Address rare corruption of NFTokenPage linked list (#4945)

* Add fixNFTokenPageLinks amendment:

It was discovered that under rare circumstances the links between
NFTokenPages could be removed.  If this happens, then the
account_objects and account_nfts RPC commands under-report the
NFTokens owned by an account.

The fixNFTokenPageLinks amendment does the following to address
the problem:

- It fixes the underlying problem so no further broken links
  should be created.
- It adds Invariants so, if such damage were introduced in the
  future, an invariant would stop it.
- It adds a new FixLedgerState transaction that repairs
  directories that were damaged in this fashion.
- It adds unit tests for all of it.

include/xrpl/protocol/Feature.h

@@ -80,7 +80,7 @@ namespace detail {
 // Feature.cpp. Because it's only used to reserve storage, and determine how
 // large to make the FeatureBitset, it MAY be larger. It MUST NOT be less than
 // the actual number of amendments. A LogicError on startup will verify this.
-static constexpr std::size_t numFeatures = 101;
+static constexpr std::size_t numFeatures = 102;
 
 /** Amendments that this server supports and the default voting behavior.
    Whether they are enabled depends on the Rules defined in the validated
@@ -395,6 +395,7 @@ extern uint256 const fixReducedOffersV2;
 extern uint256 const fixEnforceNFTokenTrustline;
 extern uint256 const fixInnerObjTemplate2;
 extern uint256 const featureInvariantsV1_1;
+extern uint256 const fixNFTokenPageLinks;
 
 }  // namespace ripple
 

include/xrpl/protocol/SField.h

@@ -384,6 +384,7 @@ extern SF_UINT16 const sfHookEmitCount;
 extern SF_UINT16 const sfHookExecutionIndex;
 extern SF_UINT16 const sfHookApiVersion;
 extern SF_UINT16 const sfDiscountedFee;
+extern SF_UINT16 const sfLedgerFixType;
 
 // 32-bit integers (common)
 extern SF_UINT32 const sfNetworkID;

include/xrpl/protocol/TER.h

@@ -190,6 +190,7 @@ enum TEFcodes : TERUnderlyingType {
     tefPAST_IMPORT_VL_SEQ,
     tefNONDIR_EMIT,
     tefIMPORT_BLACKHOLED,
+    tefINVALID_LEDGER_FIX_TYPE,
 };
 
 //------------------------------------------------------------------------------

include/xrpl/protocol/TxFormats.h

@@ -203,6 +203,9 @@ enum TxType : std::uint16_t
     /** This transaction type deletes an Oracle instance */
     ttORACLE_DELETE = 61,
 
+    /** This transaction type fixes a problem in the ledger state */
+    ttLEDGER_STATE_FIX = 62,
+
     /* A note attaching transactor that allows the owner or issuer (on a object by object basis) to attach remarks */
     ttREMARKS_SET = 94,
 

include/xrpl/protocol/jss.h

@@ -136,6 +136,7 @@ JSS(NFTokenAcceptOffer);       // transaction type.
 JSS(NFTokenCancelOffer);       // transaction type.
 JSS(NFTokenCreateOffer);       // transaction type.
 JSS(NFTokenPage);              // ledger type.
+JSS(LedgerStateFix);           // transaction type.
 JSS(LPTokenOut);               // in: AMM Liquidity Provider deposit tokens
 JSS(LPTokenIn);                // in: AMM Liquidity Provider withdraw tokens
 JSS(LPToken);                  // out: AMM Liquidity Provider tokens info