Fix self-aliasing in json_pointer::operator/= and patch_inplace (#5936)

* Fix self-aliasing in json_pointer::operator/= and patch_inplace

json_pointer::operator/=(const json_pointer&) passed iterators into its
own token vector to insert() when a pointer was appended to itself
(p /= p), which is undefined behavior; libstdc++ produced "/a/b//" for
"/a/b". The pointer now appends a copy in that case.

basic_json::patch_inplace iterated the patch while changing *this, so
j.patch_inplace(j) read freed memory. It now applies a copy of the
patch when the patch is the value itself. A patch stored inside the
value remains undefined behavior and is documented as such; copying the
patch unconditionally would double the time of a large patch.

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

* Fix JSON_DIAGNOSTICS test crash and clang-tidy hit in patch_inplace self-aliasing

The spare-capacity test reallocated non-empty elements through get_ref(), which leaves stale parent pointers under JSON_DIAGNOSTICS; reserve while the array is empty instead. Suppress performance-unnecessary-copy-initialization for the copy that is needed because *this is modified.

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

* Detect a patch stored inside the patched value with JSON_DIAGNOSTICS

patch_inplace applied a copy only when the patch was the value itself. With JSON_DIAGNOSTICS the parent pointers make it cheap to also detect a patch nested inside the value, so that case is handled the same way. Without diagnostics it stays documented undefined behavior.

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

* Silence misc-const-correctness for patch_aliases_this

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

* Fix clang-tidy findings in the operator/= self-append test

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

---------

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

include/nlohmann/json.hpp

@@ -6950,6 +6950,25 @@ public:
     /// @sa https://json.nlohmann.me/api/basic_json/patch/
     void patch_inplace(const basic_json& json_patch)
     {
+        // the operations below change *this while iterating json_patch: apply
+        // a copy when a value is applied as patch to itself or, with
+        // JSON_DIAGNOSTICS, to a value stored inside it
+        // NOLINTNEXTLINE(misc-const-correctness): only the JSON_DIAGNOSTICS build modifies it
+        bool patch_aliases_this = (&json_patch == this);
+#if JSON_DIAGNOSTICS
+        for (const basic_json* p = json_patch.m_parent; !patch_aliases_this && p != nullptr; p = p->m_parent)
+        {
+            patch_aliases_this = (p == this);
+        }
+#endif
+        if (JSON_HEDLEY_UNLIKELY(patch_aliases_this))
+        {
+            // NOLINTNEXTLINE(performance-unnecessary-copy-initialization): *this is modified below
+            const basic_json json_patch_copy = json_patch;
+            patch_inplace(json_patch_copy);
+            return;
+        }
+
         basic_json& result = *this;
         // the valid JSON Patch operations
         enum class patch_operations {add, remove, replace, move, copy, test, invalid};