solidity/docs/common-patterns.rst

388 lines
11 KiB
ReStructuredText
Raw Normal View History

2015-12-07 20:16:25 +00:00
###############
Common Patterns
###############
2016-07-08 16:21:57 +00:00
.. index:: withdrawal
2016-07-09 23:01:27 +00:00
.. _withdrawal_pattern:
2016-07-08 16:21:57 +00:00
*************************
Withdrawal from Contracts
*************************
The recommended method of sending funds after an effect
2016-07-11 14:28:20 +00:00
is using the withdrawal pattern. Although the most intuitive
method of sending Ether, as a result of an effect, is a
2018-09-25 17:08:04 +00:00
direct ``transfer`` call, this is not recommended as it
2016-07-08 16:21:57 +00:00
introduces a potential security risk. You may read
more about this on the :ref:`security_considerations` page.
2018-09-25 17:08:04 +00:00
The following is an example of the withdrawal pattern in practice in
a contract where the goal is to send the most money to the
2016-08-12 15:03:58 +00:00
contract in order to become the "richest", inspired by
`King of the Ether <https://www.kingoftheether.com/>`_.
2016-07-08 16:21:57 +00:00
In the following contract, if you are no longer the richest,
you receive the funds of the person who is now the richest.
2016-07-11 19:34:46 +00:00
.. code-block:: solidity
2016-07-11 19:34:46 +00:00
// SPDX-License-Identifier: GPL-3.0
2021-02-08 17:13:28 +00:00
pragma solidity ^0.8.4;
contract WithdrawalContract {
address public richest;
uint public mostSent;
2016-07-11 19:34:46 +00:00
2016-08-12 15:07:02 +00:00
mapping (address => uint) pendingWithdrawals;
2016-08-11 18:34:36 +00:00
2021-02-08 17:13:28 +00:00
/// The amount of Ether sent was not higher than
/// the currently highest amount.
error NotEnoughEther();
2020-06-23 16:11:34 +00:00
constructor() payable {
richest = msg.sender;
mostSent = msg.value;
2016-07-08 16:21:57 +00:00
}
2019-08-28 15:00:15 +00:00
function becomeRichest() public payable {
2021-02-08 17:13:28 +00:00
if (msg.value <= mostSent) revert NotEnoughEther();
2019-08-28 15:00:15 +00:00
pendingWithdrawals[richest] += msg.value;
richest = msg.sender;
mostSent = msg.value;
2016-07-08 16:21:57 +00:00
}
2017-12-12 18:47:30 +00:00
function withdraw() public {
2016-08-12 15:07:02 +00:00
uint amount = pendingWithdrawals[msg.sender];
// Remember to zero the pending refund before
// sending to prevent re-entrancy attacks
pendingWithdrawals[msg.sender] = 0;
payable(msg.sender).transfer(amount);
2016-07-11 19:34:46 +00:00
}
}
2016-07-11 19:34:46 +00:00
2016-09-05 14:29:08 +00:00
This is as opposed to the more intuitive sending pattern:
2016-07-11 19:34:46 +00:00
.. code-block:: solidity
2016-07-11 19:34:46 +00:00
// SPDX-License-Identifier: GPL-3.0
2021-02-08 17:13:28 +00:00
pragma solidity ^0.8.4;
contract SendContract {
address payable public richest;
uint public mostSent;
2016-08-11 18:34:36 +00:00
2021-02-08 17:13:28 +00:00
/// The amount of Ether sent was not higher than
/// the currently highest amount.
error NotEnoughEther();
2020-06-23 16:11:34 +00:00
constructor() payable {
richest = payable(msg.sender);
mostSent = msg.value;
2016-07-08 16:21:57 +00:00
}
2019-08-28 15:00:15 +00:00
function becomeRichest() public payable {
2021-02-08 17:13:28 +00:00
if (msg.value <= mostSent) revert NotEnoughEther();
2019-08-28 15:00:15 +00:00
// This line can cause problems (explained below).
richest.transfer(msg.value);
richest = payable(msg.sender);
2019-08-28 15:00:15 +00:00
mostSent = msg.value;
2016-07-08 16:21:57 +00:00
}
}
Notice that, in this example, an attacker could trap the
2016-08-11 18:34:36 +00:00
contract into an unusable state by causing ``richest`` to be
2019-12-13 15:42:24 +00:00
the address of a contract that has a receive or fallback function
which fails (e.g. by using ``revert()`` or by just
consuming more than the 2300 gas stipend transferred to them). That way,
whenever ``transfer`` is called to deliver funds to the
"poisoned" contract, it will fail and thus also ``becomeRichest``
will fail, with the contract being stuck forever.
In contrast, if you use the "withdraw" pattern from the first example,
the attacker can only cause his or her own withdraw to fail and not the
rest of the contract's workings.
2016-07-08 16:21:57 +00:00
2015-12-07 20:16:25 +00:00
.. index:: access;restricting
******************
Restricting Access
******************
Restricting access is a common pattern for contracts.
