King of the Ether: unchecked send and the pull-payment pattern (2016)

King of the Ether sold a throne. Each claimant paid more than the current price, the deposed king received the payment less a 1% fee, and the price rose by half. The contract paid with send, which forwards only 2,300 gas and returns False on failure. In February 2016 some kings used Mist’s contract wallets, whose code for receiving ether needed more than 2,300 gas; their payments failed, the contract ignored the False, crowned the next king, and kept the money.

Checking the result only moves the problem: a king who refuses every payment then reigns forever, since nobody can be crowned without paying him. The lasting fix is the pull-payment pattern: record what each deposed king is owed and let each withdraw it, with as much gas as they like, in their own transaction. A failure then harms only the one who causes it.

import matplotlib.pyplot as plt

import blockchainkit as bk

A contract wallet becomes king, then is deposed

def reign(throne_code):
    world = bk.contracts.World()
    for account in ("owner", "alice", "bob"):
        world.fund(account, 10_000)
    throne = world.deploy("owner", throne_code, 100, name="throne")
    wallet = world.deploy("alice", bk.contracts.ContractWallet, name="alice's wallet")
    world.fund(wallet, 1_000)
    world.transact("alice", wallet, "forward", throne, "claim_throne", 100)
    deposed = world.transact("bob", throne, "claim_throne", value=1_000)
    if throne_code is bk.contracts.PullKingOfTheEther:
        world.transact("alice", wallet, "forward", throne, "withdraw", 0)
    return world, throne, wallet, deposed


summary = {}
for code in (
    bk.contracts.KingOfTheEther,
    bk.contracts.CheckedKingOfTheEther,
    bk.contracts.PullKingOfTheEther,
):
    world, throne, wallet, deposed = reign(code)
    compensation = world.balance(wallet) - 900
    summary[code.__name__] = (deposed.success, compensation, world.view(throne, "king"))
    print(f"{code.__name__:22s} Bob crowned: {deposed.success}, Alice compensated: {compensation}")

assert summary["KingOfTheEther"][:2] == (True, 0)  # Crowned, but Alice lost her 990.
assert summary["CheckedKingOfTheEther"][0] is False  # Nobody can dethrone her.
assert summary["PullKingOfTheEther"][:2] == (True, 990)

KingOfTheEther         Bob crowned: True, Alice compensated: 0
CheckedKingOfTheEther  Bob crowned: False, Alice compensated: 0
PullKingOfTheEther     Bob crowned: True, Alice compensated: 990

A king who refuses payment

world = bk.contracts.World()
world.fund("mallory", 1_000)
world.fund("carol", 100_000)
throne = world.deploy("owner", bk.contracts.CheckedKingOfTheEther, 100)
usurper = world.deploy("mallory", bk.contracts.Usurper)
world.transact("mallory", usurper, "claim", throne, value=100)
offers = [150, 1_000, 10_000, 90_000]
blocked = [not world.transact("carol", throne, "claim_throne", value=v).success for v in offers]
assert all(blocked) and world.view(throne, "king") == usurper

fig, ax = plt.subplots(figsize=(8, 4))
names = list(summary)
ax.bar(names, [summary[n][1] for n in names], color=["#dc2626", "#d97706", "#16a34a"])
for i, name in enumerate(names):
    note = "Bob crowned" if summary[name][0] else "Bob refused"
    ax.text(i, 30, note, ha="center", color="white")
ax.axhline(990, color="#64748b", linestyle=":", label="what Alice was owed")
ax.set(ylabel="ether reaching the deposed king", title="Push, checked push, and pull")
ax.legend()
fig.tight_layout()

plt.show()

Exercise

In the unchecked version, where did Alice’s 990 go? Read the throne’s balance, and the call tree of Bob’s claim with plot_call_tree: which call failed, and with what error?