Report an empty array for UBJSON [$N#<n>] instead of n elements (#6079)

* Report an empty array for UBJSON `[$N#<n>` instead of n elements

A no-op (N) is not a value, so an optimized array of type N holds no
element whatever count it declares. The reader skipped the elements but
still passed the declared count to the SAX parser: a handler was told
start_array(n) and no element followed, and the DOM parser reserved
min(n, 16384) values (256 KB) for an array that stays empty. Nine bytes
(`[$N#l 0x7FFFFFFF`) therefore committed 256 KB: 2000 of them in one
input, 18 KB, committed 512 MB of heap (capacity 163,840,000 values for
the 10000 arrays of the original probe: 2.6 GB for 70 KB of input).
Untouched pages keep the resident size small, but the commit charge and
a 32-bit address space are consumed.

The reader now announces 0 elements for an optimized array of type N.
The loop that skips the elements is unchanged (it never ran). A count
that does not fit into std::size_t is still rejected by the count
reader; a count that is too large for the array type of the DOM (before:
out_of_range.408 "excessive array size" from the DOM handler) is
accepted now, because the array is empty.

Behaviour change: sax_parse reports start_array(0) for `[$N#n` (before:
n); from_ubjson no longer throws out_of_range.408 for an N-typed array
whose count exceeds the maximum size of the array type.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Clarify the reservation note for announced array counts

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/basic_json.md

@@ -67,11 +67,7 @@ basic_json(basic_json&& other) noexcept;
     - **objects**: [`object_t`](object_t.md) and all kinds of compatible associative containers such as `std::map`,
      `std::unordered_map`, `std::multimap`, and `std::unordered_multimap` with a `key_type` compatible to `string_t`
      and a `value_type` from which a `basic_json` value can be constructed.
-    - **strings**: `string_t`, string literals, and all compatible string containers can be used. Since version 3.13.0,
+    - **strings**: `string_t`, string literals, and all compatible string containers can be used.
-      this includes `#!cpp std::u8string` (C++20): its code units are stored as a JSON string and
-      [`get<std::u8string>()`](get.md) reads it back (in version 3.12.0, a `#!cpp std::u8string` was stored as an array
-      of numbers; `get<std::u8string>()` still accepts such an array). `#!cpp std::u8string_view` and `#!cpp char8_t`
-      arrays are not strings; they are stored as arrays of code units.
 
     !!! note "Rvalue containers are moved from"
 

docs/mkdocs/docs/api/basic_json/from_ubjson.md

@@ -89,8 +89,10 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va
 - Throws [out_of_range.406](../../home/exceptions.md#jsonexceptionout_of_range406) if a high-precision number, or a
   float64 when `number_float_t` is `float`, does not fit into `number_float_t`
 - Throws [out_of_range.408](../../home/exceptions.md#jsonexceptionout_of_range408) if the count of an optimized
-  container exceeds the capacity of the container (or of `std::size_t`), or if an optimized array of `Z`, `T`, or `F` has
+  container exceeds the capacity of the container (or of `std::size_t`; an array of type `N` is empty, so only the
-  more than 1,048,576 elements
+  latter applies to it), if an optimized array of `Z`, `T`, or `F` has more than 1,048,576 elements, or if the
+  optimized arrays of `Z`, `T`, and `F` in the input together have more than 1,048,576 elements plus 4 for every byte
+  read so far
 
 ## Complexity
 
@@ -132,6 +134,9 @@ Linear in the size of the input.
 - Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
 - Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.
 - Added `error_handler` parameter in version 3.13.0.
+- Report `start_array(0)` to a SAX handler for an optimized array of type `N` (`[$N#n`), whatever its count, in version
+  3.13.0.
+- Limited the total number of elements of optimized arrays of `Z`, `T`, and `F` in one input in version 3.13.0.
 
 !!! warning "Deprecation"
 

docs/mkdocs/docs/api/basic_json/get.md

@@ -130,12 +130,6 @@ overload (3).
     `#!cpp json(true).get<double>()` throw. This is the behavior of all 3.x versions;
     [`basic_json_view::get<T>()`](../basic_json_view/get.md) behaves the same.
 
-!!! note "`std::u8string` conversions"
-
-    Since version 3.13.0, `#!cpp get<std::u8string>()` (C++20) reads a JSON string; it also accepts an array of numbers,
-    the form in which version 3.12.0 stored a `#!cpp std::u8string`. Any other JSON type throws
-    [`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) ("type must be string or array").
-
 !!! note "`std::optional` conversions"
 
     Prior to version 3.13.0, `#!cpp get<std::optional<T>>()` (and other conversions to `std::optional<T>`) failed to

docs/mkdocs/docs/api/basic_json/to_ubjson.md

@@ -117,5 +117,8 @@ Linear in the size of the JSON value `j`.
 - Added `error_handler` parameter in version 3.13.0. Its default, `keep`, writes the bytes of a string or object key
   that is not valid UTF-8 unchanged, as before; `strict` (the default if
   [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled) throws `type_error.316`.
+- Writes arrays of `Z`, `T`, or `F` without the `$` type annotation (`[#n Z Z ...`) once their elements in one call
+  add up to more than 1,048,576, in version 3.13.0, so that `from_ubjson` can read the output back. Such output is
+  larger: 270,000 rows of 100 `null` values take 26.8 MB instead of 1.35 MB.
 - Throws `type_error.321` for a discarded value since version 3.13.0; previously, a discarded value nested in an
   array or object was silently skipped, producing invalid UBJSON.

docs/mkdocs/docs/features/binary_formats/ubjson.md

@@ -122,9 +122,18 @@ The library uses the following mapping from JSON values types to UBJSON types ac
     already is the value. Its declared count is therefore the only thing that decides how much memory the receiving side
     allocates, and a handful of bytes can describe billions of elements. `from_ubjson` rejects such an array with
     [`out_of_range.408`](../../home/exceptions.md#jsonexceptionout_of_range408) when the count exceeds 1,048,576
-    (`1 << 20`), and `to_ubjson` writes longer arrays of these types in the counted but untyped form: the `$` type marker is
+    (`1 << 20`), or when the counts of all such arrays in the input add up to more than 1,048,576 plus 4 for every byte
-    dropped, the `#` count is kept, and every element is written with its own marker (`[#l 1048577 Z Z ...`), so any value
+    read so far. `to_ubjson` therefore writes an array of these types in the counted but untyped form when it is longer
-    it produces can be read back. Objects and untyped counted arrays are not subject to this limit.
+    than that, or when it would bring the total of the typed arrays written so far above 1,048,576: the `$` type marker
+    is dropped, the `#` count is kept, and every element is written with its own marker (`[#l 1048577 Z Z ...`), so
+    everything it produces can be read back. Only input from another encoder, or written by version 3.12.0 and earlier of
+    this library, can exceed the limit: for example, 1,100 rows of 1,000 `null` values or 210,000 rows of 25, each with
+    a `$Z` annotation. Version 3.12.0 writes such a row in 5 bytes (6 above 255 elements), so rows of 20 or fewer such
+    values are never rejected. Objects and untyped counted arrays are not subject to this limit.
+
+    An optimized array of type `N` (no-op) is empty whatever its count is, because a no-op is not a value. Its count is
+    not passed on: `sax_parse` reports `start_array(0)` (versions before 3.13.0 reported the declared count), and the DOM
+    parser reserves nothing for it.
 
