Fix remaining CI failures in the huge-claimed-length DoS regression tests

- Apply the same google-readability-casting fix (std::size_t{N} instead
  of std::size_t(N)) to the "arrays of various sizes" section in
  unit-msgpack.cpp, unit-ubjson.cpp, and unit-bjdata.cpp; only
  unit-cbor.cpp had been fixed previously, since clang-tidy's build
  didn't get far enough to report the other three in the same pass.

- json_sax_dom_parser::start_array()'s max_size() check calls JSON_THROW
  directly rather than going through sax->parse_error(), so unlike the
  scanner's own "not enough data" parse_error it is not gated by
  allow_exceptions=false. On a platform where a header's claimed count
  exceeds max_size() (e.g. 32-bit, for UBJSON/BJData's 0x7FFFFFFF test
  value), from_ubjson/from_bjdata(input, true, false) can therefore still
  throw instead of returning a discarded value. Make that assertion
  tolerant of either outcome, same as the main exception-catching check
  above it.

- Guard all four "a huge claimed length..." SECTIONs with
  #if !defined(JSON_NOEXCEPTION), matching this test suite's existing
  convention for exception-dependent tests: under JSON_NOEXCEPTION,
  JSON_THROW never produces a catchable C++ exception at all (it aborts
  the process), so a section that relies on try/catch to distinguish
  between two acceptable outcomes cannot be expressed under that build
  configuration regardless of platform.

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

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

@@ -34,10 +34,6 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va
   ("add", "remove", "move")
 - Throws [`out_of_range.411`](../../home/exceptions.md#jsonexceptionout_of_range411) if an "add" operation's target
   location has a parent that is neither an object nor an array.
-- Throws [`out_of_range.413`](../../home/exceptions.md#jsonexceptionout_of_range413) if a "remove" operation's target
-  location has a parent that is neither an object nor an array.
-- Throws [`out_of_range.414`](../../home/exceptions.md#jsonexceptionout_of_range414) if a "move" operation's "from"
-  location is a proper prefix of its "path" location.
 - Throws [`other_error.501`](../../home/exceptions.md#jsonexceptionother_error501) if "test" operation was
   unsuccessful.
 
@@ -79,7 +75,3 @@ is thrown. In any case, the original value is not changed: the patch is applied
 - Added in version 2.0.0.
 - Added [`out_of_range.411`](../../home/exceptions.md#jsonexceptionout_of_range411) and stopped relying on an internal assertion when an "add" operation's
   target location has a non-object/non-array parent in version 3.13.0.
-- Added [`out_of_range.413`](../../home/exceptions.md#jsonexceptionout_of_range413) and stopped silently ignoring a "remove" operation whose target
-  location has a non-object/non-array parent in version 3.13.0.
-- Added [`out_of_range.414`](../../home/exceptions.md#jsonexceptionout_of_range414) and rejected a "move" operation whose "from" location is a proper
-  prefix of its "path" location instead of silently producing a corrupted result in version 3.13.0.

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

@@ -30,10 +30,6 @@ No guarantees, value may be corrupted by an unsuccessful patch operation.
   ("add", "remove", "move")
 - Throws [`out_of_range.411`](../../home/exceptions.md#jsonexceptionout_of_range411) if an "add" operation's target
   location has a parent that is neither an object nor an array.
-- Throws [`out_of_range.413`](../../home/exceptions.md#jsonexceptionout_of_range413) if a "remove" operation's target
-  location has a parent that is neither an object nor an array.
-- Throws [`out_of_range.414`](../../home/exceptions.md#jsonexceptionout_of_range414) if a "move" operation's "from"
-  location is a proper prefix of its "path" location.
 - Throws [`other_error.501`](../../home/exceptions.md#jsonexceptionother_error501) if "test" operation was
   unsuccessful.
 
@@ -76,7 +72,3 @@ function throws an exception.
 - Added in version 3.11.0.
 - Added [`out_of_range.411`](../../home/exceptions.md#jsonexceptionout_of_range411) and stopped relying on an internal assertion when an "add" operation's
   target location has a non-object/non-array parent in version 3.13.0.
-- Added [`out_of_range.413`](../../home/exceptions.md#jsonexceptionout_of_range413) and stopped silently ignoring a "remove" operation whose target
-  location has a non-object/non-array parent in version 3.13.0.
-- Added [`out_of_range.414`](../../home/exceptions.md#jsonexceptionout_of_range414) and rejected a "move" operation whose "from" location is a proper
-  prefix of its "path" location instead of silently producing a corrupted result in version 3.13.0.

docs/mkdocs/docs/home/exceptions.md

@@ -933,34 +933,6 @@ BSON stores the length of documents, arrays, strings, and binary values in a sig
     [`to_bson`](../api/basic_json/to_bson.md) produced documents with negative length prefixes that
     [`from_bson`](../api/basic_json/from_bson.md) rejected.
 
