fix(bitcoin-wallet-snap): preserve template output order when filling a PSBT

packages/bitcoin-wallet-snap/CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Keep the template output order when filling a PSBT ([#157](https://github.com/MetaMask/internal-snaps/pull/157))
+  - A template output belonging to the wallet is now only used as the drain output when it is the last output. Previously any such output was moved to the end of the transaction, silently reordering templates that place change before another output.
+  - Filling a PSBT now fails with a `ValidationError` when the built transaction does not reproduce every template output, at its original index, with its original value. Previously only the output count was compared, so a divergent transaction could be signed and broadcast.
+
 ## [2.0.1]
 
 ### Fixed

packages/bitcoin-wallet-snap/snap.manifest.json

@@ -7,7 +7,7 @@
     "url": "https://github.com/MetaMask/internal-snaps.git"
   },
   "source": {
-    "shasum": "jV649WZbbfbj3FpOMD5U/xDPuRD0t4F+pxCoy08a/O0=",
+    "shasum": "B9w8DmGsWcU8IBkEJemc1WCP46/jbdO4uBwcmDgtvg4=",
     "location": {
       "npm": {
         "filePath": "dist/bundle.js",

packages/bitcoin-wallet-snap/src/use-cases/AccountUseCases.test.ts

@@ -1595,6 +1595,146 @@ describe('AccountUseCases', () => {
       // Result should be the rebuilt PSBT with all outputs preserved
       expect(result).toBe(rebuiltPsbt);
     });
+
+    const identifiableOutput = (scriptHex: string, sats: bigint): TxOut => {
+      const scriptPubkey = mock<ScriptBuf>();
+      scriptPubkey.to_hex_string.mockReturnValue(scriptHex);
+      const value = mock<Amount>();
+      value.to_sat.mockReturnValue(sats);
+
+      return mock<TxOut>({ script_pubkey: scriptPubkey, value });
+    };
+
+    const accountOwning = (owned: ScriptBuf[]): BitcoinAccount => {
+      const account = mock<BitcoinAccount>({
+        id: 'account-id',
+        network: 'bitcoin',
+        isMine: (script: ScriptBuf) => owned.includes(script),
+        capabilities: [AccountCapability.FillPsbt],
+      });
+      account.buildTx.mockReturnValue(mockTxBuilder);
+      return account;
+    };
+
+    it('adds every template output as a fixed recipient when the wallet-owned output is not last', async () => {
+      const changeOutput = identifiableOutput('0014aaaa', 2548n);
+      const depositOutput = identifiableOutput('5120bbbb', 496774n);
+      const template = mock<Psbt>({
+        unsigned_tx: { output: [changeOutput, depositOutput] },
+        toString: () => 'templateBase64',
+      });
+      mockTxBuilder.finish.mockReturnValue(
+        mock<Psbt>({
+          unsigned_tx: { output: [changeOutput, depositOutput] },
+        }),
+      );
+      mockRepository.get.mockResolvedValueOnce(
+        accountOwning([changeOutput.script_pubkey]),
+      );
+
+      await useCases.fillPsbt('account-id', template);
+
+      expect(mockTxBuilder.drainToByScript).not.toHaveBeenCalled();
+      expect(mockTxBuilder.addRecipientByScript).toHaveBeenCalledTimes(2);
+      expect(mockTxBuilder.addRecipientByScript).toHaveBeenNthCalledWith(
+        1,
+        changeOutput.value,
+        changeOutput.script_pubkey,
+      );
+      expect(mockTxBuilder.addRecipientByScript).toHaveBeenNthCalledWith(
+        2,
+        depositOutput.value,
+        depositOutput.script_pubkey,
+      );
+    });
+
+    it('throws when the built outputs are reordered against the template', async () => {
+      const depositOutput = identifiableOutput('5120bbbb', 496774n);
