug/dwarf",
+	"debug/elf",
 	"cmd/go",
 }
 

src/cmd/go/bootstrap.go

@@ -36,7 +36,3 @@ func httpsOrHTTP(importPath string) (string, io.ReadCloser, error) {
 func parseMetaGoImports(r io.Reader) ([]metaImport, error) {
 	panic("unreachable")
 }
-
-func readnote(a, b string, t int32) ([]byte, error) {
-	return nil, nil
-}

src/cmd/go/build.go

@@ -751,9 +751,9 @@ func goFilesPackage(gofiles []string) *Package {
 }
 
 func readpkglist(shlibpath string) []*Package {
-	pkglistbytes, err := readnote(shlibpath, "GO\x00\x00", 1)
+	pkglistbytes, err := readELFNote(shlibpath, "GO\x00\x00", 1)
 	if err != nil {
-		fatalf("readnote failed: %v", err)
+		fatalf("readELFNote failed: %v", err)
 	}
 	scanner := bufio.NewScanner(bytes.NewBuffer(pkglistbytes))
 	var pkgs []*Package

src/cmd/go/note.go

@@ -2,11 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !cmd_go_bootstrap
-
-// This is not built when bootstrapping to avoid having go_bootstrap depend on
-// debug/elf.
-
 package main
 
 import (
@@ -16,21 +11,8 @@ import (
 	"io"
 )
 
-func rnd(v int32, r int32) int32 {
-	if r <= 0 {
-		return v
-	}
-	v += r - 1
-	c := v % r
-	if c < 0 {
-		c += r
-	}
-	v -= c
-	return v
-}
-
-func readwithpad(r io.Reader, sz int32) ([]byte, error) {
-	full := rnd(sz, 4)
+func readAligned4(r io.Reader, sz int32) ([]byte, error) {
+	full := (sz + 3) &^ 3
 	data := make([]byte, full)
 	_, err := io.ReadFull(r, data)
 	if err != nil {
@@ -40,7 +22,7 @@ func readwithpad(r io.Reader, sz int32) ([]byte, error) {
 	return data, nil
 }
 
-func readnote(filename, name string, typ int32) ([]byte, error) {
+func readELFNote(filename, name string, typ int32) ([]byte, error) {
 	f, err := elf.Open(filename)
 	if err != nil {
 		return nil, err
@@ -67,11 +49,11 @@ func readnote(filename, name string, typ int32) ([]byte, error) {
 			if err != nil {
 				return nil, fmt.Errorf("read type failed: %v", err)
 			}
-			noteName, err := readwithpad(r, namesize)
+			noteName, err := readAligned4(r, namesize)
 			if err != nil {
 				return nil, fmt.Errorf("read name failed: %v", err)
 			}
-			desc, err := readwithpad(r, descsize)
+			desc, err := readAligned4(r, descsize)
 			if err != nil {
 				return nil, fmt.Errorf("read desc failed: %v", err)
 			}