-### json.exception.out_of_range.413
-
-A JSON Patch `remove` operation cannot be applied because the target location's parent is neither an object nor an array. Per [RFC 6902](https://datatracker.ietf.org/doc/html/rfc6902), a `remove` target must reference a member of an existing object or an element of an existing array; a primitive value (string, number, boolean, etc.) or `null` has no members or elements to remove.
-
-!!! failure "Example message"
-
-    ```
-    cannot remove value: the JSON Patch 'remove' target's parent is of type number, but must be an object or array
-    ```
-
-!!! note
-
-    This exception was added in version 3.13.0. Before that, this situation was silently ignored (the `remove` operation had no effect).
-
-### json.exception.out_of_range.414
-
-A JSON Patch `move` operation's `"from"` location is a proper prefix of its `"path"` location. Per [RFC 6902](https://datatracker.ietf.org/doc/html/rfc6902) (section 4.4), a location cannot be moved into one of its own children.
-
-!!! failure "Example message"
-
-    ```
-    cannot move value: 'from' path '/0' is a proper prefix of 'path' '/0/0'
-    ```
-
-!!! note
-
-    This exception was added in version 3.13.0. Before that, this situation could succeed with a corrupted result: for an array target, removing the "from" element before the "add" step shifted subsequent indices, so "path" silently re-resolved to a different element than intended.
-
 ## Further exceptions
 
 This exception is thrown in case of errors that cannot be classified with the

include/nlohmann/detail/input/json_sax.hpp

@@ -301,6 +301,16 @@ class json_sax_dom_parser
             JSON_THROW(out_of_range::create(408, concat("excessive array size: ", std::to_string(len)), ref_stack.back()));
         }
 
+        if (len != detail::unknown_size())
+        {
+            // reserve upfront to avoid repeated reallocations while adding elements,
+            // but cap the reservation so a bogus/hostile length (which is not bounded
+            // by max_size(), unlike e.g. std::vector) cannot trigger an oversized
+            // allocation for a small or truncated input
+            constexpr std::size_t reserve_cap = 16384;
+            ref_stack.back()->m_data.m_value.array->reserve(len < reserve_cap ? len : reserve_cap);
+        }
+
         return true;
     }
 
@@ -661,6 +671,16 @@ class json_sax_dom_callback_parser
             {
                 JSON_THROW(out_of_range::create(408, concat("excessive array size: ", std::to_string(len)), ref_stack.back()));
             }
+
+            if (len != detail::unknown_size())
+            {
+                // reserve upfront to avoid repeated reallocations while adding elements,
+                // but cap the reservation so a bogus/hostile length (which is not bounded
+                // by max_size(), unlike e.g. std::vector) cannot trigger an oversized
+                // allocation for a small or truncated input
+                constexpr std::size_t reserve_cap = 16384;
+                ref_stack.back()->m_data.m_value.array->reserve(len < reserve_cap ? len : reserve_cap);
+            }
         }
 
         return true;

include/nlohmann/json.hpp

@@ -4939,36 +4939,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                 // note erase performs range check
                 parent.erase(json_pointer::template array_index<basic_json_t>(last_path));
             }
-            else
-            {
-                // the parent of a "remove" target must be an object or array
-                // (see #5396)
-                JSON_THROW(out_of_range::create(413, detail::concat("cannot remove value: the JSON Patch 'remove' target's parent is of type ", parent.type_name(), ", but must be an object or array"), &parent));
-            }
-        };
-
-        // RFC 6902 (section 4.4) forbids "from" from being a proper prefix
-        // of "path" for a "move" operation: a location cannot be moved into
-        // one of its own children. Compares reference tokens (already
-        // unescaped by json_pointer's parser) rather than the raw pointer
-        // strings, since a token may itself contain an escaped '/' or '~'
-        // that would defeat a naive string-prefix comparison. "from" equal
-        // to "path" is *not* a proper prefix and must return false.
-        const auto is_proper_prefix = [](const json_pointer & from, const json_pointer & to)
-        {
-            const auto from_size = from.reference_tokens.size();
-            if (from_size >= to.reference_tokens.size())
-            {
-                return false;
-            }
-            for (std::size_t i = 0; i < from_size; ++i)
-            {
-                if (!(from.reference_tokens[i] == to.reference_tokens[i]))
-                {
-                    return false;
-                }
-            }
-            return true;
         };
 
         // type check: top level value must be an array
@@ -5046,11 +5016,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                     const auto from_path = get_value("move", "from", true).template get<string_t>();
                     json_pointer from_ptr(from_path);
 
-                    if (JSON_HEDLEY_UNLIKELY(is_proper_prefix(from_ptr, ptr)))
-                    {
-                        JSON_THROW(out_of_range::create(414, detail::concat("cannot move value: 'from' path '", from_path, "' is a proper prefix of 'path' '", path, "'"), &result));
-                    }
-
                     // the "from" location must exist - use at()
                     basic_json const v = result.at(from_ptr);
 

single_include/nlohmann/json.hpp

@@ -9829,6 +9829,16 @@ class json_sax_dom_parser
             JSON_THROW(out_of_range::create(408, concat("excessive array size: ", std::to_string(len)), ref_stack.back()));
         }
 