 !!! info "Binary values"
 
@@ -180,10 +189,11 @@ The library maps UBJSON types to JSON value types as follows:
 
     Every well-formed UBJSON value can be converted to a JSON value, with these exceptions: a high-precision number
     outside the range of `number_float_t` ([`out_of_range.406`](../../home/exceptions.md#jsonexceptionout_of_range406)),
-    an optimized array of `Z`, `T`, or `F` with more than 1,048,576 elements, and a count that exceeds the capacity of the
+    optimized arrays of `Z`, `T`, or `F` with more elements than the limit allows (1,048,576 plus 4 per byte read),
-    container ([`out_of_range.408`](../../home/exceptions.md#jsonexceptionout_of_range408)). High-precision numbers that
+    and a count that exceeds the capacity of the container
-    do not fit an integer type are stored as floating-point numbers and may lose precision. Ill-formed input is rejected
+    ([`out_of_range.408`](../../home/exceptions.md#jsonexceptionout_of_range408)). High-precision numbers that do not
-    with a parse error.
+    fit an integer type are stored as floating-point numbers and may lose precision. Ill-formed input is rejected with a
+    parse error.
 
 !!! info "Duplicate object keys"
 

docs/mkdocs/docs/features/parsing/untrusted_input.md

@@ -72,10 +72,22 @@ For the binary formats, an announced size is never trusted outright:
   (reported as a parse error) instead of triggering an oversized allocation.
 - When an array announces its number of elements and the array container supports `reserve()` (as `#!cpp std::vector`,
   the default, does), the library reserves storage for at most 16384 of them upfront, regardless of how large the
-  announced count is; further elements still grow the container normally as they are read.
+  announced count is; further elements still grow the container normally as they are read. A truncated input whose
+  count is larger than its remaining bytes therefore reserves at most that much before the parse error is reported.
+  Every array element takes at least one byte of input, except in the [UBJSON](../binary_formats/ubjson.md) arrays of
+  type `Z`, `T` or `F` described below.
 - An announced array or object size that exceeds what the target container could ever hold (its `max_size()`) is
   rejected immediately as [`out_of_range.408`](../../home/exceptions.md#jsonexceptionout_of_range408), without
   attempting to allocate anything.
+- A UBJSON array of the no-op type (`[$N#<n>`) is empty whatever count it declares: a SAX handler is told
+  `start_array(0)` and the DOM parser reserves nothing.
+- A UBJSON array of type `Z`, `T` or `F` has no payload, so its announced count is not bounded by the input. One array
+  may announce at most 1,048,576 elements, and all such arrays of one input together may contain at most 1,048,576
+  elements plus 4 for every byte read so far (checked when each array's header has been read); beyond that the input is
+  rejected as [`out_of_range.408`](../../home/exceptions.md#jsonexceptionout_of_range408). For a DOM that bounds the memory these
+  arrays can need to about 80 bytes per byte of input plus about 33 MB for the first 1,048,576 elements; a SAX handler
+  receives at most that many events. Every other optimized array element, and every UBJSON object member (it has a
+  key), takes at least one byte of input.
 
 ## Strings
 

docs/mkdocs/docs/features/types/template_parameters.md

@@ -425,7 +425,7 @@ using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;
 | Type                                                                 | Reason                                                                                                  |
 |----------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------|
 | `#!cpp std::wstring`, `#!cpp std::u16string`, `#!cpp std::u32string` | the character type is not one byte wide                                                                 |
-| `#!cpp std::u8string`                                                | one byte wide, but `#!cpp char8_t` is not `#!cpp char`-compatible (as a *value*, it converts to and from a JSON string) |
+| `#!cpp std::u8string`                                                | one byte wide, but `#!cpp char8_t` is not `#!cpp char`-compatible                                       |
 | `absl::Cord`                                                         | no `value_type`, and the storage is not contiguous                                                      |
 | `QString`                                                            | no `append(const char*, size_type)`; its `QChar` is also two bytes wide, though that is never diagnosed |
 

docs/mkdocs/docs/home/exceptions.md

@@ -1050,11 +1050,17 @@ The size of an array or object in a [binary format](../features/binary_formats/i
 the size following `#` for [UBJSON](../features/binary_formats/ubjson.md)/[BJData](../features/binary_formats/bjdata.md),
 or the encoded length for [CBOR](../features/binary_formats/cbor.md).
 
-The exception is also thrown for a [UBJSON](../features/binary_formats/ubjson.md) array of a type that is encoded by its
+The exception is also thrown for [UBJSON](../features/binary_formats/ubjson.md) arrays of a type that is encoded by
-marker alone (`Z`, `T` or `F`) whose declared count exceeds 1,048,576 (`1 << 20`). Such an array has no payload, so its
+its marker alone (`Z`, `T` or `F`). Such an array has no payload, so its count alone decides how much memory is
-count alone decides how much memory is allocated, and a handful of bytes would otherwise describe billions of values.
+allocated, and a handful of bytes would otherwise describe billions of values. It is thrown if the declared count of one
-[`to_ubjson`](../api/basic_json/to_ubjson.md) writes longer arrays of these types without the `$` type annotation (the `#`
+array exceeds 1,048,576 (`1 << 20`), and if the counts of all such arrays read so far in one input add up to more than
-count and one marker per element are kept), so any value it produces can still be read back.
+1,048,576 plus 4 for every byte of the input read when the header of the last array has been read.
+[`to_ubjson`](../api/basic_json/to_ubjson.md) writes an array of these types without the `$` type annotation (the `#`
+count and one marker per element are kept) when it is longer, or when it would bring the total of the typed arrays
+written so far above 1,048,576, so everything it produces can be read back. Only input written by another encoder, or
+by version 3.12.0 and earlier of this library, can exceed the limit: for example, 1,100 rows of 1,000 `null` values or
+210,000 rows of 25, each with a `$Z` annotation. Version 3.12.0 writes such a row in 5 bytes (6 above 255 elements),
+so rows of 20 or fewer such values are never rejected.
 
 !!! failure "Example messages"
 
@@ -1070,6 +1076,9 @@ count and one marker per element are kept), so any value it produces can still b
     ```
     [json.exception.out_of_range.408] syntax error while parsing UBJSON size: excessive array size
     ```
+    ```
+    [json.exception.out_of_range.408] syntax error while parsing UBJSON size: excessive number of elements without payload
+    ```
 
 ### json.exception.out_of_range.409
 

docs/mkdocs/docs/home/releases/unreleased.md

@@ -8,16 +8,6 @@ These changes are part of the `develop` branch, but not of a release yet.
   exception is thrown during the conversion, the source may be partially moved-from, so the strong exception guarantee
   no longer holds for the source of an rvalue conversion (the JSON value itself is unaffected). Lvalues and `const`
   objects are still copied. See [`basic_json`](../../api/basic_json/basic_json.md).
-- In C++20 (library support for `char8_t`, `__cpp_lib_char8_t`), a `#!cpp std::u8string` is a JSON **string**:
-  `json j = std::u8string(u8"abc");` stores `"abc"`, and `#!cpp j.get<std::u8string>()` reads it back. Version 3.12.0
-  stored a `#!cpp std::u8string` as an array of code units (`[97,98,99]`); on the 3.13.0 development branch it was
-  already stored as a string if `std::filesystem` is available, but could not be read back. Now the result no longer
-  depends on whether `std::filesystem::path` is supported, and `#!cpp get<std::u8string>()` accepts both a JSON string
-  and the array of numbers written by 3.12.0. **Consequence:** JSON documents produced from a `#!cpp std::u8string` by
-  3.12.0 (arrays) are still read, but code that checks `is_array()` on such a value, or that reads it as
-  `#!cpp std::vector<char8_t>` / `#!cpp std::vector<std::uint8_t>`, now sees a string; to keep the array form, convert
-  explicitly, e.g., `#!cpp json(std::vector<char8_t>(s.begin(), s.end()))`. `#!cpp std::u8string_view` and
-  `#!cpp char8_t` arrays are unchanged (arrays of code units). See [`basic_json`](../../api/basic_json/basic_json.md).
 
