misc/bstr: add bstr_to_wchar for win32

Convenience to avoid strlen above other things.

misc/bstr.c

@@ -467,3 +467,23 @@ bool bstr_decode_hex(void *talloc_ctx, struct bstr hex, struct bstr *out)
     *out = (struct bstr){ .start = arr, .len = len };
     return true;
 }
+
+#ifdef _WIN32
+
+#include <windows.h>
+
+int bstr_to_wchar(void *talloc_ctx, struct bstr s, wchar_t **ret)
+{
+    int count = MultiByteToWideChar(CP_UTF8, 0, s.start, s.len, NULL, 0);
+    if (count <= 0)
+        abort();
+    wchar_t *wbuf = *ret;
+    if (!wbuf || ta_get_size(wbuf) < (count + 1) * sizeof(wchar_t))
+        wbuf = talloc_realloc(talloc_ctx, wbuf, wchar_t, count + 1);
+    MultiByteToWideChar(CP_UTF8, 0, s.start, s.len, wbuf, count);
+    wbuf[count] = L'\0';
+    *ret = wbuf;
+    return count;
+}
+
+#endif

misc/bstr.h

@@ -223,6 +223,12 @@ static inline bool bstr_eatend0(struct bstr *s, const char *prefix)
     return bstr_eatend(s, bstr0(prefix));
 }
 
+#ifdef _WIN32
+
+int bstr_to_wchar(void *talloc_ctx, struct bstr s, wchar_t **ret);
+
+#endif
+
 // create a pair (not single value!) for "%.*s" printf syntax
 #define BSTR_P(bstr) (int)((bstr).len), ((bstr).start ? (char*)(bstr).start : "")