fixDisallowIncomingV1: allow issuers to authorize trust lines (#4721)

Context: The `DisallowIncoming` amendment provides an option to block
incoming trust lines from reaching your account. The
asfDisallowIncomingTrustline AccountSet Flag, when enabled, prevents any
incoming trust line from being created. However, it was too restrictive:
it would block an issuer from authorizing a trust line, even if the
trust line already exists. Consider:

1. Issuer sets asfRequireAuth on their account.
2. User sets asfDisallowIncomingTrustline on their account.
3. User submits tx to SetTrust to Issuer.

At this point, without `fixDisallowIncomingV1` active, the issuer would
not be able to authorize the trust line.

The `fixDisallowIncomingV1` amendment, once activated, allows an issuer
to authorize a trust line even after the user sets the
asfDisallowIncomingTrustline flag, as long as the trust line already
exists.

src/ripple/app/tx/impl/SetTrust.cpp

@@ -140,7 +140,20 @@ SetTrust::preclaim(PreclaimContext const& ctx)
             return tecNO_DST;
 
         if (sleDst->getFlags() & lsfDisallowIncomingTrustline)
-            return tecNO_PERMISSION;
+        {
+            // The original implementation of featureDisallowIncoming was
+            // too restrictive.  If
+            //   o fixDisallowIncomingV1 is enabled and
+            //   o The trust line already exists
+            // Then allow the TrustSet.
+            if (ctx.view.rules().enabled(fixDisallowIncomingV1) &&
+                ctx.view.exists(keylet::line(id, uDstAccountID, currency)))
+            {
+                // pass
+            }
+            else
+                return tecNO_PERMISSION;
+        }
     }
 
     // If destination is AMM and the trustline doesn't exist then only

src/ripple/protocol/Feature.h

@@ -74,7 +74,7 @@ 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 = 62;
+static constexpr std::size_t numFeatures = 63;
 
 /** Amendments that this server supports and the default voting behavior.
    Whether they are enabled depends on the Rules defined in the validated
@@ -349,6 +349,7 @@ extern uint256 const fixNFTokenRemint;
 extern uint256 const fixReducedOffersV1;
 extern uint256 const featureClawback;
 extern uint256 const featureXChainBridge;
+extern uint256 const fixDisallowIncomingV1;
 
 }  // namespace ripple
 

src/ripple/protocol/impl/Feature.cpp

@@ -456,6 +456,7 @@ REGISTER_FIX    (fixReducedOffersV1,            Supported::yes, VoteBehavior::De
 REGISTER_FEATURE(Clawback,                      Supported::yes, VoteBehavior::DefaultNo);
 REGISTER_FEATURE(AMM,                           Supported::yes, VoteBehavior::DefaultNo);
 REGISTER_FEATURE(XChainBridge,                  Supported::yes, VoteBehavior::DefaultNo);
+REGISTER_FIX(fixDisallowIncomingV1,             Supported::yes, VoteBehavior::DefaultNo);
 
 // The following amendments are obsolete, but must remain supported
 // because they could potentially get enabled.

src/test/app/SetTrust_test.cpp

@@ -275,6 +275,48 @@ class SetTrust_test : public beast::unit_test::suite
             BEAST_EXPECT(!(flags & lsfDisallowIncomingTrustline));
         }
 
+        // fixDisallowIncomingV1
+        {
+            for (bool const withFix : {true, false})
+            {
+                auto const amend = withFix
+                    ? features | disallowIncoming
+                    : (features | disallowIncoming) - fixDisallowIncomingV1;
+
+                Env env{*this, amend};
+                auto const dist = Account("dist");
+                auto const gw = Account("gw");
+                auto const USD = gw["USD"];
+                auto const distUSD = dist["USD"];
+
+                env.fund(XRP(1000), gw, dist);
+                env.close();
+
+                env(fset(gw, asfRequireAuth));
+                env.close();
+
+                env(fset(dist, asfDisallowIncomingTrustline));
+                env.close();
+
+                env(trust(dist, USD(10000)));
+                env.close();
+
+                // withFix: can set trustline
+                // withOutFix: cannot set trustline
+                auto const trustResult =
+                    withFix ? ter(tesSUCCESS) : ter(tecNO_PERMISSION);
+                env(trust(gw, distUSD(10000)),
+                    txflags(tfSetfAuth),
+                    trustResult);
+                env.close();
+
+                auto const txResult =
+                    withFix ? ter(tesSUCCESS) : ter(tecPATH_DRY);
+                env(pay(gw, dist, USD(1000)), txResult);
+                env.close();
+            }
+        }
+
         Env env{*this, features | disallowIncoming};
 
         auto const gw = Account{"gateway"};