 ## API changes
 

include/nlohmann/detail/conversions/from_json.hpp

@@ -785,37 +785,6 @@ void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Hash, KeyE
     from_json_pair_array_to_map(j, m);
 }
 
-#if defined(__cpp_lib_char8_t)
-// a std::u8string is read from a JSON string; the array of code units written by
-// versions before 3.13.0 is still accepted
-template<typename BasicJsonType, typename Tr, typename Allocator,
-         enable_if_t<std::is_same<typename json_type_of_t<BasicJsonType>::string_t::value_type, char>::value, int> = 0>
-inline void from_json(const BasicJsonType& j, std::basic_string<char8_t, Tr, Allocator>& s)
-{
-    if (j.is_string())
-    {
-        const auto& str = string_value(j);
-        const char* const data = str.data();
-        const std::size_t size = str.size();
-        std::basic_string<char8_t, Tr, Allocator> ret(s.get_allocator());
-        ret.resize(size);
-        for (std::size_t i = 0; i < size; ++i)
-        {
-            ret[i] = static_cast<char8_t>(static_cast<unsigned char>(data[i]));
-        }
-        s = std::move(ret);
-    }
-    else if (j.is_array())
-    {
-        from_json_array_impl(j, s, priority_tag<3> {});
-    }
-    else
-    {
-        throw_type_must_be("string or array", j);
-    }
-}
-#endif
-
 #if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
 
 // Workaround for MSVC 19.51 (and possibly later): in large cpp files, the compiler may fail to resolve with generic has_from_json (issue #4996)

include/nlohmann/detail/conversions/to_json.hpp

@@ -625,20 +625,16 @@ inline void to_json(BasicJsonType& j, const T& t)
     to_json_tuple_impl(j, t, make_index_sequence<std::tuple_size<T>::value> {});
 }
 
-// a std::u8string is a JSON string (it was an array of code units before 3.13.0);
+#if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
-// defined outside the std::filesystem guard so that the result does not depend
-// on whether std::filesystem::path is supported
 #if defined(__cpp_lib_char8_t)
 template<typename BasicJsonType, typename Tr, typename Allocator>
 inline void to_json(BasicJsonType& j, const std::basic_string<char8_t, Tr, Allocator>& s)
 {
-    using string_t = typename BasicJsonType::string_t;
+    using OtherAllocator = typename std::allocator_traits<Allocator>::template rebind_alloc<char>;
-    // char may alias any object; the code units are copied unchanged
+    j = std::basic_string<char, std::char_traits<char>, OtherAllocator>(s.begin(), s.end(), s.get_allocator());
-    j = string_t(reinterpret_cast<const char*>(s.data()), s.size());
 }
 #endif
 
-#if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
 // Workaround for MSVC 19.51 (and possibly later): in large cpp files, the compiler may fail to resolve with generic has_to_json (issue #4996)
 template<typename BasicJsonType>
 struct has_to_json<BasicJsonType, std_fs::path, void> : std::true_type {};

include/nlohmann/detail/input/binary_reader.hpp

@@ -73,13 +73,46 @@ the input.
 
 This is a sanity bound rather than a security boundary, and it is far above
 any container met in practice. @ref binary_writer falls back to the
-unoptimized encoding for longer containers, so that a value serialized by
+unoptimized encoding for longer containers, and for containers that together
-this library can always be read back.
+with the ones before them would exceed @ref valueless_elements_base, so that a
+value serialized by this library can always be read back.
 
 @sa https://github.com/nlohmann/json/issues/2793
 */
 JSON_INLINE_VARIABLE constexpr std::size_t max_valueless_container_size = 1 << 20;
 
+/*!
+@brief number of elements of valueless UBJSON containers accepted on top of
+       @ref valueless_elements_per_byte
+
+The limit @ref max_valueless_container_size applies to each container on its
+own, so on its own it lets 9 bytes announce 1048576 values, and every further
+9 bytes another 1048576. The elements of all containers of a valueless type
+in one input are therefore added up, and the sum must stay below this base
+plus @ref valueless_elements_per_byte for every byte read so far. The base is
+the per-container limit, so a container that was accepted on its own before
+the sum was introduced is still accepted at the start of an input.
+*/
+JSON_INLINE_VARIABLE constexpr std::size_t valueless_elements_base = max_valueless_container_size;
+
+/*!
+@brief number of elements of valueless UBJSON containers accepted per byte of
+       input, in addition to @ref valueless_elements_base
+
+An element of a value that has a payload costs at least one byte of input.
+Valueless elements are accepted at a rate of 4 per byte. A value takes 16
+bytes of memory in a DOM, so these arrays add about 80 bytes of memory per
+byte of input (measured: 77; one `Z` per byte needs about 32), beyond what the
+same bytes cost anyway, plus the base. This bounds the arrays of valueless
+type, not the memory of a whole document. Containers written by
+@ref binary_writer never need more than the base, because it writes an array
+in the unoptimized form once the typed arrays written so far and this one add
+up to more than that. The rate matters for input written by other encoders and
+by versions before 3.13.0 only; in the data this was measured on, no input has
+more than 0.17 elements per byte.
+*/
+JSON_INLINE_VARIABLE constexpr std::size_t valueless_elements_per_byte = 4;
+
 ///////////////////
 // binary reader //
 ///////////////////
@@ -3700,6 +3733,37 @@ class binary_reader
         return emit_unsigned(number);
     }
 
+    /*!
+    @brief add the elements of a container of a valueless type to the total of
+           the input and check the total against the input read so far
+
+    A container of type 'Z', 'T' or 'F' has no payload, so its elements are the
+    only ones for which the input does not bound the count. The check is made
+    once per container, not per element.
+
+    @param[in] count  number of elements the container declares; at most
+                      @ref max_valueless_container_size
+    @return false if the total is more than @ref valueless_elements_base plus
+            @ref valueless_elements_per_byte for every byte read so far; the
+            total is not changed then
+    */
+    bool charge_valueless_elements(const std::size_t count) noexcept
+    {
+        // the limit saturates instead of overflowing when size_t is 32 bits
+        // wide and a very large input has been read; valueless_elements
+        // never exceeds the limit, so the difference cannot underflow
+        constexpr std::size_t max_size = (std::numeric_limits<std::size_t>::max)();
+        const std::size_t limit = chars_read <= (max_size - valueless_elements_base) / valueless_elements_per_byte
+                                  ? valueless_elements_base + (valueless_elements_per_byte * chars_read)
+                                  : max_size;
+        if (JSON_HEDLEY_UNLIKELY(count > limit - valueless_elements))
+        {
+            return false;
+        }
+        valueless_elements += count;
+        return true;
+    }
+
     /*!
     @return whether array creation completed
     */
@@ -3785,22 +3849,25 @@ class binary_reader
             // declared count alone decides how much is allocated; the check is
             // made before the start event so that no container is opened that
             // is then abandoned. See @ref max_valueless_container_size.
-            if (JSON_HEDLEY_UNLIKELY((size_and_type.second == 'Z' || size_and_type.second == 'T' || size_and_type.second == 'F')
+            if (JSON_HEDLEY_UNLIKELY(size_and_type.second == 'Z' || size_and_type.second == 'T' || size_and_type.second == 'F'))
-                                     && size_and_type.first > max_valueless_container_size))
+            {
+                if (JSON_HEDLEY_UNLIKELY(size_and_type.first > max_valueless_container_size))
                 {
                     return out_of_range_error(exception_id::container_too_large, "excessive array size", "size");
                 }
-
+                if (JSON_HEDLEY_UNLIKELY(!charge_valueless_elements(size_and_type.first)))
-            if (JSON_HEDLEY_UNLIKELY(!enter_array(size_and_type.first, size_and_type.second)))
                 {
-                return false;
+                    return out_of_range_error(exception_id::container_too_large, "excessive number of elements without payload", "size");
+                }
             }
 
