Add JSON_RECURSION_DEPTH_LIMIT to set how deep value walks recurse (#6081)

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

include/nlohmann/detail/recursion_depth_limit.hpp

@@ -9,9 +9,22 @@
 #pragma once
 
 #include <cstddef> // size_t
+#include <type_traits> // false_type, is_integral, remove_cv, true_type
 
 #include <nlohmann/detail/abi_macros.hpp>
 
+// JSON_RECURSION_DEPTH_LIMIT: the number of nesting levels the operations that
+// walk a value recurse into on the call stack before they continue with an
+// explicit stack (see detail::recursion_depth_limit() below).
+//
+// May be defined by the user before the first include of the library; it has
+// to be the same in every translation unit of a program (it is not part of the
+// ABI tag, see https://json.nlohmann.me/api/macros/json_recursion_depth_limit/).
+// Allowed: an integer constant expression from 0 to 254. The default is 128.
+#ifndef JSON_RECURSION_DEPTH_LIMIT
+    #define JSON_RECURSION_DEPTH_LIMIT 128
+#endif
+
 NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
@@ -20,21 +33,71 @@ namespace detail
 @brief the number of nesting levels an operation recurses into
 
 Operations that walk a value (copying, comparing, serializing, hashing, merging,
-...) recurse once
-per nesting level, which is fastest, but a value nested deeply enough would
-exhaust the call stack. So they recurse only this many levels deep and finish
-whatever lies below with an explicit stack. All of them share this limit.
+diffing, converting, destroying, and writing the binary formats CBOR,
+MessagePack, UBJSON, BJData, and BON8) recurse once per nesting level, which is
+fastest, but a value nested deeply enough would exhaust the call stack. So they
+recurse only this many levels deep and finish whatever lies below with an
+explicit stack that lives on the heap. All of them share this limit and none has
+a constant of its own. It is the value of the macro JSON_RECURSION_DEPTH_LIMIT,
+128 by default. (Parsing, the binary readers, flatten(), unflatten(), and
+to_bson() always use an explicit stack and do not read it, and the lockstep
+comparison of json_view documents has a fixed bound of its own, 48 levels.)
 
-Most of them pass the depth down as an argument. The copy constructor and the
-comparison operators cannot, as their signatures are fixed, so they count it
-in basic_json::nesting_depth() instead, a byte per thread; the limit must
-therefore stay below 255.
+Most of the operations pass the depth down as an argument. The copy constructor
+and the comparison operators cannot, as their signatures are fixed, so they
+count it in basic_json::nesting_depth() instead, a byte per thread. Only the
+levels that are descended into are counted, so the count never exceeds the limit.
+The limit must fit in the byte with some headroom, so that the count cannot wrap
+around: it must be below 255, and 254 is the largest value allowed. (More levels
+would hardly be useful: 254 levels need up to 576 KiB of stack in an unoptimized
+build already, twice the stack of the default.)
 
+The smallest value is 0: every operation then uses its explicit stack from the
+first level on and no operation recurses over the nesting of a value at all.
+That is slower, but needs a constant amount of call stack. It works because
+every use of the limit compares a depth with it (`depth >= limit`, or `<` for
+the opposite), which holds for 0 as for any other value; nothing computes
+`limit - 1` or `limit + 1`, so nothing wraps around. Each operation's explicit
+stack handles any nesting level, including the first, so none of them relies on
+having recursed first.
+
+The call stack that an operation needs is roughly a few KiB plus a cost per level
+times the limit. At the default of 128 levels, an unoptimized build needs up to
+about 290 KiB (`dump()` with Clang at -O0: about 2.3 KiB per level) and an
+optimized build up to about 48 KiB; with 254 levels it is up to 576 KiB and
+112 KiB, and with 0 it is 16 KiB or less. Lower the limit for threads with small
+stacks (RTOS tasks, fibers, musl threads), raise it to trade stack for speed on
+deeply nested values. All of this, with the measurements, is in the
+documentation of the macro.
+
+@sa https://json.nlohmann.me/api/macros/json_recursion_depth_limit/
 @sa https://github.com/nlohmann/json/issues/5387
 */
+/// the type of the value of JSON_RECURSION_DEPTH_LIMIT is bool or a character type
+template<typename T> struct recursion_depth_limit_is_char_or_bool : std::false_type {};
+template<> struct recursion_depth_limit_is_char_or_bool<bool> : std::true_type {};
+template<> struct recursion_depth_limit_is_char_or_bool<char> : std::true_type {};
+template<> struct recursion_depth_limit_is_char_or_bool<signed char> : std::true_type {};
+template<> struct recursion_depth_limit_is_char_or_bool<unsigned char> : std::true_type {};
+template<> struct recursion_depth_limit_is_char_or_bool<wchar_t> : std::true_type {};
+template<> struct recursion_depth_limit_is_char_or_bool<char16_t> : std::true_type {};
+template<> struct recursion_depth_limit_is_char_or_bool<char32_t> : std::true_type {};
+#if defined(__cpp_char8_t)
+template<> struct recursion_depth_limit_is_char_or_bool<char8_t> : std::true_type {};
+#endif
+
 constexpr std::size_t recursion_depth_limit() noexcept
 {
-    return 128;
+    // an integer: not a floating-point number, whose range check would pass for 1.5 or 254.0
+    // (bool and the character types are integer types, too, but true, false or 'a' are
+    // no way to say a limit; JSON_UBJSON_VALUELESS_ELEMENTS_PER_BYTE refuses them as well)
+    static_assert(std::is_integral<decltype(JSON_RECURSION_DEPTH_LIMIT)>::value
+                  && !recursion_depth_limit_is_char_or_bool<typename std::remove_cv<decltype(JSON_RECURSION_DEPTH_LIMIT)>::type>::value,
+                  "JSON_RECURSION_DEPTH_LIMIT must be an integer constant (not bool or a character type)");
+    static_assert(JSON_RECURSION_DEPTH_LIMIT >= 0 && JSON_RECURSION_DEPTH_LIMIT <= 254,
+                  "JSON_RECURSION_DEPTH_LIMIT must be between 0 and 254: the nesting depth is counted in a byte per thread "
+                  "and may exceed the limit by one (see https://json.nlohmann.me/api/macros/json_recursion_depth_limit/)");
+    return static_cast<std::size_t>(JSON_RECURSION_DEPTH_LIMIT);
 }
 
 }  // namespace detail