Experiment: Add invariant to enforce directory node population

- Experiment: Always delete the root

include/xrpl/ledger/helpers/NFTokenHelpers.h

@@ -20,6 +20,10 @@ removeTokenOffersWithLimit(
     Keylet const& directory,
     std::size_t maxDeletableOffers);
 
+/** Returns tesSUCCESS if NFToken has few enough offers that it can be burned */
+TER
+notTooManyOffers(ReadView const& view, uint256 const& nftokenID);
+
 /** Finds the specified token in the owner's token directory. */
 std::optional<STObject>
 findToken(ReadView const& view, AccountID const& owner, uint256 const& nftokenID);

include/xrpl/tx/invariants/InvariantCheck.h

@@ -372,6 +372,22 @@ public:
     finalize(STTx const&, TER const, XRPAmount const, ReadView const&, beast::Journal const&);
 };
 
+/**
+ * @brief Invariants: An account's directory should never be empty
+ *
+ */
+class NoEmptyDirectory
+{
+    bool bad_ = false;
+
+public:
+    void
+    visitEntry(bool, std::shared_ptr<SLE const> const&, std::shared_ptr<SLE const> const&);
+
+    bool
+    finalize(STTx const&, TER const, XRPAmount const, ReadView const&, beast::Journal const&);
+};
+
 // additional invariant checks can be declared above and then added to this
 // tuple
 using InvariantChecks = std::tuple<
@@ -399,7 +415,8 @@ using InvariantChecks = std::tuple<
     ValidLoanBroker,
     ValidLoan,
     ValidVault,
-    ValidMPTPayment>;
+    ValidMPTPayment,
+    NoEmptyDirectory>;
 
 /**
  * @brief get a tuple of all invariant checks

src/libxrpl/ledger/ApplyView.cpp

@@ -244,6 +244,7 @@ ApplyView::emptyDirDelete(Keylet const& directory)
 bool
 ApplyView::dirRemove(Keylet const& directory, std::uint64_t page, uint256 const& key, bool keepRoot)
 {
+    keepRoot = false;
     auto node = peek(keylet::page(directory, page));
 
     if (!node)

src/libxrpl/ledger/helpers/NFTokenHelpers.cpp

@@ -607,6 +607,33 @@ removeTokenOffersWithLimit(ApplyView& view, Keylet const& directory, std::size_t
     return deletedOffersCount;
 }
 
+TER
+notTooManyOffers(ReadView const& view, uint256 const& nftokenID)
+{
+    std::size_t totalOffers = 0;
+
+    {
+        Dir const buys(view, keylet::nft_buys(nftokenID));
+        for (auto iter = buys.begin(); iter != buys.end(); iter.next_page())
+        {
+            totalOffers += iter.page_size();
+            if (totalOffers > maxDeletableTokenOfferEntries)
+                return tefTOO_BIG;
+        }
+    }
+
+    {
+        Dir const sells(view, keylet::nft_sells(nftokenID));
+        for (auto iter = sells.begin(); iter != sells.end(); iter.next_page())
+        {
+            totalOffers += iter.page_size();
+            if (totalOffers > maxDeletableTokenOfferEntries)
+                return tefTOO_BIG;
+        }
+    }
+    return tesSUCCESS;
+}
+
 bool
 deleteTokenOffer(ApplyView& view, std::shared_ptr<SLE> const& offer)
 {

src/libxrpl/tx/invariants/InvariantCheck.cpp

@@ -1025,4 +1025,42 @@ NoModifiedUnmodifiableFields::finalize(
     return true;
 }
 
+//------------------------------------------------------------------------------
+
+void
+NoEmptyDirectory::visitEntry(
+    bool isDelete,
+    std::shared_ptr<SLE const> const& before,
+    std::shared_ptr<SLE const> const& after)
+{
+    if (isDelete)
+        return;
+    if (before && before->getType() != ltDIR_NODE)
+        return;
+    if (after && after->getType() != ltDIR_NODE)
+        return;
+    if (!after->isFieldPresent(sfOwner))
+        // Not an account dir
+        return;
+
+    bad_ = after->at(sfIndexes).empty();
+}
+
+bool
+NoEmptyDirectory::finalize(
+    STTx const& tx,
+    TER const result,
+    XRPAmount const,
+    ReadView const& view,
+    beast::Journal const& j)
+{
+    if (bad_)
+    {
+        JLOG(j.fatal()) << "Invariant failed: empty owner directory.";
+        return false;
+    }
+
+    return true;
+}
+
 }  // namespace xrpl