-            if (size_and_type.second == 'N')
+            // a no-op is not a value, so an array of them is empty whatever
+            // count it declares: the SAX parser is told so, instead of being
+            // made to reserve storage for a count that no input backs
+            const std::size_t announced = size_and_type.second == 'N' ? 0 : size_and_type.first;
+            if (JSON_HEDLEY_UNLIKELY(!enter_array(announced, size_and_type.second)))
             {
-                // a no-op is not a value, so a container of them holds none;
+                return false;
-                // the declared size has already been passed to the SAX parser
-                container_stack.back().remaining = 0;
             }
 
             return size_and_type.second == 0
@@ -5114,6 +5181,10 @@ class binary_reader
     /// the number of characters read
     std::size_t chars_read = 0;
 
+    /// UBJSON: the sum of the element counts of the containers of a valueless
+    /// type read so far; see @ref charge_valueless_elements
+    std::size_t valueless_elements = 0;
+
     /// whether we can assume little endianness
     const bool is_little_endian = little_endianness();
 

include/nlohmann/detail/output/binary_writer.hpp

@@ -1204,20 +1204,27 @@ class binary_writer
 
             // an optimized array of a valueless type carries no payload, so a
             // reader has nothing but the declared count to bound the allocation
-            // by and refuses an excessive one. Write the unoptimized form for
+            // by. It refuses one array of more than max_valueless_container_size
-            // those, at one byte per element, so the result can be read back.
+            // elements, and all such arrays of one input together when they add
-            // Objects are not affected: every element is preceded by its key.
+            // up to more than valueless_elements_base plus a number per byte
-            const bool valueless_type = (first_prefix == 'Z' || first_prefix == 'T' || first_prefix == 'F');
+            // read. Write the unoptimized form, at one byte per element, for
-            const bool excessive_valueless = IsArray
+            // an array that would exceed the base with the arrays written so
-                                             && valueless_type
+            // far, so that the result can be read back: the limit of a reader
-                                             && size > detail::max_valueless_container_size;
+            // is never below the base. Objects are not affected: every element
-
+            // is preceded by its key.
-            if (same_prefix && !excessive_valueless
+            if (same_prefix && !(use_bjdata && is_bjdata_excluded_type_marker(first_prefix)))
-                    && !(use_bjdata && is_bjdata_excluded_type_marker(first_prefix)))
+            {
+                const bool valueless_type = IsArray && (first_prefix == 'Z' || first_prefix == 'T' || first_prefix == 'F');
+                if (!valueless_type || size <= detail::valueless_elements_base - valueless_elements_written)
                 {
                     prefix_required = false;
                     oa.write_character(to_char_type('$'));
                     oa.write_character(first_prefix);
+                    if (valueless_type)
+                    {
+                        valueless_elements_written += size;
+                    }
+                }
             }
         }
 
@@ -5441,6 +5448,10 @@ class binary_writer
     /// how to treat a string value or object key that is not valid UTF-8
     /// (CBOR, MessagePack, UBJSON, BJData, and BSON; not BON8)
     const error_handler_t error_handler = binary_writer_default_error_handler();
+
+    /// UBJSON: the number of elements written so far in optimized arrays of a
+    /// valueless type, which a reader adds up and bounds
+    std::size_t valueless_elements_written = 0;
 };
 
 }  // namespace detail

single_include/nlohmann/json.hpp

@@ -6928,37 +6928,6 @@ void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Hash, KeyE
     from_json_pair_array_to_map(j, m);
 }
 
-#if defined(__cpp_lib_char8_t)
-// a std::u8string is read from a JSON string; the array of code units written by
-// versions before 3.13.0 is still accepted
-template<typename BasicJsonType, typename Tr, typename Allocator,
-         enable_if_t<std::is_same<typename json_type_of_t<BasicJsonType>::string_t::value_type, char>::value, int> = 0>
-inline void from_json(const BasicJsonType& j, std::basic_string<char8_t, Tr, Allocator>& s)
-{
-    if (j.is_string())
-    {
-        const auto& str = string_value(j);
-        const char* const data = str.data();
-        const std::size_t size = str.size();
-        std::basic_string<char8_t, Tr, Allocator> ret(s.get_allocator());
-        ret.resize(size);
-        for (std::size_t i = 0; i < size; ++i)
-        {
-            ret[i] = static_cast<char8_t>(static_cast<unsigned char>(data[i]));
-        }
-        s = std::move(ret);
-    }
-    else if (j.is_array())
-    {
-        from_json_array_impl(j, s, priority_tag<3> {});
-    }
-    else
-    {
-        throw_type_must_be("string or array", j);
-    }
-}
-#endif
-
 #if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
 
 // Workaround for MSVC 19.51 (and possibly later): in large cpp files, the compiler may fail to resolve with generic has_from_json (issue #4996)
@@ -8360,20 +8329,16 @@ inline void to_json(BasicJsonType& j, const T& t)
     to_json_tuple_impl(j, t, make_index_sequence<std::tuple_size<T>::value> {});
 }
 
-// a std::u8string is a JSON string (it was an array of code units before 3.13.0);
+#if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
-// defined outside the std::filesystem guard so that the result does not depend
-// on whether std::filesystem::path is supported
 #if defined(__cpp_lib_char8_t)
 template<typename BasicJsonType, typename Tr, typename Allocator>
 inline void to_json(BasicJsonType& j, const std::basic_string<char8_t, Tr, Allocator>& s)
 {
-    using string_t = typename BasicJsonType::string_t;
+    using OtherAllocator = typename std::allocator_traits<Allocator>::template rebind_alloc<char>;
-    // char may alias any object; the code units are copied unchanged
+    j = std::basic_string<char, std::char_traits<char>, OtherAllocator>(s.begin(), s.end(), s.get_allocator());
-    j = string_t(reinterpret_cast<const char*>(s.data()), s.size());
 }
 #endif
 
-#if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
 // Workaround for MSVC 19.51 (and possibly later): in large cpp files, the compiler may fail to resolve with generic has_to_json (issue #4996)
 template<typename BasicJsonType>
 struct has_to_json<BasicJsonType, std_fs::path, void> : std::true_type {};
@@ -18387,13 +18352,46 @@ the input.
 
 This is a sanity bound rather than a security boundary, and it is far above
 any container met in practice. @ref binary_writer falls back to the
-unoptimized encoding for longer containers, so that a value serialized by
+unoptimized encoding for longer containers, and for containers that together
-this library can always be read back.
+with the ones before them would exceed @ref valueless_elements_base, so that a
+value serialized by this library can always be read back.
 
