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>

diff --git a/src/internal/syscall/execenv/execenv_windows.go b/src/internal/syscall/execenv/execenv_windows.go
index a8aa1a644eeffa27151ca8839c5e9b6773026b8d..46ba12efc521b12fb05e6a36d96a08ce8eb10714 100644 (file)
--- a/src/internal/syscall/execenv/execenv_windows.go
+++ b/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
 }