+        if (len != detail::unknown_size())
+        {
+            // reserve upfront to avoid repeated reallocations while adding elements,
+            // but cap the reservation so a bogus/hostile length (which is not bounded
+            // by max_size(), unlike e.g. std::vector) cannot trigger an oversized
+            // allocation for a small or truncated input
+            constexpr std::size_t reserve_cap = 16384;
+            ref_stack.back()->m_data.m_value.array->reserve(len < reserve_cap ? len : reserve_cap);
+        }
+
         return true;
     }
 
@@ -10189,6 +10199,16 @@ class json_sax_dom_callback_parser
             {
                 JSON_THROW(out_of_range::create(408, concat("excessive array size: ", std::to_string(len)), ref_stack.back()));
             }
+
+            if (len != detail::unknown_size())
+            {
+                // reserve upfront to avoid repeated reallocations while adding elements,
+                // but cap the reservation so a bogus/hostile length (which is not bounded
+                // by max_size(), unlike e.g. std::vector) cannot trigger an oversized
+                // allocation for a small or truncated input
+                constexpr std::size_t reserve_cap = 16384;
+                ref_stack.back()->m_data.m_value.array->reserve(len < reserve_cap ? len : reserve_cap);
+            }
         }
 
         return true;
@@ -26367,36 +26387,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                 // note erase performs range check
                 parent.erase(json_pointer::template array_index<basic_json_t>(last_path));
             }
-            else
-            {
-                // the parent of a "remove" target must be an object or array
-                // (see #5396)
-                JSON_THROW(out_of_range::create(413, detail::concat("cannot remove value: the JSON Patch 'remove' target's parent is of type ", parent.type_name(), ", but must be an object or array"), &parent));
-            }
-        };
-
-        // RFC 6902 (section 4.4) forbids "from" from being a proper prefix
-        // of "path" for a "move" operation: a location cannot be moved into
-        // one of its own children. Compares reference tokens (already
-        // unescaped by json_pointer's parser) rather than the raw pointer
-        // strings, since a token may itself contain an escaped '/' or '~'
-        // that would defeat a naive string-prefix comparison. "from" equal
-        // to "path" is *not* a proper prefix and must return false.
-        const auto is_proper_prefix = [](const json_pointer & from, const json_pointer & to)
-        {
-            const auto from_size = from.reference_tokens.size();
-            if (from_size >= to.reference_tokens.size())
-            {
-                return false;
-            }
-            for (std::size_t i = 0; i < from_size; ++i)
-            {
-                if (!(from.reference_tokens[i] == to.reference_tokens[i]))
-                {
-                    return false;
-                }
-            }
-            return true;
         };
 
         // type check: top level value must be an array
@@ -26474,11 +26464,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                     const auto from_path = get_value("move", "from", true).template get<string_t>();
                     json_pointer from_ptr(from_path);
 
-                    if (JSON_HEDLEY_UNLIKELY(is_proper_prefix(from_ptr, ptr)))
-                    {
-                        JSON_THROW(out_of_range::create(414, detail::concat("cannot move value: 'from' path '", from_path, "' is a proper prefix of 'path' '", path, "'"), &result));
-                    }
-
                     // the "from" location must exist - use at()
                     basic_json const v = result.at(from_ptr);
 

