hook/iohook.c: Add iohook_open_nul_fd()

This provides a replacement for iohook_open_dummy_fd() whose error
handling is more consistent with the rest of this library.

hook/iohook.c

@@ -277,6 +277,7 @@ static void iohook_init(void)
     iohook_initted = true;
 }
 
+// Deprecated
 HANDLE iohook_open_dummy_fd(void)
 {
     iohook_init();
@@ -291,6 +292,33 @@ HANDLE iohook_open_dummy_fd(void)
             NULL);
 }
 
+HRESULT iohook_open_nul_fd(HANDLE *out)
+{
+    HANDLE fd;
+
+    assert(out != NULL);
+
+    *out = NULL;
+    iohook_init();
+
+    fd = next_CreateFileW(
+            L"NUL",
+            GENERIC_READ | GENERIC_WRITE,
+            FILE_SHARE_READ | FILE_SHARE_WRITE,
+            NULL,
+            OPEN_EXISTING,
+            FILE_FLAG_OVERLAPPED,
+            NULL);
+
+    if (fd == NULL) {
+        return HRESULT_FROM_WIN32(GetLastError());
+    }
+
+    *out = fd;
+
+    return S_OK;
+}
+
 HRESULT iohook_push_handler(iohook_fn_t fn)
 {
     iohook_fn_t *new_array;

hook/iohook.h

@@ -39,6 +39,12 @@ struct irp {
 
 typedef HRESULT (*iohook_fn_t)(struct irp *irp);
 
-HANDLE iohook_open_dummy_fd(void);
+HANDLE iohook_open_dummy_fd(void)
+#ifdef __GNUC__
+__attribute__((deprecated("Use iohook_open_nul_fd instead")))
+#endif
+;
+
+HRESULT iohook_open_nul_fd(HANDLE *fd);
 HRESULT iohook_push_handler(iohook_fn_t fn);
 HRESULT iohook_invoke_next(struct irp *irp);