core/txpool/legacypool: fix flaky test TestAllowedTxSize (#30975)

- it was failing because the maximum data length (previously `dataSize`)
was set to `txMaxSize - 213` but should had been `txMaxSize - 103` and
the last call `dataSize+1+uint64(rand.Intn(10*txMaxSize)))` would
sometimes fail depending on rand.Intn.
- Maximal transaction data size comment (invalid) replaced by code logic
to find the maximum tx length without its data length
- comments and variable naming improved for clarity
- 3rd pool add test replaced to add just 1 above the maximum length,
which is important to ensure the logic is correct

core/txpool/legacypool/legacypool_test.go

@@ -1243,33 +1243,28 @@ func TestAllowedTxSize(t *testing.T) {
 	account := crypto.PubkeyToAddress(key.PublicKey)
 	testAddBalance(pool, account, big.NewInt(1000000000))
 
-	// Compute maximal data size for transactions (lower bound).
+	// Find the maximum data length for the kind of transaction which will
-	//
+	// be generated in the pool.addRemoteSync calls below.
-	// It is assumed the fields in the transaction (except of the data) are:
+	const largeDataLength = txMaxSize - 200 // enough to have a 5 bytes RLP encoding of the data length number
-	//   - nonce     <= 32 bytes
+	txWithLargeData := pricedDataTransaction(0, pool.currentHead.Load().GasLimit, big.NewInt(1), key, largeDataLength)
-	//   - gasTip  <= 32 bytes
+	maxTxLengthWithoutData := txWithLargeData.Size() - largeDataLength // 103 bytes
-	//   - gasLimit  <= 32 bytes
+	maxTxDataLength := txMaxSize - maxTxLengthWithoutData              // 131072 - 103 = 130953 bytes
-	//   - recipient == 20 bytes
+
-	//   - value     <= 32 bytes
-	//   - signature == 65 bytes
-	// All those fields are summed up to at most 213 bytes.
-	baseSize := uint64(213)
-	dataSize := txMaxSize - baseSize
 	// Try adding a transaction with maximal allowed size
-	tx := pricedDataTransaction(0, pool.currentHead.Load().GasLimit, big.NewInt(1), key, dataSize)
+	tx := pricedDataTransaction(0, pool.currentHead.Load().GasLimit, big.NewInt(1), key, maxTxDataLength)
 	if err := pool.addRemoteSync(tx); err != nil {
 		t.Fatalf("failed to add transaction of size %d, close to maximal: %v", int(tx.Size()), err)
 	}
 	// Try adding a transaction with random allowed size
-	if err := pool.addRemoteSync(pricedDataTransaction(1, pool.currentHead.Load().GasLimit, big.NewInt(1), key, uint64(rand.Intn(int(dataSize))))); err != nil {
+	if err := pool.addRemoteSync(pricedDataTransaction(1, pool.currentHead.Load().GasLimit, big.NewInt(1), key, uint64(rand.Intn(int(maxTxDataLength+1))))); err != nil {
 		t.Fatalf("failed to add transaction of random allowed size: %v", err)
 	}
-	// Try adding a transaction of minimal not allowed size
+	// Try adding a transaction above maximum size by one
-	if err := pool.addRemoteSync(pricedDataTransaction(2, pool.currentHead.Load().GasLimit, big.NewInt(1), key, txMaxSize)); err == nil {
+	if err := pool.addRemoteSync(pricedDataTransaction(2, pool.currentHead.Load().GasLimit, big.NewInt(1), key, maxTxDataLength+1)); err == nil {
 		t.Fatalf("expected rejection on slightly oversize transaction")
 	}
-	// Try adding a transaction of random not allowed size
+	// Try adding a transaction above maximum size by more than one
-	if err := pool.addRemoteSync(pricedDataTransaction(2, pool.currentHead.Load().GasLimit, big.NewInt(1), key, dataSize+1+uint64(rand.Intn(10*txMaxSize)))); err == nil {
+	if err := pool.addRemoteSync(pricedDataTransaction(2, pool.currentHead.Load().GasLimit, big.NewInt(1), key, maxTxDataLength+1+uint64(rand.Intn(10*txMaxSize)))); err == nil {
 		t.Fatalf("expected rejection on oversize transaction")
 	}
 	// Run some sanity checks on the pool internals