 @sa https://github.com/nlohmann/json/issues/2793
 */
 JSON_INLINE_VARIABLE constexpr std::size_t max_valueless_container_size = 1 << 20;
 
+/*!
+@brief number of elements of valueless UBJSON containers accepted on top of
+       @ref valueless_elements_per_byte
+
+The limit @ref max_valueless_container_size applies to each container on its
+own, so on its own it lets 9 bytes announce 1048576 values, and every further
+9 bytes another 1048576. The elements of all containers of a valueless type
+in one input are therefore added up, and the sum must stay below this base
+plus @ref valueless_elements_per_byte for every byte read so far. The base is
+the per-container limit, so a container that was accepted on its own before
+the sum was introduced is still accepted at the start of an input.
+*/
+JSON_INLINE_VARIABLE constexpr std::size_t valueless_elements_base = max_valueless_container_size;
+
+/*!
+@brief number of elements of valueless UBJSON containers accepted per byte of
+       input, in addition to @ref valueless_elements_base
+
+An element of a value that has a payload costs at least one byte of input.
+Valueless elements are accepted at a rate of 4 per byte. A value takes 16
+bytes of memory in a DOM, so these arrays add about 80 bytes of memory per
+byte of input (measured: 77; one `Z` per byte needs about 32), beyond what the
+same bytes cost anyway, plus the base. This bounds the arrays of valueless
+type, not the memory of a whole document. Containers written by
+@ref binary_writer never need more than the base, because it writes an array
+in the unoptimized form once the typed arrays written so far and this one add
+up to more than that. The rate matters for input written by other encoders and
+by versions before 3.13.0 only; in the data this was measured on, no input has
+more than 0.17 elements per byte.
+*/
+JSON_INLINE_VARIABLE constexpr std::size_t valueless_elements_per_byte = 4;
+
 ///////////////////
 // binary reader //
 ///////////////////
@@ -22014,6 +22012,37 @@ class binary_reader
         return emit_unsigned(number);
     }
 
+    /*!
+    @brief add the elements of a container of a valueless type to the total of
+           the input and check the total against the input read so far
+
+    A container of type 'Z', 'T' or 'F' has no payload, so its elements are the
+    only ones for which the input does not bound the count. The check is made
+    once per container, not per element.
+
+    @param[in] count  number of elements the container declares; at most
+                      @ref max_valueless_container_size
+    @return false if the total is more than @ref valueless_elements_base plus
+            @ref valueless_elements_per_byte for every byte read so far; the
+            total is not changed then
+    */
+    bool charge_valueless_elements(const std::size_t count) noexcept
+    {
+        // the limit saturates instead of overflowing when size_t is 32 bits
+        // wide and a very large input has been read; valueless_elements
+        // never exceeds the limit, so the difference cannot underflow
+        constexpr std::size_t max_size = (std::numeric_limits<std::size_t>::max)();
+        const std::size_t limit = chars_read <= (max_size - valueless_elements_base) / valueless_elements_per_byte
+                                  ? valueless_elements_base + (valueless_elements_per_byte * chars_read)
+                                  : max_size;
+        if (JSON_HEDLEY_UNLIKELY(count > limit - valueless_elements))
+        {
+            return false;
+        }
+        valueless_elements += count;
+        return true;
+    }
+
     /*!
     @return whether array creation completed
     */
@@ -22099,22 +22128,25 @@ class binary_reader
             // declared count alone decides how much is allocated; the check is
             // made before the start event so that no container is opened that
             // is then abandoned. See @ref max_valueless_container_size.
-            if (JSON_HEDLEY_UNLIKELY((size_and_type.second == 'Z' || size_and_type.second == 'T' || size_and_type.second == 'F')
+            if (JSON_HEDLEY_UNLIKELY(size_and_type.second == 'Z' || size_and_type.second == 'T' || size_and_type.second == 'F'))
-                                     && size_and_type.first > max_valueless_container_size))
+            {
+                if (JSON_HEDLEY_UNLIKELY(size_and_type.first > max_valueless_container_size))
                 {
                     return out_of_range_error(exception_id::container_too_large, "excessive array size", "size");
                 }
-
+                if (JSON_HEDLEY_UNLIKELY(!charge_valueless_elements(size_and_type.first)))
-            if (JSON_HEDLEY_UNLIKELY(!enter_array(size_and_type.first, size_and_type.second)))
                 {
-                return false;
+                    return out_of_range_error(exception_id::container_too_large, "excessive number of elements without payload", "size");
+                }
             }
 
-            if (size_and_type.second == 'N')
+            // a no-op is not a value, so an array of them is empty whatever
+            // count it declares: the SAX parser is told so, instead of being
+            // made to reserve storage for a count that no input backs
+            const std::size_t announced = size_and_type.second == 'N' ? 0 : size_and_type.first;
+            if (JSON_HEDLEY_UNLIKELY(!enter_array(announced, size_and_type.second)))
             {
-                // a no-op is not a value, so a container of them holds none;
+                return false;
-                // the declared size has already been passed to the SAX parser
-                container_stack.back().remaining = 0;
             }
 
             return size_and_type.second == 0
@@ -23428,6 +23460,10 @@ class binary_reader
     /// the number of characters read
     std::size_t chars_read = 0;
 
+    /// UBJSON: the sum of the element counts of the containers of a valueless
+    /// type read so far; see @ref charge_valueless_elements
+    std::size_t valueless_elements = 0;
+
     /// whether we can assume little endianness
     const bool is_little_endian = little_endianness();
 
@@ -30581,20 +30617,27 @@ class binary_writer
 
             // an optimized array of a valueless type carries no payload, so a
             // reader has nothing but the declared count to bound the allocation
-            // by and refuses an excessive one. Write the unoptimized form for
+            // by. It refuses one array of more than max_valueless_container_size
-            // those, at one byte per element, so the result can be read back.
+            // elements, and all such arrays of one input together when they add
-            // Objects are not affected: every element is preceded by its key.
+            // up to more than valueless_elements_base plus a number per byte
-            const bool valueless_type = (first_prefix == 'Z' || first_prefix == 'T' || first_prefix == 'F');
+            // read. Write the unoptimized form, at one byte per element, for
-            const bool excessive_valueless = IsArray
+            // an array that would exceed the base with the arrays written so
-                                             && valueless_type
+            // far, so that the result can be read back: the limit of a reader
-                                             && size > detail::max_valueless_container_size;
+            // is never below the base. Objects are not affected: every element
-
+            // is preceded by its key.
-            if (same_prefix && !excessive_valueless
+            if (same_prefix && !(use_bjdata && is_bjdata_excluded_type_marker(first_prefix)))
-                    && !(use_bjdata && is_bjdata_excluded_type_marker(first_prefix)))
+            {
+                const bool valueless_type = IsArray && (first_prefix == 'Z' || first_prefix == 'T' || first_prefix == 'F');
+                if (!valueless_type || size <= detail::valueless_elements_base - valueless_elements_written)
                 {
                     prefix_required = false;
                     oa.write_character(to_char_type('$'));
                     oa.write_character(first_prefix);
+                    if (valueless_type)
+                    {
+                        valueless_elements_written += size;
+                    }
+                }
             }
         }
 
@@ -34818,6 +34861,10 @@ class binary_writer
     /// how to treat a string value or object key that is not valid UTF-8
     /// (CBOR, MessagePack, UBJSON, BJData, and BSON; not BON8)
     const error_handler_t error_handler = binary_writer_default_error_handler();
+
+    /// UBJSON: the number of elements written so far in optimized arrays of a
+    /// valueless type, which a reader adds up and bounds
+    std::size_t valueless_elements_written = 0;
 };
 
 }  // namespace detail

tests/CMakeLists.txt

@@ -364,16 +364,6 @@ json_test_add_test_for(src/unit-class_parser.cpp
     MAIN test_main CXX_STANDARDS ${test_cxx_standards} ${test_force}
 )
 
