windows: add token environment functions

This brings the x/sys package into parity with the capabilities provided
indirectly in CL 176619, and adds a helper to make it useful.

Change-Id: I81f2d205bbb0c2b2c530b2bd991c1e6ff30cc94e
Reviewed-on: https://go-review.googlesource.com/c/sys/+/176620
Run-TryBot: Jason Donenfeld <Jason@zx2c4.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

windows/env_windows.go

@@ -6,7 +6,11 @@
 
 package windows
 
-import "syscall"
+import (
+	"syscall"
+	"unicode/utf16"
+	"unsafe"
+)
 
 func Getenv(key string) (value string, found bool) {
 	return syscall.Getenv(key)
@@ -24,6 +28,34 @@ func Environ() []string {
 	return syscall.Environ()
 }
 
+// Returns a default environment associated with the token, rather than the current
+// process. If inheritExisting is true, then this environment also inherits the
+// environment of the current process.
+func (token Token) Environ(inheritExisting bool) (env []string, err error) {
+	var block *uint16
+	err = CreateEnvironmentBlock(&block, token, inheritExisting)
+	if err != nil {
+		return nil, err
+	}
+	defer DestroyEnvironmentBlock(block)
+	blockp := uintptr(unsafe.Pointer(block))
+	for {
+		entry := (*[(1 << 30) - 1]uint16)(unsafe.Pointer(blockp))[:]
+		for i, v := range entry {
+			if v == 0 {
+				entry = entry[:i]
+				break
+			}
+		}
+		if len(entry) == 0 {
+			break
+		}
+		env = append(env, string(utf16.Decode(entry)))
+		blockp += 2 * (uintptr(len(entry)) + 1)
+	}
+	return env, nil
+}
+
 func Unsetenv(key string) error {
 	return syscall.Unsetenv(key)
 }