os, syscall: implement Getwd on darwin using getattrlist

Fixes #4807

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/12349044

src/pkg/os/getwd.go

@@ -22,8 +22,10 @@ func Getwd() (pwd string, err error) {
 	// If the operating system provides a Getwd call, use it.
 	if syscall.ImplementsGetwd {
 		s, e := syscall.Getwd()
+		if e != syscall.ENOTSUP {
 			return s, NewSyscallError("getwd", e)
 		}
+	}
 
 	// Otherwise, we're trying to find our way back to ".".
 	dot, err := Stat(".")

src/pkg/syscall/syscall_bsd.go

@@ -17,17 +17,6 @@ import (
 	"unsafe"
 )
 
-/*
- * Pseudo-system calls
- */
-
-// The const provides a compile-time constant so clients
-// can adjust to whether there is a working Getwd and avoid
-// even linking this function into the binary.  See ../os/getwd.go.
-const ImplementsGetwd = false
-
-func Getwd() (string, error) { return "", ENOTSUP }
-
 /*
  * Wrapped
  */

src/pkg/syscall/syscall_darwin.go

@@ -12,7 +12,28 @@
 
 package syscall
 
-import "unsafe"
+import (
+	errorspkg "errors"
+	"unsafe"
+)
+
+const ImplementsGetwd = true
+
+func Getwd() (string, error) {
+	buf := make([]byte, 2048)
+	attrs, err := getAttrList(".", attrList{CommonAttr: attrCmnFullpath}, buf, 0)
+	if err == nil && len(attrs) == 1 && len(attrs[0]) >= 2 {
+		wd := string(attrs[0])
+		// Sanity check that it's an absolute path and ends
+		// in a null byte, which we then strip.
+		if wd[0] == '/' && wd[len(wd)-1] == 0 {
+			return wd[:len(wd)-1], nil
+		}
+	}
+	// If pkg/os/getwd.go gets ENOTSUP, it will fall back to the
+	// slow algorithm.
+	return "", ENOTSUP
+}
 
 type SockaddrDatalink struct {
 	Len    uint8
@@ -86,6 +107,76 @@ func ParseDirent(buf []byte, max int, names []string) (consumed int, count int,
 func PtraceAttach(pid int) (err error) { return ptrace(PT_ATTACH, pid, 0, 0) }
 func PtraceDetach(pid int) (err error) { return ptrace(PT_DETACH, pid, 0, 0) }
 
+const (
+	attrBitMapCount = 5
+	attrCmnFullpath = 0x08000000
+)
+
+type attrList struct {
+	bitmapCount uint16
+	_           uint16
+	CommonAttr  uint32
+	VolAttr     uint32
+	DirAttr     uint32
+	FileAttr    uint32
+	Forkattr    uint32
+}
+
+func getAttrList(path string, attrList attrList, attrBuf []byte, options uint) (attrs [][]byte, err error) {
+	if len(attrBuf) < 4 {
+		return nil, errorspkg.New("attrBuf too small")
+	}
+	attrList.bitmapCount = attrBitMapCount
+
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return nil, err
+	}
+
+	_, _, e1 := Syscall6(
+		SYS_GETATTRLIST,
+		uintptr(unsafe.Pointer(_p0)),
+		uintptr(unsafe.Pointer(&attrList)),
+		uintptr(unsafe.Pointer(&attrBuf[0])),
+		uintptr(len(attrBuf)),
+		uintptr(options),
+		0,
+	)
+	if e1 != 0 {
+		return nil, e1
+	}
+	size := *(*uint32)(unsafe.Pointer(&attrBuf[0]))
+
+	// dat is the section of attrBuf that contains valid data,
+	// without the 4 byte length header. All attribute offsets
+	// are relative to dat.
+	dat := attrBuf
+	if int(size) < len(attrBuf) {
+		dat = dat[:size]
+	}
+	dat = dat[4:] // remove length prefix
+
+	for i := uint32(0); int(i) < len(dat); {
+		header := dat[i:]
+		if len(header) < 8 {
+			return attrs, errorspkg.New("truncated attribute header")
+		}
+		datOff := *(*int32)(unsafe.Pointer(&header[0]))
+		attrLen := *(*uint32)(unsafe.Pointer(&header[4]))
+		if datOff < 0 || uint32(datOff)+attrLen > uint32(len(dat)) {
+			return attrs, errorspkg.New("truncated results; attrBuf too small")
+		}
+		end := uint32(datOff) + attrLen
+		attrs = append(attrs, dat[datOff:end])
+		i = end
+		if r := i % 4; r != 0 {
+			i += (4 - r)
+		}
+	}
+	return
+}
+
 //sysnb pipe() (r int, w int, err error)
 
 func Pipe(p []int) (err error) {

src/pkg/syscall/syscall_no_getwd.go

+// 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.
+
+// +build freebsd netbsd openbsd
+
+package syscall
+
+const ImplementsGetwd = false
+
+func Getwd() (string, error) { return "", ENOTSUP }