-# test the conversions again without std::filesystem::path support: a
-# std::u8string must be a JSON string in this configuration, too
-json_test_set_test_options(test-conversions2_no_filesystem
-    COMPILE_DEFINITIONS JSON_HAS_FILESYSTEM=0 JSON_HAS_EXPERIMENTAL_FILESYSTEM=0
-)
-json_test_add_test_for(src/unit-conversions2.cpp
-    NAME test-conversions2_no_filesystem
-    MAIN test_main CXX_STANDARDS ${test_cxx_standards} ${test_force}
-)
-
 # test diagnostic positions again without regular diagnostics (JSON pointer paths)
 json_test_set_test_options(test-diagnostic-positions_only
     COMPILE_DEFINITIONS JSON_DIAGNOSTICS=0

tests/src/unit-alt-string.cpp

@@ -536,20 +536,4 @@ TEST_CASE("alternative string type with json_view")
     }
 }
 
-// a std::u8string is a JSON string, whatever the string type of the JSON value
-// JSON_HAS_CPP_20 JSON_HAS_CPP_23 (do not remove; see note at top of file)
-#if defined(__cpp_lib_char8_t)
-TEST_CASE("alternative string type and std::u8string")
-{
-    const std::u8string s = u8"abc";
-    const alt_json j = s;
-    CHECK(j.is_string());
-    CHECK(j.get<alt_string>() == alt_string("abc"));
-    CHECK(j.get<std::u8string>() == s);
-
-    // the array of code units written by versions before 3.13.0
-    CHECK(alt_json::array({0x61, 0x62, 0x63}).get<std::u8string>() == s);
-}
-#endif
-
 DOCTEST_CLANG_SUPPRESS_WARNING_POP

tests/src/unit-conversions2.cpp

@@ -27,12 +27,10 @@ using nlohmann::json;
 #include <deque>
 #include <forward_list>
 #include <list>
-#include <map>
 #include <set>
 #include <unordered_map>
 #include <unordered_set>
 #include <valarray>
-#include <vector>
 
 // NLOHMANN_JSON_SERIALIZE_ENUM uses a static std::pair
 DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
@@ -709,13 +707,24 @@ TEST_CASE("std::filesystem::path")
 }
 #endif
 
-// a std::u8string is a JSON string since 3.13.0 (it was an array of code units
+// the ADL to_json overload for std::u8string only exists under the same guard
-// before); independent of whether std::filesystem::path is supported, see the
+// as std::filesystem::path support (it is otherwise only reached indirectly,
-// test-conversions2_no_filesystem target
+// via std::filesystem::path::u8string()) -- mirror both #if conditions from
-// JSON_HAS_CPP_20 JSON_HAS_CPP_23 (do not remove; see note at top of file)
+// include/nlohmann/detail/conversions/to_json.hpp exactly
+#if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
 #if defined(__cpp_lib_char8_t)
 TEST_CASE("std::u8string")
 {
+    SECTION("ascii")
+    {
+        const std::u8string s = u8"Path";
+        json const j = s;
+
+        CHECK(j.template get<std::string>() == "Path");
+    }
+
+    SECTION("utf-8")
+    {
         // use \u universal-character-names (rather than raw \x byte escapes
         // or literal non-ASCII source bytes) to compose the multi-byte UTF-8
         // encoding -- MSVC treats \x escapes used that way inside a u8
@@ -723,101 +732,14 @@ TEST_CASE("std::u8string")
         // our CI configs promote to an error; \u is portable and produces
         // the exact same encoded bytes without depending on the source
         // file's encoding
-    const std::u8string utf8 = u8"P\u011B\u0161ina";
+        const std::u8string s = u8"P\u011B\u0161ina";
-
-    SECTION("ascii")
-    {
-        const std::u8string s = u8"Path";
         json const j = s;
 
-        CHECK(j.is_string());
-        CHECK(j.template get<std::string>() == "Path");
-    }
-
-    SECTION("utf-8")
-    {
-        json const j = utf8;
-
-        CHECK(j.is_string());
         CHECK(j.template get<std::string>() == "P\xc4\x9b\xc5\xa1ina");
     }
-
-    SECTION("round trip")
-    {
-        json const j = utf8;
-        CHECK(j.template get<std::u8string>() == utf8);
-
-        std::u8string back;
-        j.get_to(back);
-        CHECK(back == utf8);
-
-        // from a string
-        json const j_string = "P\xc4\x9b\xc5\xa1ina";
-        CHECK(j_string.template get<std::u8string>() == utf8);
-
-        // empty, and with an embedded NUL
-        CHECK(json(std::u8string()).template get<std::u8string>().empty());
-        const std::u8string with_nul(u8"a\0b", 3);
-        CHECK(json(with_nul).template get<std::u8string>() == with_nul);
-
-        // the lvalue and rvalue paths, and in containers
-        std::u8string lvalue = utf8;
-        CHECK(json(lvalue).template get<std::u8string>() == utf8);
-        CHECK(json(std::move(lvalue)).template get<std::u8string>() == utf8);
-        const std::vector<std::u8string> vec = {utf8, u8"x"};
-        json const j_vec = vec;
-        CHECK(j_vec.is_array());
-        CHECK(j_vec[0].is_string());
-        CHECK(j_vec.template get<std::vector<std::u8string>>() == vec);
-        const std::map<std::string, std::u8string> map = {{"k", utf8}};
-        CHECK(json(map).template get<std::map<std::string, std::u8string>>() == map);
-    }
-
-    SECTION("array of code units (written by versions before 3.13.0)")
-    {
-        json const j = json::array({0x50, 0xC4, 0x9B, 0xC5, 0xA1, 0x69, 0x6E, 0x61});
-        CHECK(j.template get<std::u8string>() == utf8);
-        CHECK(json::array().template get<std::u8string>().empty());
-        CHECK(json::parse("[97, 98]").template get<std::u8string>() == u8"ab");
-
-        std::u8string s = u8"overwritten";
-        j.get_to(s);
-        CHECK(s == utf8);
-    }
-
-    SECTION("other types")
-    {
-#if !defined(JSON_NOEXCEPTION)
-        CHECK_THROWS_WITH_AS(json(1).template get<std::u8string>(), "[json.exception.type_error.302] type must be string or array, but is number", json::type_error);
-        CHECK_THROWS_WITH_AS(json(nullptr).template get<std::u8string>(), "[json.exception.type_error.302] type must be string or array, but is null", json::type_error);
-        CHECK_THROWS_AS(json::object().template get<std::u8string>(), json::type_error);
-        // an element that is not a number
-        CHECK_THROWS_AS(json::array({"a"}).template get<std::u8string>(), json::type_error);
-#endif
-    }
-
-    SECTION("ordered_json")
-    {
-        nlohmann::ordered_json const j = utf8;
-        CHECK(j.is_string());
-        CHECK(j.template get<std::string>() == "P\xc4\x9b\xc5\xa1ina");
-        CHECK(j.template get<std::u8string>() == utf8);
-        CHECK(nlohmann::ordered_json::array({0x61, 0x62}).template get<std::u8string>() == u8"ab");
-    }
-
-    SECTION("std::u8string_view and char8_t arrays are unchanged (arrays of code units)")
-    {
-        const std::u8string_view view = utf8;
-        json const j_view = view;
-        CHECK(j_view.is_array());
-        CHECK(j_view.size() == 8);
-
-        const char8_t arr[] = u8"ab";
-        json const j_arr = arr;
-        CHECK(j_arr == json::array({0x61, 0x62, 0}));
-    }
 }
 #endif
+#endif
 
