internal/syscall/execenv: refactor handling env variables

Discover while working on CL 471335.

Change-Id: I006077a5aa93cafb7be47813ab0c4714bb00d774
Reviewed-on: https://go-review.googlesource.com/c/go/+/471435
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/internal/syscall/execenv/execenv_windows.go

@@ -25,30 +25,24 @@ func Default(sys *syscall.SysProcAttr) (env []string, err error) {
 	if sys == nil || sys.Token == 0 {
 		return syscall.Environ(), nil
 	}
-	var block *uint16
-	err = windows.CreateEnvironmentBlock(&block, sys.Token, false)
+	var blockp *uint16
+	err = windows.CreateEnvironmentBlock(&blockp, sys.Token, false)
 	if err != nil {
 		return nil, err
 	}
-	defer windows.DestroyEnvironmentBlock(block)
-	blockp := uintptr(unsafe.Pointer(block))
-	for {
+	defer windows.DestroyEnvironmentBlock(blockp)
 
+	const size = unsafe.Sizeof(*blockp)
+	for *blockp != 0 { // environment block ends with empty string
 		// find NUL terminator
-		end := unsafe.Pointer(blockp)
+		end := unsafe.Add(unsafe.Pointer(blockp), size)
 		for *(*uint16)(end) != 0 {
-			end = unsafe.Pointer(uintptr(end) + 2)
+			end = unsafe.Add(end, size)
 		}
 
-		n := (uintptr(end) - uintptr(unsafe.Pointer(blockp))) / 2
-		if n == 0 {
-			// environment block ends with empty string
-			break
-		}
-
-		entry := (*[(1 << 30) - 1]uint16)(unsafe.Pointer(blockp))[:n:n]
+		entry := unsafe.Slice(blockp, (uintptr(end)-uintptr(unsafe.Pointer(blockp)))/2)
 		env = append(env, string(utf16.Decode(entry)))
-		blockp += 2 * (uintptr(len(entry)) + 1)
+		blockp = (*uint16)(unsafe.Add(end, size))
 	}
 	return
 }