+      const opReturnOutput = identifiableOutput('6a3ecccc', 0n);
+      const template = mock<Psbt>({
+        unsigned_tx: { output: [depositOutput, opReturnOutput] },
+        toString: () => 'templateBase64',
+      });
+      mockTxBuilder.finish.mockReturnValue(
+        mock<Psbt>({
+          unsigned_tx: {
+            output: [
+              opReturnOutput,
+              identifiableOutput('0014aaaa', 2548n),
+              depositOutput,
+            ],
+          },
+        }),
+      );
+      mockRepository.get.mockResolvedValueOnce(accountOwning([]));
+
+      await expect(useCases.fillPsbt('account-id', template)).rejects.toThrow(
+        'Built PSBT does not preserve the template outputs',
+      );
+    });
+
+    it('throws when a built output value diverges from the template', async () => {
+      const depositOutput = identifiableOutput('5120bbbb', 496774n);
+      const template = mock<Psbt>({
+        unsigned_tx: { output: [depositOutput] },
+        toString: () => 'templateBase64',
+      });
+      mockTxBuilder.finish.mockReturnValue(
+        mock<Psbt>({
+          unsigned_tx: { output: [identifiableOutput('5120bbbb', 1n)] },
+        }),
+      );
+      mockRepository.get.mockResolvedValueOnce(accountOwning([]));
+
+      await expect(useCases.fillPsbt('account-id', template)).rejects.toThrow(
+        'Built PSBT does not preserve the template outputs',
+      );
+    });
+
+    it('accepts a built PSBT that appends a change output after the template outputs', async () => {
+      const depositOutput = identifiableOutput('5120bbbb', 496774n);
+      const opReturnOutput = identifiableOutput('6a3ecccc', 0n);
+      const template = mock<Psbt>({
+        unsigned_tx: { output: [depositOutput, opReturnOutput] },
+        toString: () => 'templateBase64',
+      });
+      const builtPsbt = mock<Psbt>({
+        unsigned_tx: {
+          output: [
+            depositOutput,
+            opReturnOutput,
+            identifiableOutput('0014aaaa', 2548n),
+          ],
+        },
+      });
+      mockTxBuilder.finish.mockReturnValue(builtPsbt);
+      mockRepository.get.mockResolvedValueOnce(accountOwning([]));
+
+      expect(await useCases.fillPsbt('account-id', template)).toBe(builtPsbt);
+    });
+
+    it('accepts the drained output taking a value the template did not specify', async () => {
+      const depositOutput = identifiableOutput('5120bbbb', 496774n);
+      const changeOutput = identifiableOutput('0014aaaa', 1000n);
+      const template = mock<Psbt>({
+        unsigned_tx: { output: [depositOutput, changeOutput] },
+        toString: () => 'templateBase64',
+      });
+      const builtPsbt = mock<Psbt>({
+        unsigned_tx: {
+          output: [depositOutput, identifiableOutput('0014aaaa', 2548n)],
+        },
+      });
+      mockTxBuilder.finish.mockReturnValue(builtPsbt);
+      mockRepository.get.mockResolvedValueOnce(
+        accountOwning([changeOutput.script_pubkey]),
+      );
+
+      expect(await useCases.fillPsbt('account-id', template)).toBe(builtPsbt);
+      expect(mockTxBuilder.drainToByScript).toHaveBeenCalledWith(
+        changeOutput.script_pubkey,
+      );
+    });
   });
 
   describe('computeFee', () => {

packages/bitcoin-wallet-snap/src/use-cases/AccountUseCases.ts

@@ -779,47 +779,47 @@ export class AccountUseCases {
     const frozenUTXOs = await this.#repository.getFrozenUTXOs(account.id);
     const feeRateToUse = feeRate ?? (await this.getFallbackFeeRate(account));
 