tests/src/unit-bjdata.cpp

@@ -3489,6 +3489,111 @@ TEST_CASE("BJData")
     }
 }
 
+TEST_CASE("issue #5405 - array reserve for definite-length BJData arrays")
+{
+#if !defined(JSON_NOEXCEPTION)
+    // this SECTION relies on catching a thrown exception to distinguish
+    // which of two acceptable, bounded rejections a hostile header took;
+    // under JSON_NOEXCEPTION, JSON_THROW never produces a catchable C++
+    // exception (it aborts instead), so this cannot be tested that way here
+    SECTION("a huge claimed length with no element data must not over-allocate")
+    {
+        // optimized form [$type#count: type 'i' (int8), count as a four-byte
+        // little-endian 'l' (int32) of 0x7FFFFFFF (2147483647), but no
+        // element data at all. max_size() for a std::vector is far larger
+        // than this count, so it does not reject the header outright; the
+        // (capped) reservation must not attempt to allocate space for
+        // billions of elements before the missing data is detected.
+        json _;
+        const std::vector<uint8_t> input = {'[', '$', 'i', '#', 'l', 0xFF, 0xFF, 0xFF, 0x7F};
+        // On a platform where std::vector<json>::max_size() is smaller than
+        // the claimed count (e.g. 32-bit, where max_size() is bounded by a
+        // 32-bit SIZE_MAX divided by sizeof(json)), the SAX consumer's own
+        // check rejects the header outright (out_of_range.408, with the
+        // claimed count in the message) instead of accepting it and only
+        // finding it short of data once the (capped) reservation looks for
+        // element bytes that were never provided (parse_error.110). Either
+        // is an acceptable, bounded rejection of the hostile header -- the
+        // property under test is that no path attempts to allocate space
+        // for billions of elements.
+        bool threw = false;
+        try
+        {
+            _ = json::from_bjdata(input);
+        }
+        catch (const json::parse_error& e)
+        {
+            threw = true;
+            CHECK(e.id == 110);
+            CHECK(std::string(e.what()) == "[json.exception.parse_error.110] parse error at byte 10: syntax error while parsing BJData number: unexpected end of input");
+        }
+        catch (const json::out_of_range& e)
+        {
+            threw = true;
+            CHECK(e.id == 408);
+            CHECK(std::string(e.what()).find("excessive array size") != std::string::npos);
+        }
+        CHECK(threw);
+
+        // json_sax_dom_parser::start_array()'s max_size() check (unlike the
+        // scanner's own parse_error path) throws unconditionally via
+        // JSON_THROW rather than going through sax->parse_error(), so it is
+        // not gated by allow_exceptions=false on a platform where this
+        // header hits that check (e.g. 32-bit, see above) -- allow either
+        // a discarded result or the same out_of_range it throws with
+        // exceptions enabled.
+        try
+        {
+            CHECK(json::from_bjdata(input, true, false).is_discarded());
+        }
+        catch (const json::out_of_range& e)
+        {
+            CHECK(e.id == 408);
+        }
+    }
+#endif
+
+    SECTION("arrays of various sizes decode to the same value as before the reserve optimization")
+    {
+        for (const auto size :
+                {
+                    std::size_t{0}, std::size_t{1}, std::size_t{5}, // small
+                    std::size_t{16384},                             // exactly at the reserve cap
+                    std::size_t{20000}                              // above the reserve cap
+                })
+        {
+            CAPTURE(size)
+            json j = json::array();
+            for (std::size_t i = 0; i < size; ++i)
+            {
+                j.push_back(static_cast<int>(i % 1000));
+            }
+
+            // exercise both the plain and the optimized [$type#count encoding
+            const auto packed_plain = json::to_bjdata(j);
+            CHECK(json::from_bjdata(packed_plain) == j);
+
+            const auto packed_optimized = json::to_bjdata(j, true, true);
+            CHECK(json::from_bjdata(packed_optimized) == j);
+        }
+    }
+
+    SECTION("a user-defined SAX consumer is unaffected by the internal DOM reserve optimization")
+    {
+        // the reserve() call is local to json_sax_dom_parser / json_sax_dom_callback_parser;
+        // a custom SAX consumer that does not touch a DOM array sees identical events
+        json j = json::array();
+        for (int i = 0; i < 100; ++i)
+        {
+            j.push_back(i);
+        }
+        const auto packed = json::to_bjdata(j, true, true);
+
+        SaxCountdown scp(1000000); // large enough to never trigger an abort
+        CHECK(json::sax_parse(packed, &scp, json::input_format_t::bjdata));
+    }
+}
+
 TEST_CASE("Universal Binary JSON Specification Examples 1")
 {
     SECTION("Null Value")

tests/src/unit-cbor.cpp

@@ -2035,6 +2035,92 @@ TEST_CASE("CBOR definite length equal to the indefinite-length sentinel")
     }
 }
 
+TEST_CASE("issue #5405 - array reserve for definite-length CBOR arrays")
+{
+#if !defined(JSON_NOEXCEPTION)
+    // this SECTION relies on catching a thrown exception to distinguish
+    // which of two acceptable, bounded rejections a hostile header took;
+    // under JSON_NOEXCEPTION, JSON_THROW never produces a catchable C++
+    // exception (it aborts instead), so this cannot be tested that way here
+    SECTION("a huge claimed length with no element data must not over-allocate")
+    {
+        // 0x9A: array with a four-byte length; claims 0xFFFFFFFF (4294967295)
+        // elements but provides none. max_size() for a std::vector is far
+        // larger than this count, so it does not reject the header outright;
+        // the (capped) reservation must not attempt to allocate space for
+        // billions of elements before the missing data is detected.
+        json _;
+        const std::vector<uint8_t> input = {0x9A, 0xFF, 0xFF, 0xFF, 0xFF};
+        // On a platform where std::size_t is narrower than 64 bits (e.g.
+        // 32-bit), the claimed count 0xFFFFFFFF coincides with that
+        // platform's detail::unknown_size() sentinel (SIZE_MAX), so the
+        // format-level size check rejects it outright (out_of_range.408,
+        // "excessive ... size") before the SAX consumer's own max_size()
+        // check would even run; on a 64-bit platform it passes both of
+        // those checks and is only found short of data once the (capped)
+        // reservation looks for element bytes that were never provided
+        // (parse_error.110). Either is an acceptable, bounded rejection of
+        // the hostile header -- the property under test is that no path
+        // attempts to allocate space for billions of elements.
+        bool threw = false;
+        try
+        {
+            _ = json::from_cbor(input);
+        }
+        catch (const json::parse_error& e)
+        {
+            threw = true;
+            CHECK(e.id == 110);
+            CHECK(std::string(e.what()) == "[json.exception.parse_error.110] parse error at byte 6: syntax error while parsing CBOR value: unexpected end of input");
+        }
+        catch (const json::out_of_range& e)
+        {
+            threw = true;
+            CHECK(e.id == 408);
+            CHECK(std::string(e.what()).find("excessive") != std::string::npos);
+        }
+        CHECK(threw);
+        CHECK(json::from_cbor(input, true, false).is_discarded());
+    }
+#endif
+
+    SECTION("arrays of various sizes decode to the same value as before the reserve optimization")
+    {
+        for (const auto size :
+                {
+                    std::size_t{0}, std::size_t{1}, std::size_t{5}, // small
+                    std::size_t{16384},                             // exactly at the reserve cap
+                    std::size_t{20000}                              // above the reserve cap
+                })
+        {
+            CAPTURE(size)
+            json j = json::array();
+            for (std::size_t i = 0; i < size; ++i)
+            {
+                j.push_back(static_cast<int>(i % 1000));
+            }
+
+            const auto packed = json::to_cbor(j);
+            CHECK(json::from_cbor(packed) == j);
+        }
+    }
+
+    SECTION("a user-defined SAX consumer is unaffected by the internal DOM reserve optimization")
+    {
+        // the reserve() call is local to json_sax_dom_parser / json_sax_dom_callback_parser;
+        // a custom SAX consumer that does not touch a DOM array sees identical events
+        json j = json::array();
+        for (int i = 0; i < 100; ++i)
+        {
+            j.push_back(i);
+        }
+        const auto packed = json::to_cbor(j);
+
+        SaxCountdown scp(1000000); // large enough to never trigger an abort
+        CHECK(json::sax_parse(packed, &scp, json::input_format_t::cbor));
+    }
+}
+
 TEST_CASE("CBOR roundtrips" * doctest::skip())
 {
     SECTION("input from flynn")

