Remove directory size limit (#5935)

This change introduces the `fixDirectoryLimit` amendment to remove the directory pages limit. We found that the directory size limit is easier to hit than originally assumed, and there is no good reason to keep this limit, since the object reserve provides the necessary incentive to avoid creating unnecessary objects on the ledger.

src/libxrpl/ledger/ApplyView.cpp

@@ -22,6 +22,9 @@
 #include <xrpl/ledger/ApplyView.h>
 #include <xrpl/protocol/Protocol.h>
 
+#include <limits>
+#include <type_traits>
+
 namespace ripple {
 
 std::optional<std::uint64_t>
@@ -91,8 +94,21 @@ ApplyView::dirAdd(
         return page;
     }
 
+    // We rely on modulo arithmetic of unsigned integers (guaranteed in
+    // [basic.fundamental] paragraph 2) to detect page representation overflow.
+    // For signed integers this would be UB, hence static_assert here.
+    static_assert(std::is_unsigned_v<decltype(page)>);
+    // Defensive check against breaking changes in compiler.
+    static_assert([]<typename T>(std::type_identity<T>) constexpr -> T {
+        T tmp = std::numeric_limits<T>::max();
+        return ++tmp;
+    }(std::type_identity<decltype(page)>{}) == 0);
+    ++page;
     // Check whether we're out of pages.
-    if (++page >= dirNodeMaxPages)
+    if (page == 0)
+        return std::nullopt;
+    if (!rules().enabled(fixDirectoryLimit) &&
+        page >= dirNodeMaxPages)  // Old pages limit
         return std::nullopt;
 
     // We are about to create a new node; we'll link it to