cmd/addr2line, cmd/nm: factor object reading into cmd/internal/objfile

To do in another CL: make cmd/objdump use cmd/internal/objfile too.

There is a package placement decision in this CL:
cmd/internal/objfile instead of internal/objfile.
I chose to put internal under cmd to make clear (and enforce)
that no standard library packages should use this
(it's a bit dependency-heavy).

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/123910043

src/cmd/internal/objfile/macho.go

@@ -0,0 +1,92 @@
+// Copyright 2013 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Parsing of Mach-O executables (OS X).
+
+package objfile
+
+import (
+	"debug/macho"
+	"fmt"
+	"os"
+	"sort"
+)
+
+type machoFile struct {
+	macho *macho.File
+}
+
+func openMacho(r *os.File) (rawFile, error) {
+	f, err := macho.NewFile(r)
+	if err != nil {
+		return nil, err
+	}
+	return &machoFile{f}, nil
+}
+
+func (f *machoFile) symbols() ([]Sym, error) {
+	if f.macho.Symtab == nil {
+		return nil, fmt.Errorf("missing symbol table")
+	}
+
+	// Build sorted list of addresses of all symbols.
+	// We infer the size of a symbol by looking at where the next symbol begins.
+	var addrs []uint64
+	for _, s := range f.macho.Symtab.Syms {
+		addrs = append(addrs, s.Value)
+	}
+	sort.Sort(uint64s(addrs))
+
+	var syms []Sym
+	for _, s := range f.macho.Symtab.Syms {
+		sym := Sym{Name: s.Name, Addr: s.Value, Code: '?'}
+		i := sort.Search(len(addrs), func(x int) bool { return addrs[x] > s.Value })
+		if i < len(addrs) {
+			sym.Size = int64(addrs[i] - s.Value)
+		}
+		if s.Sect == 0 {
+			sym.Code = 'U'
+		} else if int(s.Sect) <= len(f.macho.Sections) {
+			sect := f.macho.Sections[s.Sect-1]
+			switch sect.Seg {
+			case "__TEXT":
+				sym.Code = 'R'
+			case "__DATA":
+				sym.Code = 'D'
+			}
+			switch sect.Seg + " " + sect.Name {
+			case "__TEXT __text":
+				sym.Code = 'T'
+			case "__DATA __bss", "__DATA __noptrbss":
+				sym.Code = 'B'
+			}
+		}
+		syms = append(syms, sym)
+	}
+
+	return syms, nil
+}
+
+func (f *machoFile) pcln() (textStart uint64, symtab, pclntab []byte, err error) {
+	if sect := f.macho.Section("__text"); sect != nil {
+		textStart = sect.Addr
+	}
+	if sect := f.macho.Section("__gosymtab"); sect != nil {
+		if symtab, err = sect.Data(); err != nil {
+			return 0, nil, nil, err
+		}
+	}
+	if sect := f.macho.Section("__gopclntab"); sect != nil {
+		if pclntab, err = sect.Data(); err != nil {
+			return 0, nil, nil, err
+		}
+	}
+	return textStart, symtab, pclntab, nil
+}
+
+type uint64s []uint64
+
+func (x uint64s) Len() int           { return len(x) }
+func (x uint64s) Swap(i, j int)      { x[i], x[j] = x[j], x[i] }
+func (x uint64s) Less(i, j int) bool { return x[i] < x[j] }