Merge pull request #7812 from BitGo/WIN-8461

fix(sdk-coin-flrp): implement dynamic fee calculation for C-chain import transactions

Author: mohd-kashif

modules/sdk-coin-flrp/src/lib/ImportInCTxBuilder.ts

@@ -164,14 +164,32 @@ export class ImportInCTxBuilder extends AtomicInCTransactionBuilder {
 
     const { inputs, amount, credentials } = this.createInputs();
 
-    // Calculate fee
+    // Calculate import cost units (matching AVAXP's costImportTx approach)
+    // Create a temporary transaction to calculate the actual cost units
+    const tempOutput = new evmSerial.Output(
+      new Address(this.transaction._to[0]),
+      new BigIntPr(amount),
+      new Id(new Uint8Array(Buffer.from(this.transaction._assetId, 'hex')))
+    );
+    const tempImportTx = new evmSerial.ImportTx(
+      new Int(this.transaction._networkID),
+      new Id(new Uint8Array(Buffer.from(this.transaction._blockchainID, 'hex'))),
+      new Id(new Uint8Array(this._externalChainId)),
+      inputs,
+      [tempOutput]
+    );
+
+    // Calculate the import cost units (matches AVAXP's feeSize from costImportTx)
+    const feeSize = this.calculateImportCost(tempImportTx);
+
+    // Multiply feeRate by cost units (matching AVAXP: fee = feeRate.muln(feeSize))
     const feeRate = BigInt(this.transaction._fee.feeRate);
-    const feeSize = this.calculateFeeSize();
     const fee = feeRate * BigInt(feeSize);
+
     this.transaction._fee.fee = fee.toString();
     this.transaction._fee.size = feeSize;
 
-    // Create EVM output using proper FlareJS class
+    // Create EVM output using proper FlareJS class with amount minus fee
     const output = new evmSerial.Output(
       new Address(this.transaction._to[0]),
       new BigIntPr(amount - fee),
@@ -274,7 +292,43 @@ export class ImportInCTxBuilder extends AtomicInCTransactionBuilder {
   }
 
   /**
-   * Calculate the fee size for the transaction
+   * Calculate the import cost for C-chain import transactions
+   * Matches AVAXP's costImportTx formula:
+   * - Base byte cost: transactionSize * txBytesGas (1 gas per byte)
+   * - Per-input cost: numInputs * costPerSignature (1000 per signature) * threshold
+   * - Fixed fee: 10000
+   *
+   * This returns cost "units" to be multiplied by feeRate, matching AVAXP's approach:
+   * AVAXP: fee = feeRate.muln(costImportTx(tx))
+   * FLRP:  fee = feeRate * calculateImportCost(tx)
+   *
+   * @param tx The ImportTx to calculate the cost for
+   * @returns The total cost units
+   */
+  private calculateImportCost(tx: evmSerial.ImportTx): number {
+    const codec = avmSerial.getAVMManager().getDefaultCodec();
+    const txBytes = tx.toBytes(codec);
+
+    // Base byte cost: 1 gas per byte (matching AVAX txBytesGas)
+    const txBytesGas = 1;
+    let bytesCost = txBytes.length * txBytesGas;
+
+    // Per-input cost: costPerSignature (1000) per signature
+    const costPerSignature = 1000;
+    const numInputs = tx.importedInputs.length;
+    const numSignatures = this.transaction._threshold; // Each input requires threshold signatures
+    const inputCost = numInputs * costPerSignature * numSignatures;
+    bytesCost += inputCost;
+
+    // Fixed fee component
+    const fixedFee = 10000;
+    const totalCost = bytesCost + fixedFee;
+
+    return totalCost;
+  }
+
+  /**
+   * Calculate the fee size for the transaction (for backwards compatibility)
    * For C-chain imports, the feeRate is treated as an absolute fee value
    */
   private calculateFeeSize(tx?: evmSerial.ImportTx): number {

modules/sdk-coin-flrp/test/resources/transactionData/importInC.ts

@@ -1,13 +1,13 @@
 export const IMPORT_IN_C = {
-  txhash: '2we2yuz575k7BnVgX4AdiheL9xTDpVSi8f8tRpD4C7SwPxR7YB',
+  txhash: '2Q5RkxF2eRK3KCzDaijoScyunahbEvt6ai6YZipmShQTPryfky',
   unsignedHex:
-    '0x0000000000000000007278db5c30bed04c05ce209179812850bbb3fe6d46d7eef3744d814c0da5552479000000000000000000000000000000000000000000000000000000000000000000000001fcea1c0e2cb7e3d77c993eb74ee05d98c24325ded1918e8a0595c96a789e2f790000000158734f94af871c3d131b56131b6fb7a0291eacadd261e69dfb42a9cdf6f7fddd00000005000000001dcd65000000000200000000000000010000000117dbd11b9dd1c9be337353db7c14f9fb3662e5b5000000001d8119c058734f94af871c3d131b56131b6fb7a0291eacadd261e69dfb42a9cdf6f7fddd0000000100000009000000020000000000000000000000000000000000000000000000000000000000000000000000000000000000000000003329be7d01cd3ebaae6654d7327dd9f17a2e158100000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000003452faa4',
+    '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',
   halfSignedSignature:
     '0xd365ef7ce45aebc4e81bc03f600867f515cebb25c4a0e8e1f06d9fe0a00d41fd2efac6c6df392e5f92e271c57486e39425537da7cafbb085cd1bd21aff06955d00',
   halfSigntxHex:
-    '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',
+    '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',
   fullSigntxHex:
-    '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',
+    '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',
   fullSignedSignature:
     '0x70d2ca9711622142610ddd347e482cbe5dc45aeafe66876bb82bfd57581300045b8457d804cc1b8f2efc10401367e5919b1912ee26d2d48c06cf82dc3f146acd00',
 