 #if !defined(JSON_NOEXCEPTION)
 namespace

tests/src/unit-ubjson.cpp

@@ -19,8 +19,12 @@
 #include <nlohmann/json.hpp>
 using nlohmann::json;
 
+#include <cstdio>
 #include <iostream>
+#include <sstream>
+#include <utility>
 #include <fstream>
+#include <limits>
 #include <list>
 #include <set>
 #include "make_test_data_available.hpp"
@@ -2436,6 +2440,341 @@ TEST_CASE("UBJSON optimized arrays of a valueless type are bounded")
     }
 }
 
+namespace
+{
+/// SAX consumer that counts the values and keeps the id of a parse error
+struct ValuelessEventCounter : public nlohmann::json_sax<json>
+{
+    bool null() override
+    {
+        return ++values < stop_after;
+    }
+    bool boolean(bool /*val*/) override
+    {
+        return ++values < stop_after;
+    }
+    bool number_integer(json::number_integer_t /*val*/) override
+    {
+        ++values;
+        return true;
+    }
+    bool number_unsigned(json::number_unsigned_t /*val*/) override
+    {
+        ++values;
+        return true;
+    }
+    bool number_float(json::number_float_t /*val*/, const std::string& /*s*/) override
+    {
+        ++values;
+        return true;
+    }
+    bool string(std::string& /*val*/) override
+    {
+        ++values;
+        return true;
+    }
+    bool binary(json::binary_t& /*val*/) override
+    {
+        ++values;
+        return true;
+    }
+    bool start_object(std::size_t /*elements*/) override
+    {
+        return true;
+    }
+    bool key(std::string& /*val*/) override
+    {
+        return true;
+    }
+    bool end_object() override
+    {
+        return true;
+    }
+    bool start_array(std::size_t /*elements*/) override
+    {
+        return true;
+    }
+    bool end_array() override
+    {
+        return true;
+    }
+    bool parse_error(std::size_t /*position*/, const std::string& /*last_token*/, const json::exception& ex) override
+    {
+        error_id = ex.id;
+        return false;
+    }
+
+    /// the handler refuses the value that makes this many
+    std::size_t stop_after = (std::numeric_limits<std::size_t>::max)();
+    std::size_t values = 0;
+    int error_id = 0;
+};
+
+/// appends `[$<marker>#l<count>` (nine bytes) to @a v
+void append_valueless_array(std::vector<std::uint8_t>& v, const char marker, const std::uint32_t count)
+{
+    v.push_back('[');
+    v.push_back('$');
+    v.push_back(static_cast<std::uint8_t>(marker));
+    v.push_back('#');
+    v.push_back('l');
+    v.push_back(static_cast<std::uint8_t>(count >> 24));
+    v.push_back(static_cast<std::uint8_t>(count >> 16));
+    v.push_back(static_cast<std::uint8_t>(count >> 8));
+    v.push_back(static_cast<std::uint8_t>(count));
+}
+
+/// `[` followed by one `[$<marker>#l<count>` per entry of @a counts and `]`
+std::vector<std::uint8_t> valueless_arrays(const char marker, const std::vector<std::uint32_t>& counts)
+{
+    std::vector<std::uint8_t> v = {'['};
+    for (const auto count : counts)
+    {
+        append_valueless_array(v, marker, count);
+    }
+    v.push_back(']');
+    return v;
+}
+} // namespace
+
+TEST_CASE("UBJSON the valueless elements of a whole input are bounded")
+{
+    // The per-container limit alone let nine bytes (`[$Z#l` and a count of
+    // 1048576) announce a million values, and every further nine bytes another
+    // million. The elements of all containers of type 'Z', 'T' or 'F' in one
+    // input are now added up: they may not exceed 1048576 plus 4 for every
+    // byte read when the container's header has been read.
+    //
+    // The tests count events with a SAX handler where they can: building and
+    // comparing values of a million elements is what makes them slow under
+    // Valgrind, so there is one rejection and one round trip with a DOM.
+    json _;
+    const char* const budget_message = "[json.exception.out_of_range.408] syntax error while parsing UBJSON size: excessive number of elements without payload";
+
+    // two arrays that exceed the limit: the second one, with its header read
+    // at byte 19, may hold 4 * 19 = 76 values on top of the base of 1048576
+    const std::uint32_t over_first = 524288;
+    const std::uint32_t over_second = 1048576 + 76 - over_first + 1;
+
+    SECTION("the input of the original report is rejected")
+    {
+        // 362 bytes (40 arrays of 1048576 values) used to allocate 661 MB; the
+        // second array is the one that is refused
+        const auto input = valueless_arrays('Z', {over_first, over_second});
+        CHECK_THROWS_WITH_AS(_ = json::from_ubjson(input), budget_message, json::out_of_range&);
+    }
+
+    SECTION("every input adapter applies the budget")
+    {
+        const auto input = valueless_arrays('Z', {over_first, over_second});
+
+        // stream; not strict, which does not matter
+        std::stringstream stream;
+        stream.write(reinterpret_cast<const char*>(input.data()), static_cast<std::streamsize>(input.size()));
+        ValuelessEventCounter from_stream;
+        CHECK(!json::sax_parse(stream, &from_stream, json::input_format_t::ubjson, false));
+        CHECK(from_stream.error_id == 408);
+
+        // a container that is not contiguous; the counts of arrays of
+        // different types are added up
+        std::vector<std::uint8_t> mixed = {'['};
+        append_valueless_array(mixed, 'T', over_first);
+        append_valueless_array(mixed, 'F', over_second);
+        mixed.push_back(']');
+        const std::list<std::uint8_t> as_list(mixed.begin(), mixed.end());
+        ValuelessEventCounter from_list;
+        CHECK(!json::sax_parse(as_list.begin(), as_list.end(), &from_list, json::input_format_t::ubjson));
+        CHECK(from_list.error_id == 408);
+        CHECK(from_list.values == over_first);
+
+        // FILE*
+        FILE* const file = std::tmpfile(); // NOLINT(cppcoreguidelines-owning-memory)
+        REQUIRE(file != nullptr);
+        CHECK(std::fwrite(input.data(), 1, input.size(), file) == input.size());
+        std::rewind(file);
+        ValuelessEventCounter from_file;
+        CHECK(!json::sax_parse(file, &from_file, json::input_format_t::ubjson));
+        CHECK(from_file.error_id == 408);
+        std::fclose(file); // NOLINT(cert-err33-c,cppcoreguidelines-owning-memory)
+    }
+
+    SECTION("the limit is exact and the bytes read raise it")
+    {
+        // [ [$Z#l 524288] [$U#l 4096 + 4096 bytes] [$Z#l count] ]: chars_read
+        // is 1 + 9 + (9 + 4096) + 9 = 4124 when the last header has been read,
+        // so the last array may hold 4 * 4124 = 16496 values on top of the
+        // base of 1048576 less the 524288 of the first array
+        const std::uint32_t last_allowed = 1048576 + 16496 - 524288;
+        for (const std::uint32_t count :
+                {
+                    last_allowed, last_allowed + 1
+                })
+        {
+            CAPTURE(count)
+            std::vector<std::uint8_t> input = {'['};
+            append_valueless_array(input, 'Z', 524288);
+            input.insert(input.end(), {'[', '$', 'U', '#', 'l', 0x00, 0x00, 0x10, 0x00});
+            input.insert(input.end(), 4096, 0x01);
+            append_valueless_array(input, 'Z', count);
+            input.push_back(']');
