windows: add ShellExecute

This is the way to do things like execute a process elevated with UAC
and interact with that whole system. It turns out to be quite important
for writing Windows software.

Change-Id: I5e05dc9b89ea308d42ac86ba563fd01922fc940c
Reviewed-on: https://go-review.googlesource.com/c/sys/+/178898
Run-TryBot: Jason Donenfeld <Jason@zx2c4.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

windows/syscall_windows.go

@@ -172,6 +172,7 @@ func NewCallbackCDecl(fn interface{}) uintptr {
 //sys	CancelIoEx(s Handle, o *Overlapped) (err error)
 //sys	CreateProcess(appName *uint16, commandLine *uint16, procSecurity *SecurityAttributes, threadSecurity *SecurityAttributes, inheritHandles bool, creationFlags uint32, env *uint16, currentDir *uint16, startupInfo *StartupInfo, outProcInfo *ProcessInformation) (err error) = CreateProcessW
 //sys	OpenProcess(da uint32, inheritHandle bool, pid uint32) (handle Handle, err error)
+//sys	ShellExecute(hwnd Handle, verb *uint16, file *uint16, args *uint16, cwd *uint16, showCmd int32) (err error) = shell32.ShellExecuteW
 //sys	TerminateProcess(handle Handle, exitcode uint32) (err error)
 //sys	GetExitCodeProcess(handle Handle, exitcode *uint32) (err error)
 //sys	GetStartupInfo(startupInfo *StartupInfo) (err error) = GetStartupInfoW

windows/zsyscall_windows.go

@@ -37,8 +37,8 @@ func errnoErr(e syscall.Errno) error {
 var (
 	modadvapi32 = NewLazySystemDLL("advapi32.dll")
 	modkernel32 = NewLazySystemDLL("kernel32.dll")
-	moduserenv  = NewLazySystemDLL("userenv.dll")
 	modshell32  = NewLazySystemDLL("shell32.dll")
+	moduserenv  = NewLazySystemDLL("userenv.dll")
 	modmswsock  = NewLazySystemDLL("mswsock.dll")
 	modcrypt32  = NewLazySystemDLL("crypt32.dll")
 	modws2_32   = NewLazySystemDLL("ws2_32.dll")
@@ -110,6 +110,7 @@ var (
 	procCancelIoEx                         = modkernel32.NewProc("CancelIoEx")
 	procCreateProcessW                     = modkernel32.NewProc("CreateProcessW")
 	procOpenProcess                        = modkernel32.NewProc("OpenProcess")
+	procShellExecuteW                      = modshell32.NewProc("ShellExecuteW")
 	procTerminateProcess                   = modkernel32.NewProc("TerminateProcess")
 	procGetExitCodeProcess                 = modkernel32.NewProc("GetExitCodeProcess")
 	procGetStartupInfoW                    = modkernel32.NewProc("GetStartupInfoW")
@@ -1075,6 +1076,18 @@ func OpenProcess(da uint32, inheritHandle bool, pid uint32) (handle Handle, err
 	return
 }
 
+func ShellExecute(hwnd Handle, verb *uint16, file *uint16, args *uint16, cwd *uint16, showCmd int32) (err error) {
+	r1, _, e1 := syscall.Syscall6(procShellExecuteW.Addr(), 6, uintptr(hwnd), uintptr(unsafe.Pointer(verb)), uintptr(unsafe.Pointer(file)), uintptr(unsafe.Pointer(args)), uintptr(unsafe.Pointer(cwd)), uintptr(showCmd))
+	if r1 == 0 {
+		if e1 != 0 {
+			err = errnoErr(e1)
+		} else {
+			err = syscall.EINVAL
+		}
+	}
+	return
+}
+
 func TerminateProcess(handle Handle, exitcode uint32) (err error) {
 	r1, _, e1 := syscall.Syscall(procTerminateProcess.Addr(), 2, uintptr(handle), uintptr(exitcode), 0)
 	if r1 == 0 {