Fix how the state of the fork block is handled

packages/hardhat-core/src/internal/hardhat-network/provider/fork/ForkStateManager.ts

@@ -36,7 +36,8 @@ const notSupportedError = (method: string) =>
 
 export class ForkStateManager implements PStateManager {
   private _state: State = ImmutableMap();
-  private _stateRoot: string = randomHash();
+  private _initialStateRoot: string = randomHash();
+  private _stateRoot: string = this._initialStateRoot;
   private _stateRootToState: Map<string, State> = new Map();
   private _originalStorageCache: Map<string, Buffer> = new Map();
   private _stateCheckpoints: string[] = [];
@@ -50,6 +51,15 @@ export class ForkStateManager implements PStateManager {
     this._state = ImmutableMap();
   }
 
+  /**
+   * The forked block needs special handling because the genesis accounts are added to it.
+   * This is meant to be called when the initial state is final, so that _initialStateRoot
+   * points to this starting fork state.
+   */
+  public updateInitialStateRoot() {
+    this._stateRootToState.set(this._initialStateRoot, this._state);
+  }
+
   public copy(): ForkStateManager {
     const fsm = new ForkStateManager(
       this._jsonRpcClient,
@@ -319,6 +329,10 @@ export class ForkStateManager implements PStateManager {
     if (this._stateCheckpoints.length !== 0) {
       throw checkpointedError("setBlockContext");
     }
+    if (blockNumber.eq(this._forkBlockNumber)) {
+      this._setStateRoot(toBuffer(this._initialStateRoot));
+      return;
+    }
     if (blockNumber.gt(this._forkBlockNumber)) {
       this._setStateRoot(stateRoot);
       return;

packages/hardhat-core/src/internal/hardhat-network/provider/utils/putGenesisAccounts.ts

@@ -11,4 +11,6 @@ export async function putGenesisAccounts(
     const { address, account } = makeAccount(ga);
     await stateManager.putAccount(address, account);
   }
+
+  stateManager.updateInitialStateRoot();
 }

packages/hardhat-core/test/internal/hardhat-network/helpers/transactions.ts

@@ -33,12 +33,13 @@ export async function deployContract(
 }
 
 export async function sendTxToZeroAddress(
-  provider: EthereumProvider
+  provider: EthereumProvider,
+  from?: string
 ): Promise<string> {
   const accounts = await provider.send("eth_accounts");
 
   const burnTxParams = {
-    from: accounts[0],
+    from: from ?? accounts[0],
     to: zeroAddress(),
     value: numberToRpcQuantity(1),
     gas: numberToRpcQuantity(21000),

packages/hardhat-core/test/internal/hardhat-network/provider/modules/eth.ts

@@ -697,6 +697,25 @@ describe("Eth module", function () {
             `Received invalid block number ${futureBlock}. Latest block number is ${firstBlock}`
           );
         });
+
+        it("Should return the initial balance for the genesis accounts in the previous block after a transaction", async function () {
+          const blockNumber = await this.provider.send("eth_blockNumber");
+          const account = DEFAULT_ACCOUNTS_ADDRESSES[0];
+
+          const initialBalanceBeforeTx = await this.provider.send(
+            "eth_getBalance",
+            [account, blockNumber]
+          );
+          assert.equal(initialBalanceBeforeTx, "0xde0b6b3a7640000");
+
+          await sendTxToZeroAddress(this.provider, account);
+
+          const initialBalanceAfterTx = await this.provider.send(
+            "eth_getBalance",
+            [account, blockNumber]
+          );
+          assert.equal(initialBalanceAfterTx, "0xde0b6b3a7640000");
+        });
       });
 
       describe("eth_getBlockByHash", async function () {