+            ValuelessEventCounter sax;
+            const bool accepted = count == last_allowed;
+            CHECK(json::sax_parse(input, &sax, json::input_format_t::ubjson) == accepted);
+            CHECK(sax.error_id == (accepted ? 0 : 408));
+            CHECK(sax.values == 524288u + 4096u + (accepted ? count : 0u));
+        }
+
+        // an array of 1048576 values on its own is accepted as before
+        const std::vector<std::uint8_t> single = {'[', '$', 'Z', '#', 'l', 0x00, 0x10, 0x00, 0x00};
+        // (the handler ends the parse at the first value, so that the million
+        // values are not read)
+        ValuelessEventCounter sax;
+        sax.stop_after = 1;
+        CHECK(!json::sax_parse(single, &sax, json::input_format_t::ubjson));
+        CHECK(sax.error_id == 0);
+        CHECK(sax.values == 1);
+    }
+
+    SECTION("to_ubjson writes the typed form up to the base only")
+    {
+        // 1048 rows of 1000 values fit under the base of 1048576, the 1049th
+        // would not: its elements are written one by one. Written all typed,
+        // 1100 rows of 6 bytes would be refused by a reader (1075 rows are the
+        // most it accepts). The rows hold null, true or false, and the
+        // fallback does not depend on which
+        std::vector<std::uint8_t> encoded;
+        {
+            json table = json::array();
+            for (int row = 0; row < 1100; ++row)
+            {
+                const json cell = row % 3 == 0 ? json(nullptr) : json(row % 3 == 1);
+                table.push_back(json(1000, cell));
+            }
+            encoded = json::to_ubjson(table, true, true);
+        }
+        // outer array: `[$[#I` and the count (7 bytes), then 1048 typed rows of 6
+        // bytes (`$Z#I` and two bytes) and 52 rows of `#I`, two bytes, and 1000 `Z`
+        CHECK(encoded.size() == 7 + (1048 * 6) + (52 * (4 + 1000)));
+
+        // the output is read back
+        ValuelessEventCounter sax;
+        CHECK(json::sax_parse(encoded, &sax, json::input_format_t::ubjson));
+        CHECK(sax.error_id == 0);
+        CHECK(sax.values == 1100u * 1000u);
+    }
+
+    SECTION("BJData is not affected")
+    {
+        // BJData has no optimized arrays of these types, and the writer does
+        // not charge them
+        json table = json::array();
+        for (int row = 0; row < 3; ++row)
+        {
+            table.push_back(json(10, nullptr));
+        }
+        const auto encoded = json::to_bjdata(table, true, true);
+        CHECK(json::from_bjdata(encoded) == table);
+
+        const std::vector<std::uint8_t> input = {'[', '$', 'Z', '#', 'l', 0x00, 0x10, 0x00, 0x00};
+        CHECK_THROWS_AS(_ = json::from_bjdata(input), json::parse_error&);
+    }
+
+    SECTION("an object of a valueless type needs a key for every value")
+    {
+        // an object member costs a key, so the count is bounded by the input
+        // and is not charged to the budget
+        const std::vector<std::uint8_t> input = {'{', '$', 'Z', '#', 'l', 0x00, 0x10, 0x00, 0x00};
+        ValuelessEventCounter sax;
+        CHECK(!json::sax_parse(input, &sax, json::input_format_t::ubjson));
+        CHECK(sax.error_id == 110);
+        CHECK(sax.values == 0);
+    }
+}
+
+TEST_CASE("UBJSON an optimized array of no-ops is empty whatever its count")
+{
+    // A no-op ('N') is not a value, so `[$N#<n>` is an array without elements
+    // for every n. The count used to be passed on to the SAX parser, which
+    // told it to expect n elements, and the DOM parser reserved storage for
+    // them: 16384 values (256 KB) for a count of 2^31 in 9 bytes of input.
+    SECTION("the SAX parser is told that the array is empty")
+    {
+        for (const std::vector<std::uint8_t>& input :
+                {
+                    std::vector<std::uint8_t> {'[', '$', 'N', '#', 'i', 2},
+                    std::vector<std::uint8_t> {'[', '$', 'N', '#', 'I', 0x7F, 0xFF},
+                    std::vector<std::uint8_t> {'[', '$', 'N', '#', 'l', 0x7F, 0xFF, 0xFF, 0xFF},
+                    std::vector<std::uint8_t> {'[', '$', 'N', '#', 'i', 0}
+                })
+        {
+            SaxEventLogger logger;
+            CHECK(json::sax_parse(input, &logger, json::input_format_t::ubjson));
+            CHECK(logger.events == std::vector<std::string>({"start_array(0)", "end_array()"}));
+        }
+
+        // between other values, the next value is read as usual
+        const std::vector<std::uint8_t> nested = {'[', '[', '$', 'N', '#', 'l', 0x7F, 0xFF, 0xFF, 0xFF, 'T', '[', '$', 'N', '#', 'i', 3, ']'};
+        SaxEventLogger logger;
+        CHECK(json::sax_parse(nested, &logger, json::input_format_t::ubjson));
+        CHECK(logger.events == std::vector<std::string>({"start_array()", "start_array(0)", "end_array()", "boolean(true)", "start_array(0)", "end_array()", "end_array()"}));
+    }
+
+    SECTION("the DOM parser reserves nothing")
+    {
+        // 2000 arrays of 9 bytes that announce 2^31 elements each: with a
+        // reservation of 16384 values per array this took about 500 MB
+        std::vector<std::uint8_t> input = {'['};
+        for (int i = 0; i < 2000; ++i)
+        {
+            input.insert(input.end(), {'[', '$', 'N', '#', 'l', 0x7F, 0xFF, 0xFF, 0xFF});
+        }
+        input.push_back(']');
+
+        const json j = json::from_ubjson(input);
+        REQUIRE(j.size() == 2000);
+        std::size_t capacity = 0;
+        for (const auto& element : j)
+        {
+            CHECK(element.is_array());
+            CHECK(element.empty());
+            capacity += element.get_ref<const json::array_t&>().capacity();
+        }
+        CHECK(capacity == 0);
+
+        // the 7-byte form inside an optimized array of arrays
+        std::vector<std::uint8_t> typed = {'[', '$', '[', '#', 'I', 0x07, 0xD0};
+        for (int i = 0; i < 2000; ++i)
+        {
+            typed.insert(typed.end(), {'$', 'N', '#', 'I', 0x7F, 0xFF});
+        }
+        const json k = json::from_ubjson(typed);
+        REQUIRE(k.size() == 2000);
+        capacity = 0;
+        for (const auto& element : k)
+        {
+            CHECK(element.empty());
+            capacity += element.get_ref<const json::array_t&>().capacity();
+        }
+        CHECK(capacity == 0);
+    }
+
+    SECTION("a count that no container could hold is no longer an error")
+    {
+        // the array is empty, so nothing has to fit: 0x7FFFFFFF used to be
+        // reported as out_of_range.408 where it exceeds the maximum size of
+        // an array, and is accepted now
+        const std::vector<std::uint8_t> input = {'[', '$', 'N', '#', 'l', 0x7F, 0xFF, 0xFF, 0xFF};
+        CHECK(json::from_ubjson(input) == json::array());
+
+        // a count that does not fit into std::size_t still is
+#if SIZE_MAX == 0xffffffff
+        const std::vector<std::uint8_t> too_large = {'[', '$', 'N', '#', 'L', 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00};
+        json _;
+        CHECK_THROWS_AS(_ = json::from_ubjson(too_large), json::out_of_range&);
+#endif
+    }
+}
+
 TEST_CASE("issue #5405 - array reserve for definite-length UBJSON arrays")
 {
 #if !defined(JSON_NOEXCEPTION)