+    const templateOutputs = templatePsbt.unsigned_tx.output;
+    const lastOutput = templateOutputs[templateOutputs.length - 1];
+    // The drain output is always appended last, so only a trailing output of ours can keep its position. Any other output of ours stays a fixed recipient.
+    const drainIndex =
+      lastOutput && account.isMine(lastOutput.script_pubkey)
+        ? templateOutputs.length - 1
+        : -1;
+
+    let builtPsbt: Psbt;
     try {
       let builder = account
         .buildTx()
         .feeRate(feeRateToUse)
         .unspendable(frozenUTXOs)
         .untouchedOrdering(); // we need to strictly adhere to the template output order. Many protocols use the order (e.g: 1: deposit, 2: OP_RETURN, 3: change)
 
-      for (const txout of templatePsbt.unsigned_tx.output) {
-        // if the PSBT contains an output that is sending to ourselves, we change its value. If the PSBT contains no change outputs, one will automatically be added.
-        if (account.isMine(txout.script_pubkey)) {
-          builder = builder.drainToByScript(txout.script_pubkey);
-        } else {
-          builder = builder.addRecipientByScript(
-            txout.value,
-            txout.script_pubkey,
-          );
-        }
-      }
-      let builtPsbt = builder.finish();
+      templateOutputs.forEach((txout, index) => {
+        // the drain output takes the excess, so its template value is replaced. If the template has no output of ours, one will automatically be added.
+        builder =
+          index === drainIndex
+            ? builder.drainToByScript(txout.script_pubkey)
+            : builder.addRecipientByScript(txout.value, txout.script_pubkey);
+      });
+      builtPsbt = builder.finish();
 
-      if (
-        builtPsbt.unsigned_tx.output.length <
-        templatePsbt.unsigned_tx.output.length
-      ) {
+      if (builtPsbt.unsigned_tx.output.length < templateOutputs.length) {
         // Second attempt: use fixed recipients for all outputs
         builder = account
           .buildTx()
           .feeRate(feeRateToUse)
           .unspendable(frozenUTXOs)
           .untouchedOrdering();
 
-        for (const txout of templatePsbt.unsigned_tx.output) {
+        for (const txout of templateOutputs) {
           builder = builder.addRecipientByScript(
             txout.value,
             txout.script_pubkey,
           );
         }
         builtPsbt = builder.finish();
       }
-
-      return builtPsbt;
     } catch (error) {
       const causeMessage = (error as Error)?.message ?? 'unknown cause';
       throw new ValidationError(
@@ -832,6 +832,54 @@ export class AccountUseCases {
         error,
       );
     }
+
+    this.#assertTemplateOutputsPreserved(
+      account,
+      templatePsbt,
+      builtPsbt,
+      drainIndex,
+      feeRateToUse,
+    );
+
+    return builtPsbt;
+  }
+
+  #assertTemplateOutputsPreserved(
+    account: BitcoinAccount,
+    templatePsbt: Psbt,
+    builtPsbt: Psbt,
+    drainIndex: number,
+    feeRate: number,
+  ): void {
+    const templateOutputs = templatePsbt.unsigned_tx.output;
+    const builtOutputs = builtPsbt.unsigned_tx.output;
+
+    const preserved =
+      builtOutputs.length >= templateOutputs.length &&
+      templateOutputs.every((txout, index) => {
+        const builtOutput = builtOutputs[index];
+        if (!builtOutput) {
+          return false;
+        }
+        return (
+          builtOutput.script_pubkey.to_hex_string() ===
+            txout.script_pubkey.to_hex_string() &&
+          (index === drainIndex ||
+            builtOutput.value.to_sat() === txout.value.to_sat())
+        );
+      });
+
+    if (!preserved) {
+      throw new ValidationError(
+        'Built PSBT does not preserve the template outputs',
+        {
+          id: account.id,
+          templatePsbt: templatePsbt.toString(),
+          builtPsbt: builtPsbt.toString(),
+          feeRate,
+        },
+      );
+    }
   }
 
   async #broadcast(