Use best rate when ILP packet amount is zero

src/lib/quoter.js

@@ -141,6 +141,18 @@ class Quoter {
     }, liquidityQuote)
   }
 
+  async findBestPathForSourceAmount (sourceLedger, destination, sourceAmount) {
+    const hop = this.localTables.findBestHopForSourceAmount(
+      sourceLedger, destination, sourceAmount)
+
+    return {
+      destinationLedger: hop.bestRoute.nextLedger,
+      destinationCreditAccount: hop.bestRoute.isLocal ? destination : hop.bestHop,
+      sourceAmount: sourceAmount.toString(),
+      destinationAmount: hop.bestRoute.curve.amountAt(sourceAmount).toString()
+    }
+  }
+
   /**
    * Note that this function, like most of the ilp-connector code, uses the terms
    * source, destination, and final, to refer to what we would normally call
@@ -172,8 +184,7 @@ class Quoter {
       destinationLedger: quote.route.nextLedger,
       destinationCreditAccount: quote.hop,
       sourceAmount: quote.sourceAmount,
-      destinationAmount: headCurve.amountAt(quote.sourceAmount).toString(),
-      finalAmount: quote.liquidityCurve.amountAt(quote.sourceAmount).toString()
+      destinationAmount: headCurve.amountAt(quote.sourceAmount).toString()
     }
   }
 }

src/lib/route-builder.js

@@ -176,8 +176,12 @@ class RouteBuilder {
       ilpPacket.account, ilpPacket.amount)
 
     const sourceLedger = sourceTransfer.ledger
-    const nextHop = await this.quoter.findBestPathForFinalAmount(
-      sourceLedger, ilpPacket.account, ilpPacket.amount)
+
+    // If the ILP amount field is zero, it means we should forward the maximum
+    // we are willing to.
+    const nextHop = (ilpPacket.amount === '0')
+      ? await this.quoter.findBestPathForSourceAmount(sourceLedger, ilpPacket.account, sourceTransfer.amount)
+      : await this.quoter.findBestPathForFinalAmount(sourceLedger, ilpPacket.account, ilpPacket.amount)
     if (!nextHop) {
       log.info('could not find quote for source transfer: ' + JSON.stringify(sourceTransfer))
       throw new IncomingTransferError(ilpErrors.F02_Unreachable({

test/paymentsSpec.js

@@ -280,6 +280,37 @@ describe('Payments', function () {
     })
   })
 
+  it('uses best rate when ilp packet amount = 0', async function () {
+    const sendSpy = sinon.spy(this.mockPlugin2, 'sendTransfer')
+    await this.mockPlugin1.emitAsync('incoming_prepare', {
+      id: '5857d460-2a46-4545-8311-1539d99e78e8',
+      direction: 'incoming',
+      ledger: 'mock.test1.',
+      amount: '100',
+      executionCondition: 'ni:///sha-256;I3TZF5S3n0-07JWH0s8ArsxPmVP6s-0d0SqxR6C3Ifk?fpt=preimage-sha-256&cost=6',
+      expiresAt: (new Date(START_DATE + 1000)).toISOString(),
+      ilp: packet.serializeIlpPayment({
+        account: 'mock.test2.bob',
+        amount: '0'
+      }).toString('base64')
+    })
+
+    sinon.assert.calledOnce(sendSpy)
+    sinon.assert.calledWithMatch(sendSpy, {
+      direction: 'outgoing',
+      ledger: 'mock.test2.',
+      to: 'mock.test2.bob',
+      amount: '94',
+      executionCondition: 'ni:///sha-256;I3TZF5S3n0-07JWH0s8ArsxPmVP6s-0d0SqxR6C3Ifk?fpt=preimage-sha-256&cost=6',
+      expiresAt: (new Date(START_DATE)).toISOString(),
+      noteToSelf: {
+        source_transfer_id: '5857d460-2a46-4545-8311-1539d99e78e8',
+        source_transfer_ledger: 'mock.test1.',
+        source_transfer_amount: '100'
+      }
+    })
+  })
+
   it('supports optimistic mode', async function () {
     const sendSpy = sinon.spy(this.mockPlugin2, 'sendTransfer')
     await this.mockPlugin1.emitAsync('incoming_transfer', {

test/routeBuilderSpec.js

@@ -168,6 +168,23 @@ describe('RouteBuilder', function () {
       assert.deepEqual(destinationTransfer.ilp, ilpPacket)
     })
 
+    it('uses best rate when ilp packet amount = 0', async function () {
+      const ilpPacket = packet.serializeIlpPayment({
+        account: bobB,
+        amount: '0'
+      }).toString('base64')
+      const destinationTransfer = await this.builder.getDestinationTransfer({
+        id: 'fd7ecefd-8eb8-4e16-b7c8-b67d9d6995f5',
+        ledger: ledgerA,
+        direction: 'incoming',
+        account: aliceA,
+        amount: '100',
+        ilp: ilpPacket
+      })
+      assert.deepEqual(destinationTransfer.amount, '50')
+      assert.deepEqual(destinationTransfer.to, 'eur-ledger.bob')
+    })
+
     it('throws "Insufficient Source Amount" when the amount is too low', async function () {
       const ilpPacket = packet.serializeIlpPayment({
         account: bobB,