archive/zip: make zip understand os.FileMode.

Fixes implicit dependency on underlying os file modes.

R=rsc, r, n13m3y3r, gustavo, adg
CC=golang-dev
https://golang.org/cl/5440130

src/pkg/archive/zip/struct.go

@@ -12,7 +12,7 @@ This package does not support ZIP64 or disk spanning.
 package zip
 
 import (
-	"errors"
+	"os"
 	"time"
 )
 
@@ -32,7 +32,11 @@ const (
 	dataDescriptorLen        = 12
 
 	// Constants for the first byte in CreatorVersion
-	creatorUnix = 3
+	creatorFAT    = 0
+	creatorUnix   = 3
+	creatorNTFS   = 11
+	creatorVFAT   = 14
+	creatorMacOSX = 19
 )
 
 type FileHeader struct {
@@ -98,17 +102,85 @@ func (h *FileHeader) ModTime() time.Time {
 	return msDosTimeToTime(h.ModifiedDate, h.ModifiedTime)
 }
 
+// traditional names for Unix constants
+const (
+	s_IFMT  = 0xf000
+	s_IFDIR = 0x4000
+	s_IFREG = 0x8000
+	s_ISUID = 0x800
+	s_ISGID = 0x400
+
+	msdosDir      = 0x10
+	msdosReadOnly = 0x01
+)
+
 // Mode returns the permission and mode bits for the FileHeader.
 // An error is returned in case the information is not available.
-func (h *FileHeader) Mode() (mode uint32, err error) {
-	if h.CreatorVersion>>8 == creatorUnix {
-		return h.ExternalAttrs >> 16, nil
+func (h *FileHeader) Mode() (mode os.FileMode, err error) {
+	switch h.CreatorVersion >> 8 {
+	case creatorUnix, creatorMacOSX:
+		mode = unixModeToFileMode(h.ExternalAttrs >> 16)
+	case creatorNTFS, creatorVFAT, creatorFAT:
+		mode = msdosModeToFileMode(h.ExternalAttrs)
 	}
-	return 0, errors.New("file mode not available")
+	if len(h.Name) > 0 && h.Name[len(h.Name)-1] == '/' {
+		mode |= os.ModeDir
+	}
+	return mode, nil
 }
 
 // SetMode changes the permission and mode bits for the FileHeader.
-func (h *FileHeader) SetMode(mode uint32) {
+func (h *FileHeader) SetMode(mode os.FileMode) {
 	h.CreatorVersion = h.CreatorVersion&0xff | creatorUnix<<8
-	h.ExternalAttrs = mode << 16
+	h.ExternalAttrs = fileModeToUnixMode(mode) << 16
+
+	// set MSDOS attributes too, as the original zip does.
+	if mode&os.ModeDir != 0 {
+		h.ExternalAttrs |= msdosDir
+	}
+	if mode&0200 == 0 {
+		h.ExternalAttrs |= msdosReadOnly
+	}
+}
+
+func msdosModeToFileMode(m uint32) (mode os.FileMode) {
+	if m&msdosDir != 0 {
+		mode = os.ModeDir | 0777
+	} else {
+		mode = 0666
+	}
+	if m&msdosReadOnly != 0 {
+		mode &^= 0222
+	}
+	return mode
+}
+
+func fileModeToUnixMode(mode os.FileMode) uint32 {
+	var m uint32
+	if mode&os.ModeDir != 0 {
+		m = s_IFDIR
+	} else {
+		m = s_IFREG
+	}
+	if mode&os.ModeSetuid != 0 {
+		m |= s_ISUID
+	}
+	if mode&os.ModeSetgid != 0 {
+		m |= s_ISGID
+	}
+	return m | uint32(mode&0777)
+}
+
+func unixModeToFileMode(m uint32) os.FileMode {
+	var mode os.FileMode
+	if m&s_IFMT == s_IFDIR {
+		mode |= os.ModeDir
+	}
+	if m&s_ISGID != 0 {
+		mode |= os.ModeSetgid
+	}
+	if m&s_ISUID != 0 {
+		mode |= os.ModeSetuid
+	}
+	return mode | os.FileMode(m&0777)
 }