Note that you can never restrict any human or computer
from reading the content of your transactions or
your contract's state. You can make it a bit harder
by using encryption, but if your contract is supposed
to read the data, so will everyone else.
You can restrict read access to your contract's state
by **other contracts**. That is actually the default
2019-01-17 20:59:31 +00:00
unless you declare your state variables ``public``.
2015-12-07 20:16:25 +00:00
Furthermore, you can restrict who can make modifications
to your contract's state or call your contract's
2017-10-04 12:35:05 +00:00
functions and this is what this section is about.
2015-12-07 20:16:25 +00:00
.. index:: function;modifier
The use of **function modifiers** makes these
restrictions highly readable.
.. code-block:: solidity
2015-12-07 20:16:25 +00:00
// SPDX-License-Identifier: GPL-3.0
2021-02-08 17:13:28 +00:00
pragma solidity ^0.8.4;
2015-12-07 20:16:25 +00:00
contract AccessRestriction {
// These will be assigned at the construction
// phase, where `msg.sender` is the account
// creating this contract.
address public owner = msg.sender;
uint public creationTime = block.timestamp;
2015-12-07 20:16:25 +00:00
2021-02-08 17:13:28 +00:00
// Now follows a list of errors that
// this contract can generate together
// with a textual explanation in special
// comments.
/// Sender not authorized for this
/// operation.
error Unauthorized();
/// Function called too early.
error TooEarly();
/// Not enough Ether sent with function call.
error NotEnoughEther();
2015-12-07 20:16:25 +00:00
// Modifiers can be used to change
// the body of a function.
// If this modifier is used, it will
// prepend a check that only passes
// if the function is called from
// a certain address.
modifier onlyBy(address _account)
{
2021-02-08 17:13:28 +00:00
if (msg.sender != _account)
revert Unauthorized();
2016-09-05 12:54:50 +00:00
// Do not forget the "_;"! It will
2015-12-07 20:16:25 +00:00
// be replaced by the actual function
2016-09-05 14:29:08 +00:00
// body when the modifier is used.
2016-09-05 12:54:50 +00:00
_;
2015-12-07 20:16:25 +00:00
}
/// Make `_newOwner` the new owner of this
/// contract.
function changeOwner(address _newOwner)
2017-12-12 18:47:30 +00:00
public
2015-12-07 20:16:25 +00:00
onlyBy(owner)
{
owner = _newOwner;
}
modifier onlyAfter(uint _time) {
2021-02-08 17:13:28 +00:00
if (block.timestamp < _time)
revert TooEarly();
2016-09-05 12:54:50 +00:00
_;
2015-12-07 20:16:25 +00:00
}
/// Erase ownership information.
/// May only be called 6 weeks after
/// the contract has been created.
function disown()
2017-12-12 18:47:30 +00:00
public
2015-12-07 20:16:25 +00:00
onlyBy(owner)
onlyAfter(creationTime + 6 weeks)
{
delete owner;
}
// This modifier requires a certain
// fee being associated with a function call.
// If the caller sent too much, he or she is
// refunded, but only after the function body.
2016-09-05 14:29:08 +00:00
// This was dangerous before Solidity version 0.4.0,
// where it was possible to skip the part after `_;`.
2015-12-07 20:16:25 +00:00
modifier costs(uint _amount) {
2021-02-08 17:13:28 +00:00
if (msg.value < _amount)
revert NotEnoughEther();
2016-09-05 12:54:50 +00:00
_;
2015-12-07 20:16:25 +00:00
if (msg.value > _amount)
payable(msg.sender).transfer(msg.value - _amount);
2015-12-07 20:16:25 +00:00
}
function forceOwnerChange(address _newOwner)
2017-12-12 18:47:30 +00:00
public
payable
2015-12-07 20:16:25 +00:00
costs(200 ether)
{
owner = _newOwner;
// just some example condition
if (uint160(owner) & 0 == 1)
2016-09-05 14:29:08 +00:00
// This did not refund for Solidity
// before version 0.4.0.
2015-12-07 20:16:25 +00:00
return;
2016-09-05 14:29:08 +00:00
// refund overpaid fees
2015-12-07 20:16:25 +00:00
}
}
A more specialised way in which access to function
calls can be restricted will be discussed
in the next example.
.. index:: state machine
*************
State Machine
*************
Contracts often act as a state machine, which means
that they have certain **stages** in which they behave
differently or in which different functions can
be called. A function call often ends a stage
and transitions the contract into the next stage
(especially if the contract models **interaction**).
It is also common that some stages are automatically
reached at a certain point in **time**.
An example for this is a blind auction contract which
starts in the stage "accepting blinded bids", then
transitions to "revealing bids" which is ended by
2017-04-14 03:57:14 +00:00
"determine auction outcome".