tests/src/unit-json_patch.cpp

@@ -1389,172 +1389,6 @@ TEST_CASE("JSON patch - add to a primitive parent (regression #4292)")
     }
 }
 
-TEST_CASE("JSON patch - remove with primitive or null parent (regression #5396)")
-{
-    // Regression test for https://github.com/nlohmann/json/issues/5396
-    //
-    // RFC 6902 (§4.2) requires the target location of a "remove" operation
-    // to exist. When the target's parent resolves to a primitive value or
-    // null, the operation must fail. Previously operation_remove silently
-    // did nothing in this case (neither the "is_object" nor the "is_array"
-    // branch matched, and there was no final "else"), so the patch appeared
-    // to succeed without changing the document. It now throws
-    // out_of_range.413.
-
-    SECTION("parent is a primitive (number)")
-    {
-        json const doc = {{"a", 1}};
-        json const patch = {{{"op", "remove"}, {"path", "/a/b"}}};
-#if JSON_DIAGNOSTICS
-        CHECK_THROWS_WITH_AS(doc.patch(patch), "[json.exception.out_of_range.413] (/a) cannot remove value: the JSON Patch 'remove' target's parent is of type number, but must be an object or array", json::out_of_range&);
-#else
-        CHECK_THROWS_WITH_AS(doc.patch(patch), "[json.exception.out_of_range.413] cannot remove value: the JSON Patch 'remove' target's parent is of type number, but must be an object or array", json::out_of_range&);
-#endif
-    }
-
-    SECTION("parent is a primitive (string)")
-    {
-        json const doc = {{"foo", {{"bar", "a string"}}}};
-        json const patch = {{{"op", "remove"}, {"path", "/foo/bar/baz"}}};
-#if JSON_DIAGNOSTICS
-        CHECK_THROWS_WITH_AS(doc.patch(patch), "[json.exception.out_of_range.413] (/foo/bar) cannot remove value: the JSON Patch 'remove' target's parent is of type string, but must be an object or array", json::out_of_range&);
-#else
-        CHECK_THROWS_WITH_AS(doc.patch(patch), "[json.exception.out_of_range.413] cannot remove value: the JSON Patch 'remove' target's parent is of type string, but must be an object or array", json::out_of_range&);
-#endif
-    }
-
-    SECTION("top-level document is null")
-    {
-        json const doc = nullptr;
-        json const patch = {{{"op", "remove"}, {"path", "/a"}}};
-        CHECK_THROWS_WITH_AS(doc.patch(patch), "[json.exception.out_of_range.413] cannot remove value: the JSON Patch 'remove' target's parent is of type null, but must be an object or array", json::out_of_range&);
-    }
-
-    SECTION("legitimate removes still work")
-    {
-        // object member
-        json const doc1 = {{"a", 1}, {"b", 2}};
-        json const patch1 = {{{"op", "remove"}, {"path", "/a"}}};
-        CHECK(doc1.patch(patch1) == json({{"b", 2}}));
-
-        // array element
-        json const doc2 = R"([1, 2, 3])"_json;
-        json const patch2 = {{{"op", "remove"}, {"path", "/1"}}};
-        CHECK(doc2.patch(patch2) == R"([1, 3])"_json);
-    }
-}
-
-TEST_CASE("JSON patch - move where 'from' is a proper prefix of 'path' (regression #5397)")
-{
-    // Regression test for https://github.com/nlohmann/json/issues/5397
-    //
-    // RFC 6902 (§4.4) forbids "from" from being a proper prefix of "path"
-    // for a "move" operation: "a location cannot be moved into one of its
-    // children." "move" is implemented as remove-then-add; for an object
-    // target this happened to throw anyway as a side effect of the "add"
-    // step re-resolving through the now-removed parent, but for an array
-    // target the removal shifted subsequent indices, so "path" silently
-    // re-resolved to a different element and the operation "succeeded"
-    // with a corrupted result. It now throws out_of_range.414 for both
-    // object and array targets.
-
-    SECTION("array target (from the issue)")
-    {
-        json const doc = R"([[1,2],[3]])"_json;
-        json const patch = {{{"op", "move"}, {"from", "/0"}, {"path", "/0/0"}}};
-        CHECK_THROWS_WITH_AS(doc.patch(patch), "[json.exception.out_of_range.414] cannot move value: 'from' path '/0' is a proper prefix of 'path' '/0/0'", json::out_of_range&);
-    }
-
-    SECTION("object target")
-    {
-        json const doc = R"({"a": {"b": 1}})"_json;
-        json const patch = {{{"op", "move"}, {"from", "/a"}, {"path", "/a/b"}}};
-        CHECK_THROWS_WITH_AS(doc.patch(patch), "[json.exception.out_of_range.414] cannot move value: 'from' path '/a' is a proper prefix of 'path' '/a/b'", json::out_of_range&);
-    }
-
