[libcxx] Error out if __libcpp_mbsrtowcs_l fails to convert anything in __time_get_storage

[mstorsjo]

Summary

If __libcpp_mbsrtowcs_l outputs zero wchar_t's for week days or month names (due to errors in the locale function setup), these are matched all the time in __time_get_storage::__analyze, ending up in an infinite loop, allocating more memory until killed.

[ldionne]

Can we add a test for this alongside the fix?

[mstorsjo]

To test the issue itself, you'd need a platform with broken locale functions, and the issue is triggered by a number of the existing testcases.

The locale functions were broken in such a way on windows, triggering this issue, prior to D69505 - and that patch fixed around 11 of the previously broken tests.

Before the patch that fixed the windows issue, the symptom of the problem was libc++ getting stuck in infinite loops; with this patch it would error out much more cleanly, which would have made it a bit easier to locate the root cause behind D69505.

So this patch doesn't fix any issue, it just changes a bad failure case (infinite loop) into a more manageable one (throwing an error).

libcxx/src/locale.cpp

__time_get_storage<wchar_t>::init(const ctype<wchar_t>& ct)
     // __weeks_
     for (int i = 0; i < 7; ++i)
     {
         t.tm_wday = i;
         strftime_l(buf, countof(buf), "%A", &t, __loc_);
         mb = mbstate_t();
         const char* bb = buf;
         size_t j = __libcpp_mbsrtowcs_l(wbuf, &bb, countof(wbuf), &mb, __loc_);
-        if (j == size_t(-1))
+        if (j == size_t(-1) || j == 0)
             __throw_runtime_error("locale not supported");
         wbe = wbuf + j;
         __weeks_[i].assign(wbuf, wbe);
         strftime_l(buf, countof(buf), "%a", &t, __loc_);
         mb = mbstate_t();
         bb = buf;
         j = __libcpp_mbsrtowcs_l(wbuf, &bb, countof(wbuf), &mb, __loc_);
-        if (j == size_t(-1))
+        if (j == size_t(-1) || j == 0)
             __throw_runtime_error("locale not supported");
         wbe = wbuf + j;
         __weeks_[i+7].assign(wbuf, wbe);
     }
     // __months_
     for (int i = 0; i < 12; ++i)
     {
         t.tm_mon = i;
         strftime_l(buf, countof(buf), "%B", &t, __loc_);
         mb = mbstate_t();
         const char* bb = buf;
         size_t j = __libcpp_mbsrtowcs_l(wbuf, &bb, countof(wbuf), &mb, __loc_);
-        if (j == size_t(-1))
+        if (j == size_t(-1) || j == 0)
             __throw_runtime_error("locale not supported");
         wbe = wbuf + j;
         __months_[i].assign(wbuf, wbe);
         strftime_l(buf, countof(buf), "%b", &t, __loc_);
         mb = mbstate_t();
         bb = buf;
         j = __libcpp_mbsrtowcs_l(wbuf, &bb, countof(wbuf), &mb, __loc_);
-        if (j == size_t(-1))
+        if (j == size_t(-1) || j == 0)
             __throw_runtime_error("locale not supported");
         wbe = wbuf + j;
         __months_[i+12].assign(wbuf, wbe);
     }
     // __am_pm_
     t.tm_hour = 1;
     strftime_l(buf, countof(buf), "%p", &t, __loc_);
     mb = mbstate_t();