2015-12-07 20:16:25 +00:00
.. index:: function;modifier
Function modifiers can be used in this situation
to model the states and guard against
incorrect usage of the contract.
Example
=======
In the following example,
the modifier ``atStage`` ensures that the function can
2015-12-07 20:16:25 +00:00
only be called at a certain stage.
Automatic timed transitions
2020-12-28 11:06:40 +00:00
are handled by the modifier ``timedTransitions``, which
2015-12-07 20:16:25 +00:00
should be used for all functions.
.. note::
**Modifier Order Matters**.
If atStage is combined
with timedTransitions, make sure that you mention
it after the latter, so that the new stage is
taken into account.
Finally, the modifier ``transitionNext`` can be used
2015-12-07 20:16:25 +00:00
to automatically go to the next stage when the
function finishes.
.. note::
**Modifier May be Skipped**.
2016-09-05 14:29:08 +00:00
This only applies to Solidity before version 0.4.0:
2015-12-07 20:16:25 +00:00
Since modifiers are applied by simply replacing
code and not by using a function call,
the code in the transitionNext modifier
can be skipped if the function itself uses
return. If you want to do that, make sure
2016-09-05 14:29:08 +00:00
to call nextStage manually from those functions.
Starting with version 0.4.0, modifier code
2016-08-16 14:26:57 +00:00
will run even if the function explicitly returns.
2015-12-07 20:16:25 +00:00
.. code-block:: solidity
2015-12-07 20:16:25 +00:00
// SPDX-License-Identifier: GPL-3.0
2021-02-08 17:13:28 +00:00
pragma solidity ^0.8.4;
2015-12-07 20:16:25 +00:00
contract StateMachine {
enum Stages {
AcceptingBlindedBids,
RevealBids,
AnotherStage,
AreWeDoneYet,
Finished
}
2021-02-08 17:13:28 +00:00
/// Function cannot be called at this time.
error FunctionInvalidAtThisStage();
2016-05-18 15:05:28 +00:00
2015-12-07 20:16:25 +00:00
// This is the current stage.
Stages public stage = Stages.AcceptingBlindedBids;
uint public creationTime = block.timestamp;
2015-12-07 20:16:25 +00:00
modifier atStage(Stages _stage) {
2021-02-08 17:13:28 +00:00
if (stage != _stage)
revert FunctionInvalidAtThisStage();
2016-09-05 12:54:50 +00:00
_;
2015-12-07 20:16:25 +00:00
}
2016-05-18 15:05:28 +00:00
2015-12-07 20:16:25 +00:00
function nextStage() internal {
stage = Stages(uint(stage) + 1);
}
2016-05-18 15:05:28 +00:00
2015-12-07 20:16:25 +00:00
// Perform timed transitions. Be sure to mention
// this modifier first, otherwise the guards
// will not take the new stage into account.
modifier timedTransitions() {
if (stage == Stages.AcceptingBlindedBids &&
block.timestamp >= creationTime + 10 days)
2015-12-07 20:16:25 +00:00
nextStage();
if (stage == Stages.RevealBids &&
block.timestamp >= creationTime + 12 days)
2015-12-07 20:16:25 +00:00
nextStage();
// The other stages transition by transaction
2016-09-05 12:54:50 +00:00
_;
2015-12-07 20:16:25 +00:00
}
2016-05-13 14:32:35 +00:00
2015-12-07 20:16:25 +00:00
// Order of the modifiers matters here!
function bid()
2017-12-12 18:47:30 +00:00
public
2016-09-05 14:29:08 +00:00
payable
2015-12-07 20:16:25 +00:00
timedTransitions
atStage(Stages.AcceptingBlindedBids)
{
// We will not implement that here
}
2016-05-18 15:05:28 +00:00
2015-12-07 20:16:25 +00:00
function reveal()
2017-12-12 18:47:30 +00:00
public
2015-12-07 20:16:25 +00:00
timedTransitions
atStage(Stages.RevealBids)
{
}
// This modifier goes to the next stage
// after the function is done.
modifier transitionNext()
{
2016-09-05 12:54:50 +00:00
_;
2015-12-07 20:16:25 +00:00
nextStage();
}
2016-05-18 15:05:28 +00:00
2015-12-07 20:16:25 +00:00
function g()
2017-12-12 18:47:30 +00:00
public
2015-12-07 20:16:25 +00:00
timedTransitions
atStage(Stages.AnotherStage)
transitionNext
{
}
2016-05-18 15:05:28 +00:00
2015-12-07 20:16:25 +00:00
function h()
2017-12-12 18:47:30 +00:00
public
2015-12-07 20:16:25 +00:00
timedTransitions
atStage(Stages.AreWeDoneYet)
transitionNext
{
}
2016-05-18 15:05:28 +00:00
2015-12-07 20:16:25 +00:00
function i()
2017-12-12 18:47:30 +00:00
public
2015-12-07 20:16:25 +00:00
timedTransitions
atStage(Stages.Finished)
{
}
}