[libc++] Fix money_get::do_get with huge input (#126273)

`money_get::do_get` needs to be fixed to handle extremely huge input
(e.g. more than 100 digits).
1. `__double_or_nothing` needs to copy the contents of the stack buffer
on the initial allocation.
2. The `sscanf` call in `do_get` needs to scan the dynamic buffer if
dynamic allocation happens.

Author: frederick-vs-ja

libcxx/include/locale

@@ -2386,6 +2386,8 @@ _LIBCPP_HIDE_FROM_ABI void __double_or_nothing(unique_ptr<_Tp, void (*)(void*)>&
     std::__throw_bad_alloc();
   if (__owns)
     __b.release();
+  else
+    std::memcpy(__t, __b.get(), __cur_cap);
   __b = unique_ptr<_Tp, void (*)(void*)>(__t, free);
   __new_cap /= sizeof(_Tp);
   __n = __b.get() + __n_off;
@@ -2581,20 +2583,22 @@ _InputIterator money_get<_CharT, _InputIterator>::do_get(
     char_type __atoms[sizeof(__src) - 1];
     __ct.widen(__src, __src + (sizeof(__src) - 1), __atoms);
     char __nbuf[__bz];
-    char* __nc = __nbuf;
+    char* __nc          = __nbuf;
+    const char* __nc_in = __nc;
     unique_ptr<char, void (*)(void*)> __h(nullptr, free);
     if (__wn - __wb.get() > __bz - 2) {
       __h.reset((char*)malloc(static_cast<size_t>(__wn - __wb.get() + 2)));
       if (__h.get() == nullptr)
         std::__throw_bad_alloc();
-      __nc = __h.get();
+      __nc    = __h.get();
+      __nc_in = __nc;
     }
     if (__neg)
       *__nc++ = '-';
     for (const char_type* __w = __wb.get(); __w < __wn; ++__w, ++__nc)
       *__nc = __src[std::find(__atoms, std::end(__atoms), *__w) - __atoms];
     *__nc = char();
-    if (sscanf(__nbuf, "%Lf", &__v) != 1)
+    if (sscanf(__nc_in, "%Lf", &__v) != 1)
       std::__throw_runtime_error("money_get error");
   }
   if (__b == __e)

libcxx/test/std/localization/locale.categories/category.monetary/locale.money.get/locale.money.get.members/get_long_double_overlong.pass.cpp

@@ -0,0 +1,115 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <locale>
+
+// class money_get<charT, InputIterator>
+
+// iter_type get(iter_type b, iter_type e, bool intl, ios_base& iob,
+//               ios_base::iostate& err, long double& v) const;
+
+// Ensure that money_get::do_get correct works when the input doesn't fit into the stack buffer
+// (100 characters currently).
+
+// XFAIL: FROZEN-CXX03-HEADERS-FIXME
+
+#include <cassert>
+#include <cstddef>
+#include <ios>
+#include <locale>
+#include <streambuf>
+#include <string>
+
+#include "make_string.h"
+#include "test_macros.h"
+#include "test_iterators.h"
+
+template <class CharT>
+class my_basic_facet : public std::money_get<CharT, cpp17_input_iterator<const CharT*> > {
+private:
+  typedef std::money_get<CharT, cpp17_input_iterator<const CharT*> > Base;
+
+public:
+  explicit my_basic_facet(std::size_t refs = 0) : Base(refs) {}
+};
+
+template <class CharT>
+void test() {
+  struct digit_result_case {
+    std::size_t digit;
+    long double result;
+  };
+  const digit_result_case digit_result_cases[] = {
+      {60, 2.0E60L}, {120, 2.0E120L}, {180, 2.0E180L}, {240, 2.0E240L}, {300, 2.0E300L}};
+
+  std::ios ios(0);
+  {
+    const my_basic_facet<CharT> f(1);
+    for (std::size_t i = 0; i != sizeof(digit_result_cases) / sizeof(digit_result_cases[0]); ++i) {
+      {
+        std::basic_string<CharT> v = MAKE_STRING(CharT, "2");
+        v.append(digit_result_cases[i].digit, static_cast<CharT>('0'));
+
+        typedef cpp17_input_iterator<const CharT*> I;
+        long double ex;
+        std::ios_base::iostate err = std::ios_base::goodbit;
+        I iter                     = f.get(I(v.data()), I(v.data() + v.size()), false, ios, err, ex);
+        assert(base(iter) == v.data() + v.size());
+        assert(err == std::ios_base::eofbit);
+        assert(ex == digit_result_cases[i].result);
+      }
+      {
+        std::basic_string<CharT> v = MAKE_STRING(CharT, "-2");
+        v.append(digit_result_cases[i].digit, static_cast<CharT>('0'));
+
+        typedef cpp17_input_iterator<const CharT*> I;
+        long double ex;
+        std::ios_base::iostate err = std::ios_base::goodbit;
+        I iter                     = f.get(I(v.data()), I(v.data() + v.size()), false, ios, err, ex);
+        assert(base(iter) == v.data() + v.size());
+        assert(err == std::ios_base::eofbit);
+        assert(ex == -digit_result_cases[i].result);
+      }
+      {
+        std::basic_string<CharT> v = MAKE_STRING(CharT, "0.");
+        v.append(digit_result_cases[i].digit, static_cast<CharT>('0'));
+        v += MAKE_CSTRING(CharT, "2");
+
+        typedef cpp17_input_iterator<const CharT*> I;
+        long double ex;
+        std::ios_base::iostate err = std::ios_base::goodbit;
+        I iter                     = f.get(I(v.data()), I(v.data() + v.size()), false, ios, err, ex);
+        assert(base(iter) == v.data() + 1);
+        assert(err == std::ios_base::goodbit);
+        assert(ex == 0.0L);
+      }
+      {
+        std::basic_string<CharT> v = MAKE_STRING(CharT, "-0.");
+        v.append(digit_result_cases[i].digit, static_cast<CharT>('0'));
+        v += MAKE_CSTRING(CharT, "2");
+
+        typedef cpp17_input_iterator<const CharT*> I;
+        long double ex;
+        std::ios_base::iostate err = std::ios_base::goodbit;
+        I iter                     = f.get(I(v.data()), I(v.data() + v.size()), false, ios, err, ex);
+        assert(base(iter) == v.data() + 2);
+        assert(err == std::ios_base::goodbit);
+        assert(ex == 0.0L);
+      }
+    }
+  }
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+
+  return 0;
+}