feat: add Connection.getFeeForMessage (#22128)

* web3.js: add Connection.getFeeForMessage

* throw if value is null

* fix null value

* fix types

web3.js/src/connection.ts

@@ -3023,6 +3023,27 @@ export class Connection {
     };
   }
 
+  /**
+   * Fetch the fee for a message from the cluster, return with context
+   */
+  async getFeeForMessage(
+    message: Message,
+    commitment?: Commitment,
+  ): Promise<RpcResponseAndContext<number>> {
+    const wireMessage = message.serialize().toString('base64');
+    const args = this._buildArgs([wireMessage], commitment);
+    const unsafeRes = await this._rpcRequest('getFeeForMessage', args);
+
+    const res = create(unsafeRes, jsonRpcResultAndContext(nullable(number())));
+    if ('error' in res) {
+      throw new Error('failed to get slot: ' + res.error.message);
+    }
+    if (res.result === null) {
+      throw new Error('invalid blockhash');
+    }
+    return res.result as unknown as RpcResponseAndContext<number>;
+  }
+
   /**
    * Fetch a recent blockhash from the cluster
    * @return {Promise<{blockhash: Blockhash, feeCalculator: FeeCalculator}>}

web3.js/test/connection.test.ts

@@ -2278,6 +2278,38 @@ describe('Connection', () => {
     expect(feeCalculator.lamportsPerSignature).to.eq(5000);
   });
 
+  it('get fee for message', async () => {
+    const accountFrom = Keypair.generate();
+    const accountTo = Keypair.generate();
+
+    const {blockhash} = await helpers.recentBlockhash({connection});
+
+    const transaction = new Transaction({
+      feePayer: accountFrom.publicKey,
+      recentBlockhash: blockhash,
+    }).add(
+      SystemProgram.transfer({
+        fromPubkey: accountFrom.publicKey,
+        toPubkey: accountTo.publicKey,
+        lamports: 10,
+      }),
+    );
+    const message = transaction.compileMessage();
+
+    await mockRpcResponse({
+      method: 'getFeeForMessage',
+      params: [
+        message.serialize().toString('base64'),
+        {commitment: 'confirmed'},
+      ],
+      value: 5000,
+      withContext: true,
+    });
+
+    const fee = (await connection.getFeeForMessage(message, 'confirmed')).value;
+    expect(fee).to.eq(5000);
+  });
+
   it('get block time', async () => {
     await mockRpcResponse({
       method: 'getBlockTime',