fix: disable geth preemptive empty block mining logic

When using PoW, geth begins mining on empty blocks before attempting to
select which transactions to include. This optimization is not needed,
as we require that every L2 block have exactly one transaction. This
commit disables this behavior an modifies the tests accordingly. The
test is also expanded to be stricter wrt to testing number of txs and
uncles on all blocks.

l2geth/miner/worker.go

@@ -352,7 +352,7 @@ func (w *worker) newWorkLoop(recommit time.Duration) {
 		select {
 		case <-w.startCh:
 			clearPending(w.chain.CurrentBlock().NumberU64())
-			commit(false, commitInterruptNewHead)
+			commit(true, commitInterruptNewHead)
 
 		// Remove this code for the OVM implementation. It is responsible for
 		// cleaning up memory with the call to `clearPending`, so be sure to

l2geth/miner/worker_test.go

@@ -328,15 +328,35 @@ func TestStreamUncleBlock(t *testing.T) {
 	taskIndex := 0
 	w.newTaskHook = func(task *task) {
 		if task.block.NumberU64() == 2 {
-			// The first task is an empty task, the second
-			// one has 1 pending tx, the third one has 1 tx
-			// and 1 uncle.
-			if taskIndex == 2 {
+			// The first task has 1 pending tx, the second one has 1
+			// tx and 1 uncle.
+			numTxs := len(task.block.Transactions())
+			numUncles := len(task.block.Uncles())
+
+			switch taskIndex {
+			case 0:
+				if numTxs != 1 {
+					t.Errorf("expected 1 tx in first task, got: %d", numTxs)
+				}
+				if numUncles != 0 {
+					t.Errorf("expected no uncles in first task, got: %d", numUncles)
+				}
+
+			case 1:
+				if numTxs != 1 {
+					t.Errorf("expected 1 tx in second task, got: %d", numTxs)
+				}
+				if numUncles != 1 {
+					t.Errorf("expected 1 uncle in second task, got: %d", numUncles)
+				}
 				have := task.block.Header().UncleHash
 				want := types.CalcUncleHash([]*types.Header{b.uncleBlock.Header()})
 				if have != want {
 					t.Errorf("uncle hash mismatch: have %s, want %s", have.Hex(), want.Hex())
 				}
+
+			default:
+				t.Errorf("only expected two tasks")
 			}
 			taskCh <- struct{}{}
 			taskIndex += 1
@@ -350,12 +370,10 @@ func TestStreamUncleBlock(t *testing.T) {
 	}
 	w.start()
 
-	for i := 0; i < 2; i += 1 {
-		select {
-		case <-taskCh:
-		case <-time.NewTimer(time.Second).C:
-			t.Error("new task timeout")
-		}
+	select {
+	case <-taskCh:
+	case <-time.NewTimer(time.Second).C:
+		t.Error("new task timeout")
 	}
 
 	w.postSideBlock(core.ChainSideEvent{Block: b.uncleBlock})