fix: TransactionMessage.decompile() now counts the correct number of unsigned, writable accounts (#28990)

* Test that `TransactionMessage.decompile()` can decompile a legacy `Message`

* `TransactionMessage.decompile()` now correctly accounts for the number of writable unsigned accounts

web3.js/src/transaction/message.ts

@@ -49,7 +49,9 @@ export class TransactionMessage {
     assert(numWritableSignedAccounts > 0, 'Message header is invalid');
 
     const numWritableUnsignedAccounts =
-      message.staticAccountKeys.length - numReadonlyUnsignedAccounts;
+      message.staticAccountKeys.length -
+      numRequiredSignatures -
+      numReadonlyUnsignedAccounts;
     assert(numWritableUnsignedAccounts >= 0, 'Message header is invalid');
 
     const accountKeys = message.getAccountKeys(args);

web3.js/test/transaction-tests/message.test.ts

@@ -3,12 +3,13 @@ import {expect} from 'chai';
 import {sha256} from '@noble/hashes/sha256';
 
 import {
+  Transaction,
   TransactionInstruction,
   TransactionMessage,
 } from '../../src/transaction';
 import {PublicKey} from '../../src/publickey';
 import {AddressLookupTableAccount} from '../../src/programs';
-import {MessageV0} from '../../src/message';
+import {Message, MessageV0} from '../../src/message';
 
 function createTestKeys(count: number): Array<PublicKey> {
   return new Array(count).fill(0).map(() => PublicKey.unique());
@@ -31,7 +32,77 @@ function createTestLookupTable(
 }
 
 describe('TransactionMessage', () => {
-  it('decompile', () => {
+  it('decompiles a legacy message', () => {
+    const keys = createTestKeys(7);
+    const recentBlockhash = bs58.encode(sha256('test'));
+    const payerKey = keys[0];
+    const instructions = [
+      new TransactionInstruction({
+        programId: keys[5],
+        keys: [
+          {pubkey: keys[0], isSigner: true, isWritable: true},
+          {pubkey: keys[6], isSigner: false, isWritable: false},
+          {pubkey: keys[1], isSigner: false, isWritable: true},
+          {pubkey: keys[3], isSigner: false, isWritable: false},
+          {pubkey: keys[4], isSigner: false, isWritable: false},
+          {pubkey: keys[2], isSigner: false, isWritable: false},
+        ],
+        data: Buffer.alloc(1),
+      }),
+    ];
+
+    const message = Message.compile({
+      instructions,
+      payerKey,
+      recentBlockhash,
+    });
+
+    expect(() => TransactionMessage.decompile(message)).not.to.throw(
+      'Failed to get account keys because address table lookups were not resolved',
+    );
+
+    const decompiledMessage = TransactionMessage.decompile(message);
+
+    expect(decompiledMessage.payerKey).to.eql(payerKey);
+    expect(decompiledMessage.recentBlockhash).to.eq(recentBlockhash);
+    expect(decompiledMessage.instructions).to.eql(instructions);
+  });
+
+  // Regression test for https://github.com/solana-labs/solana/issues/28900
+  it('decompiles a legacy message the same way as the old API', () => {
+    const accountKeys = createTestKeys(7);
+    const legacyMessage = new Message({
+      header: {
+        numRequiredSignatures: 1,
+        numReadonlySignedAccounts: 0,
+        numReadonlyUnsignedAccounts: 5,
+      },
+      recentBlockhash: bs58.encode(sha256('test')),
+      accountKeys,
+      instructions: [
+        {
+          accounts: [0, 6, 1, 3, 4, 2],
+          data: '',
+          programIdIndex: 5,
+        },
+      ],
+    });
+
+    const transactionFromLegacyAPI = Transaction.populate(legacyMessage);
+    const transactionMessage = TransactionMessage.decompile(legacyMessage);
+
+    expect(transactionMessage.payerKey).to.eql(
+      transactionFromLegacyAPI.feePayer,
+    );
+    expect(transactionMessage.instructions).to.eql(
+      transactionFromLegacyAPI.instructions,
+    );
+    expect(transactionMessage.recentBlockhash).to.eql(
+      transactionFromLegacyAPI.recentBlockhash,
+    );
+  });
+
+  it('decompiles a V0 message', () => {
     const keys = createTestKeys(7);
     const recentBlockhash = bs58.encode(sha256('test'));
     const payerKey = keys[0];