@@ -39,7 +39,7 @@ export const IMPORT_IN_C = {
   to: '0x17Dbd11B9dD1c9bE337353db7C14f9fb3662E5B5',
   sourceChainId: 'vE8M98mEQH6wk56sStD1ML8HApTgSqfJZLk9gQ3Fsd4i6m3Bi',
   threshold: 2,
-  fee: '5000000',
+  fee: '409', // feeRate multiplier: 5,000,000 (desired fee) ÷ ~12,228 (cost units) ≈ 409
   locktime: 0,
   INVALID_CHAIN_ID: 'wrong chain id',
   VALID_C_CHAIN_ID: 'yH8D7ThNJkxmtkuv2jgBa4P1Rn3Qpr4pPr7QYNfcdoS6k6HWp',

modules/sdk-coin-flrp/test/unit/lib/importInCTxBuilder.ts

@@ -52,6 +52,71 @@ describe('Flrp Import In C Tx Builder', () => {
     txHash: testData.txhash,
   });
 
+  describe('dynamic fee calculation', () => {
+    it('should calculate proper fee using feeRate multiplier (AVAXP approach) to avoid "insufficient unlocked funds" error', async () => {
+      const amount = '100000000'; // 100M nanoFLRP (0.1 FLR)
+      const feeRate = '1'; // 1 nanoFLRP per cost unit (matching AVAXP's feeRate usage)
+
+      const utxo: DecodedUtxoObj = {
+        outputID: 0,
+        amount: amount,
+        txid: '2vPMx8P63adgBae7GAWFx7qvJDwRmMnDCyKddHRBXWhysjX4BP',
+        outputidx: '0',
+        addresses: [
+          '0x3329be7d01cd3ebaae6654d7327dd9f17a2e1581',
+          '0x7e918a5e8083ae4c9f2f0ed77055c24bf3665001',
+          '0xc7324437c96c7c8a6a152da2385c1db5c3ab1f91',
+        ],
+        threshold: 2,
+      };
+
+      const txBuilder = factory
+        .getImportInCBuilder()
+        .threshold(2)
+        .fromPubKey(testData.pAddresses)
+        .utxos([utxo])
+        .to(testData.to)
+        .feeRate(feeRate) as any;
+
+      const tx = await txBuilder.build();
+
+      const calculatedFee = BigInt((tx as any).fee.fee);
+      const feeRateBigInt = BigInt(feeRate);
+
+      // The fee should be approximately: feeRate × (txSize + inputCost + fixedFee)
+      // For 1 input, threshold=2, ~228 bytes: 1 × (228 + 2000 + 10000) = 12,228
+      const expectedMinCost = 12000; // Minimum cost units (conservative estimate)
+      const expectedMaxCost = 13000; // Maximum cost units (with some buffer)
+
+      const expectedMinFee = feeRateBigInt * BigInt(expectedMinCost);
+      const expectedMaxFee = feeRateBigInt * BigInt(expectedMaxCost);
+
+      // Verify fee is in the expected range
+      assert(
+        calculatedFee >= expectedMinFee,
+        `Fee ${calculatedFee} should be at least ${expectedMinFee} (feeRate × minCost)`
+      );
+      assert(
+        calculatedFee <= expectedMaxFee,
+        `Fee ${calculatedFee} should not exceed ${expectedMaxFee} (feeRate × maxCost)`
+      );
+
+      // Verify the output amount is positive (no "insufficient funds" error)
+      const outputs = tx.outputs;
+      outputs.length.should.equal(1);
+      const outputAmount = BigInt(outputs[0].value);
+      assert(
+        outputAmount > BigInt(0),
+        'Output amount should be positive - transaction should not fail with insufficient funds'
+      );
+
+      // Verify the math: input - output = fee
+      const inputAmount = BigInt(amount);
+      const calculatedOutput = inputAmount - calculatedFee;
+      assert(outputAmount === calculatedOutput, 'Output should equal input minus total fee');
+    });
+  });
+
   describe('on-chain verified transactions', () => {
     it('should verify on-chain tx id for signed C-chain import', async () => {
       const signedImportHex =