Fix #262: reject non-positive pricePerToken when parsing a babel fee box

appkit/src/test/scala/org/ergoplatform/appkit/BabelFeeSpec.scala

@@ -1,6 +1,7 @@
 package org.ergoplatform.appkit
 
 import org.ergoplatform.appkit.babelfee.{BabelFeeBoxContract, BabelFeeBoxState, BabelFeeOperations}
+import org.ergoplatform.appkit.impl.ErgoTreeContract
 import org.ergoplatform.sdk.{ErgoId, ErgoToken, SecretString}
 import org.scalatest.matchers.should.Matchers
 import org.scalatest.propspec.AnyPropSpec
@@ -104,6 +105,39 @@ class BabelFeeSpec extends AnyPropSpec with Matchers with ScalaCheckDrivenProper
     }
   }
 
+  property("babel fee box with non-positive price is rejected") {
+    val ergoClient = createMockedErgoClient(MockData(Nil, Nil))
+    ergoClient.execute { ctx: BlockchainContext =>
+      val creator = address
+
+      // builds a babel fee box directly (bypassing BabelFeeBoxStateBuilder, which
+      // validates the price) the way a box created outside appkit appears on chain
+      def babelBoxWithPrice(price: Long): InputBox = {
+        ctx.newTxBuilder.outBoxBuilder
+          .value(Parameters.OneErg)
+          .contract(new ErgoTreeContract(
+            new BabelFeeBoxContract(ErgoId.create(mockTokenId)).getErgoTree, ctx.getNetworkType))
+          .registers(ErgoValue.of(SigmaProp.createFromAddress(creator)), ErgoValue.of(price))
+          .build().convertToInputWith(mockTokenId, 0)
+      }
+
+      // a price of zero divides by zero in getMaxTokenAmountToBuy and
+      // calcTokensToSellForErgAmount, a negative price makes them return nonsense:
+      // both must be rejected when the state is parsed from a box
+      intercept[IllegalStateException] {
+        new BabelFeeBoxState(babelBoxWithPrice(0))
+      }
+      intercept[IllegalStateException] {
+        new BabelFeeBoxState(babelBoxWithPrice(-1))
+      }
+
+      // a regular price still parses and calculates as before
+      val state = new BabelFeeBoxState(babelBoxWithPrice(Parameters.MinFee))
+      state.getPricePerToken shouldBe Parameters.MinFee
+      state.calcTokensToSellForErgAmount(Parameters.MinFee) shouldBe 1
+    }
+  }
+
   property("fetch babel fee boxes") {
     val ergoClient = createMockedErgoClient(MockData(Nil, Nil))
     ergoClient.execute { ctx: BlockchainContext =>

lib-api/src/main/java/org/ergoplatform/appkit/babelfee/BabelFeeBoxState.java

@@ -37,6 +37,13 @@ public BabelFeeBoxState(TransactionBox ergoBox) {
         List<ErgoValue<?>> registers = ergoBox.getRegisters();
         boxCreator = new SigmaProp((sigma.SigmaProp) registers.get(0).getValue());
         pricePerToken = (Long) registers.get(1).getValue();
+        if (pricePerToken <= 0) {
+            // a box can be created outside appkit with any register values, so unlike
+            // BabelFeeBoxStateBuilder this parse path cannot assume a valid price:
+            // zero would divide by zero in getMaxTokenAmountToBuy and
+            // calcTokensToSellForErgAmount, a negative price makes them return nonsense
+            throw new IllegalStateException("Price per token of babel fee box must be greater than 0, but was " + pricePerToken);
+        }
         this.tokenId = new BabelFeeBoxContract(ergoBox.getErgoTree()).getTokenId();
 
         if (!ergoBox.getTokens().isEmpty()) {
@@ -52,6 +59,8 @@ public BabelFeeBoxState(TransactionBox ergoBox) {
     }
 
     BabelFeeBoxState(long pricePerToken, ErgoId tokenId, SigmaProp boxCreator, long value, long tokenAmount) {
+        if (pricePerToken <= 0)
+            throw new IllegalArgumentException("pricePerToken must be greater than 0");
         this.pricePerToken = pricePerToken;
         this.tokenId = tokenId;
         this.boxCreator = boxCreator;