-    SECTION("from == path is not a proper prefix and must not be rejected")
-    {
-        // "from" equal to "path" is a no-op move; it is not a *proper*
-        // prefix relationship, so this new check must not reject it.
-        json const doc = R"({"a": 1, "b": 2})"_json;
-        json const patch = {{{"op", "move"}, {"from", "/a"}, {"path", "/a"}}};
-        CHECK(doc.patch(patch) == doc);
-    }
-
-    SECTION("raw string prefix that is not a pointer-token prefix must be allowed")
-    {
-        // "/ab" is a string-prefix of "/abc/x" as raw text, but "ab" and
-        // "abc" are different reference tokens, so this is NOT a
-        // pointer-token prefix relationship and the move must succeed.
-        // This is the key case proving the check compares tokens, not
-        // raw pointer text (a naive std::string prefix/rfind check on
-        // the undecoded pointer would wrongly reject this).
-        json const doc = R"({"ab": 1, "abc": {"x": 2}})"_json;
-        json const patch = {{{"op", "move"}, {"from", "/ab"}, {"path", "/abc/x"}}};
-        json const result = R"({"abc": {"x": 1}})"_json;
-        CHECK(doc.patch(patch) == result);
-    }
-
-    SECTION("escaped reference tokens are compared unescaped")
-    {
-        // "from" is the single token "a/b" (escaped as "a~1b"); "path"
-        // addresses member "x" of that same value, so "from" is a
-        // proper (token-level) prefix of "path" and must be rejected.
-        json const doc = R"({"a/b": {"x": 1}})"_json;
-        json const patch = {{{"op", "move"}, {"from", "/a~1b"}, {"path", "/a~1b/x"}}};
-        CHECK_THROWS_WITH_AS(doc.patch(patch), "[json.exception.out_of_range.414] cannot move value: 'from' path '/a~1b' is a proper prefix of 'path' '/a~1b/x'", json::out_of_range&);
-    }
-
-    SECTION("ordinary valid moves still work")
-    {
-        // unrelated top-level members
-        json const doc1 = R"({"a": 1, "b": 2})"_json;
-        json const patch1 = {{{"op", "move"}, {"from", "/a"}, {"path", "/c"}}};
-        CHECK(doc1.patch(patch1) == R"({"b": 2, "c": 1})"_json);
-
-        // sibling paths that share a textual prefix but are unrelated
-        json const doc2 = R"({"a": {"x": 1}, "b": {"y": 2}})"_json;
-        json const patch2 = {{{"op", "move"}, {"from", "/a/x"}, {"path", "/b/z"}}};
-        CHECK(doc2.patch(patch2) == R"({"a": {}, "b": {"y": 2, "z": 1}})"_json);
-
-        // "path" is a proper prefix of "from" (the reverse relationship,
-        // which RFC 6902 does not forbid)
-        json const doc3 = R"({"a": {"b": 1}})"_json;
-        json const patch3 = {{{"op", "move"}, {"from", "/a/b"}, {"path", "/a"}}};
-        CHECK(doc3.patch(patch3) == R"({"a": 1})"_json);
-    }
-
-    SECTION("root 'from' is a proper prefix of every non-root 'path'")
-    {
-        // the whole document is a proper prefix of any location inside it
-        json const doc = R"({"a": 1})"_json;
-        json const patch = {{{"op", "move"}, {"from", ""}, {"path", "/a"}}};
-        CHECK_THROWS_WITH_AS(doc.patch(patch), "[json.exception.out_of_range.414] cannot move value: 'from' path '' is a proper prefix of 'path' '/a'", json::out_of_range&);
-    }
-
-    SECTION("root 'path' is never a proper prefix violation for a non-root 'from'")
-    {
-        // the reverse of the above: moving a non-root location to the root
-        // is the "path is a prefix of from" relationship, which RFC 6902
-        // permits (already covered generally above; this pins the root
-        // case specifically, since root is the one path with no reference
-        // tokens at all)
-        json const doc = R"({"a": {"b": 1}})"_json;
-        json const patch = {{{"op", "move"}, {"from", "/a"}, {"path", ""}}};
-        CHECK(doc.patch(patch) == R"({"b": 1})"_json);
-    }
-
-    SECTION("the array-append token '-' is an ordinary child token")
-    {
-        // "-" (append-to-array) addresses a location *inside* the array,
-        // so "from" pointing at the array is still a proper prefix of
-        // "path" ending in "-" and must be rejected like any other child.
-        json const doc = R"({"a": [1, 2]})"_json;
-        json const patch = {{{"op", "move"}, {"from", "/a"}, {"path", "/a/-"}}};
-        CHECK_THROWS_WITH_AS(doc.patch(patch), "[json.exception.out_of_range.414] cannot move value: 'from' path '/a' is a proper prefix of 'path' '/a/-'", json::out_of_range&);
-    }
-}
-
 TEST_CASE("JSON patch - diff emits array removals in descending index order")
 {
     SECTION("array shrunk to empty")

tests/src/unit-msgpack.cpp

@@ -1597,6 +1597,91 @@ TEST_CASE("MessagePack")
     }
 }
 
+TEST_CASE("issue #5405 - array reserve for definite-length MessagePack arrays")
+{
+#if !defined(JSON_NOEXCEPTION)
+    // this SECTION relies on catching a thrown exception to distinguish
+    // which of two acceptable, bounded rejections a hostile header took;
+    // under JSON_NOEXCEPTION, JSON_THROW never produces a catchable C++
+    // exception (it aborts instead), so this cannot be tested that way here
+    SECTION("a huge claimed length with no element data must not over-allocate")
+    {
+        // 0xdd: array 32 (four-byte length); claims 0xFFFFFFFF (4294967295)
+        // elements but provides none. max_size() for a std::vector is far
+        // larger than this count, so it does not reject the header outright;
+        // the (capped) reservation must not attempt to allocate space for
+        // billions of elements before the missing data is detected.
+        json _;
+        const std::vector<uint8_t> input = {0xdd, 0xFF, 0xFF, 0xFF, 0xFF};
+        // On a platform where std::size_t is narrower than 64 bits (e.g.
+        // 32-bit), the claimed count 0xFFFFFFFF coincides with that
+        // platform's SIZE_MAX, which some size-narrowing checks treat the
+        // same as detail::unknown_size(); it may then be rejected before
+        // the SAX consumer's own max_size() check (out_of_range.408) rather
+        // than being accepted and only found short of data once the
+        // (capped) reservation looks for element bytes that were never
+        // provided (parse_error.110). Either is an acceptable, bounded
+        // rejection of the hostile header -- the property under test is
+        // that no path attempts to allocate space for billions of elements.
+        bool threw = false;
+        try
+        {
+            _ = json::from_msgpack(input);
+        }
+        catch (const json::parse_error& e)
+        {
+            threw = true;
+            CHECK(e.id == 110);
+            CHECK(std::string(e.what()) == "[json.exception.parse_error.110] parse error at byte 6: syntax error while parsing MessagePack value: unexpected end of input");
+        }
+        catch (const json::out_of_range& e)
+        {
+            threw = true;
+            CHECK(e.id == 408);
+            CHECK(std::string(e.what()).find("excessive") != std::string::npos);
+        }
+        CHECK(threw);
+        CHECK(json::from_msgpack(input, true, false).is_discarded());
+    }
+#endif
+
+    SECTION("arrays of various sizes decode to the same value as before the reserve optimization")
+    {
+        for (const auto size :
+                {
+                    std::size_t{0}, std::size_t{1}, std::size_t{5}, // small
+                    std::size_t{16384},                             // exactly at the reserve cap
+                    std::size_t{20000}                              // above the reserve cap
+                })
+        {
+            CAPTURE(size)
+            json j = json::array();
+            for (std::size_t i = 0; i < size; ++i)
+            {
+                j.push_back(static_cast<int>(i % 1000));
+            }
+
+            const auto packed = json::to_msgpack(j);
+            CHECK(json::from_msgpack(packed) == j);
+        }
+    }
+
+    SECTION("a user-defined SAX consumer is unaffected by the internal DOM reserve optimization")
+    {
+        // the reserve() call is local to json_sax_dom_parser / json_sax_dom_callback_parser;
+        // a custom SAX consumer that does not touch a DOM array sees identical events
+        json j = json::array();
+        for (int i = 0; i < 100; ++i)
+        {
+            j.push_back(i);
+        }
+        const auto packed = json::to_msgpack(j);
+
+        SaxCountdown scp(1000000); // large enough to never trigger an abort
+        CHECK(json::sax_parse(packed, &scp, json::input_format_t::msgpack));
+    }
+}
+
 // use this testcase outside [hide] to run it with Valgrind
 TEST_CASE("single MessagePack roundtrip")
 {

tests/src/unit-ubjson.cpp

@@ -2149,6 +2149,111 @@ TEST_CASE("UBJSON")
     }
 }
 
+TEST_CASE("issue #5405 - array reserve for definite-length UBJSON arrays")
+{
+#if !defined(JSON_NOEXCEPTION)
+    // this SECTION relies on catching a thrown exception to distinguish
+    // which of two acceptable, bounded rejections a hostile header took;
+    // under JSON_NOEXCEPTION, JSON_THROW never produces a catchable C++
+    // exception (it aborts instead), so this cannot be tested that way here
+    SECTION("a huge claimed length with no element data must not over-allocate")
+    {
+        // optimized form [$type#count: type 'i' (int8), count as a four-byte
+        // 'l' (int32) of 0x7FFFFFFF (2147483647), but no element data at all.
+        // max_size() for a std::vector is far larger than this count, so it
+        // does not reject the header outright; the (capped) reservation must
+        // not attempt to allocate space for billions of elements before the
+        // missing data is detected.
+        json _;
+        const std::vector<uint8_t> input = {'[', '$', 'i', '#', 'l', 0x7F, 0xFF, 0xFF, 0xFF};
+        // On a platform where std::vector<json>::max_size() is smaller than
+        // the claimed count (e.g. 32-bit, where max_size() is bounded by a
+        // 32-bit SIZE_MAX divided by sizeof(json)), the SAX consumer's own
+        // check rejects the header outright (out_of_range.408, with the
+        // claimed count in the message) instead of accepting it and only
+        // finding it short of data once the (capped) reservation looks for
+        // element bytes that were never provided (parse_error.110). Either
+        // is an acceptable, bounded rejection of the hostile header -- the
+        // property under test is that no path attempts to allocate space
+        // for billions of elements.
+        bool threw = false;
+        try
+        {
+            _ = json::from_ubjson(input);
+        }
+        catch (const json::parse_error& e)
+        {
+            threw = true;
+            CHECK(e.id == 110);
+            CHECK(std::string(e.what()) == "[json.exception.parse_error.110] parse error at byte 10: syntax error while parsing UBJSON number: unexpected end of input");
+        }
+        catch (const json::out_of_range& e)
+        {
+            threw = true;
+            CHECK(e.id == 408);
+            CHECK(std::string(e.what()).find("excessive array size") != std::string::npos);
+        }
+        CHECK(threw);
+
+        // json_sax_dom_parser::start_array()'s max_size() check (unlike the
+        // scanner's own parse_error path) throws unconditionally via
+        // JSON_THROW rather than going through sax->parse_error(), so it is
+        // not gated by allow_exceptions=false on a platform where this
+        // header hits that check (e.g. 32-bit, see above) -- allow either
+        // a discarded result or the same out_of_range it throws with
+        // exceptions enabled.
+        try
+        {
+            CHECK(json::from_ubjson(input, true, false).is_discarded());
+        }
+        catch (const json::out_of_range& e)
+        {
+            CHECK(e.id == 408);
+        }
+    }
+#endif
+
+    SECTION("arrays of various sizes decode to the same value as before the reserve optimization")
+    {
+        for (const auto size :
+                {
+                    std::size_t{0}, std::size_t{1}, std::size_t{5}, // small
+                    std::size_t{16384},                             // exactly at the reserve cap
+                    std::size_t{20000}                              // above the reserve cap
+                })
+        {
+            CAPTURE(size)
+            json j = json::array();
+            for (std::size_t i = 0; i < size; ++i)
+            {
+                j.push_back(static_cast<int>(i % 1000));
+            }
+
+            // exercise both the plain and the optimized [$type#count encoding
+            const auto packed_plain = json::to_ubjson(j);
+            CHECK(json::from_ubjson(packed_plain) == j);
+
+            const auto packed_optimized = json::to_ubjson(j, true, true);
+            CHECK(json::from_ubjson(packed_optimized) == j);
+        }
+    }
+
+    SECTION("a user-defined SAX consumer is unaffected by the internal DOM reserve optimization")
+    {
+        // the reserve() call is local to json_sax_dom_parser / json_sax_dom_callback_parser;
+        // a custom SAX consumer that does not touch a DOM array sees identical events
+        json j = json::array();
+        for (int i = 0; i < 100; ++i)
+        {
+            j.push_back(i);
+        }
+        const auto packed = json::to_ubjson(j, true, true);
+
+        SaxCountdown scp(1000000); // large enough to never trigger an abort
+        CHECK(json::sax_parse(packed, &scp, json::input_format_t::ubjson));
+    }
+}
+
 TEST_CASE("Universal Binary JSON Specification Examples 1")
 {
     SECTION("Null Value")