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What is smart contract testing? Smart contract testing means performing detailed analysis and evaluation of a smart contract to assess the quality of its source code during the development cycle. Testing a smart contract makes it easier to identify bugs and vulnerabilities and reduces the possibility of software errors that could lead to costly exploits.
Smart contract testing takes many forms, with different methods offering benefits. Strategies for testing Ethereum smart contracts can be classified into two broad categories: automated testing and manual testing. Automated testing Automated testing involves using automated tools to carry out scripted testing of smart contracts. This technique relies on automated software that can execute repeated tests to find defects in smart contracts.
Automated testing is efficient, uses fewer resources, and promises higher levels of coverage than manual analysis. Automated testing tools can also be configured with test data, allowing them to compare predicted behaviors with actual results. Manual testing Manual testing is human-aided and involves an individual who executes testing steps manually.
Manual testing of smart contracts requires considerable skill and a considerable investment of time, money, and effort. Moreover, manual testing can sometimes be susceptible to the problems of human error. However, applying manual testing to smart contracts can also be beneficial. Code audits harness human intelligence to find defects in contract code that might go undetected during automated testing.
Manual-testing your smart contracts can also reveal vulnerabilities that exist outside the code, but can still affect it. For example, a smart contract audit can discover vulnerabilities arising from flawed interaction with off-chain components. Why is it important to test smart contracts? Testing smart contracts is important for the following reasons: 1. Smart contracts are high-value applications Smart contracts often deal with high-value financial assets, especially in industries like decentralized finance DeFi , and valuable items, such as non-fungible tokens NFTs.
As such, minor vulnerabilities in smart contracts can and often lead to massive, irrecoverable losses for users. Comprehensive testing can, however, expose errors in smart contract code and reduce security risks before deployment. While traditional developers may be used to fixing software bugs after launching, Ethereum development leaves little room for patching security flaws once a smart contract is live on the blockchain. While upgradeability mechanisms for smart contracts, such as proxy patterns, these can be difficult to implement.
Besides reducing immutability and introducing complexity, upgrades often demand complex governance processes. For the most part, upgrades should be considered a last resort and avoided unless necessary. Detecting potential vulnerabilities and flaws in your smart contract during the pre-launch phase reduces the need for a logic upgrade. Automated testing for smart contracts 1. Functional testing Functional testing verifies the functionality of a smart contract and provides assurance that each function in the code works as expected.
Functional testing requires understanding how your smart contract should behave in certain conditions. Then you can test each function by running computations with selected values and comparing the returned output with the expected output. Functional testing covers three methods: unit testing, integration testing, and system testing. Unit testing Unit testing involves testing individual components in a smart contract for correctness.
A unit test is simple, quick to run, and provides a clear idea of what went wrong if the test fails. Unit tests are crucial for smart contract development, especially if you need to add new logic to the code. You can verify the behavior of each function and confirm that it executes as intended. Running a unit test often requires creating assertions—simple, informal statements specifying requirements for a smart contract. Unit testing can then be used to test each assertion and see if it holds true under execution.
Examples of contract-related assertions include: i. In integration testing, individual components of the smart contract are tested together. Then you'll learn about Ethereum and smart contracts, and we'll cover Ethereum virtual machine EVM in detail. We'll also delve into the mechanisms of advanced smart contracts, taking a practical approach. You'll also learn how to develop your own cryptocurrency from scratch in order to understand the business behind ICO.
Further on, you'll get to know the key concepts of the Solidity programming language, enabling you to build decentralized blockchain-based applications. We'll also look at enterprise use cases, where you'll build a decentralized microblogging site. At the end of this book, we discuss blockchain-as-a-service, the dark web marketplace, and various advanced topics so you can get well versed with the blockchain principles and ecosystem.
Instructions and Navigation All of the code is organized into folders.
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Returns false on error. WARNING: unsafe—recipient can accidentally or maliciously use up all your gas, causing your contract to halt with an OOG exception; always check the return value of call. Built-in functions Other functions worth noting are: addmod, mulmod For modulo addition and multiplication. Similar to any object in an object-oriented language, the contract is a container that includes data and methods. Solidity offers two other object types that are similar to a contract: interface An interface definition is structured exactly like a contract, except none of the functions are defined, they are only declared.
This type of declaration is often called a stub; it tells you the functions' arguments and return types without any implementation. An interface specifies the "shape" of a contract; when inherited, each of the functions declared by the interface must be defined by the child. Functions Within a contract, we define functions that can be called by an EOA transaction or another contract. In our Faucet example, we have two functions: withdraw and the unnamed fallback function.
One function in each contract may be defined without a name, in which case it is the fallback function, which is called when no other function is named. The fallback function cannot have any arguments or return anything. In our Faucet example, both functions are defined as public.
They can be called by derived contracts those that inherit this one. Keep in mind that the terms internal and private are somewhat misleading. Any function or data inside a contract is always visible on the public blockchain, meaning that anyone can see the code or data. The keywords described here only affect how and when a function can be called. The second set of keywords pure, constant, view, payable affect the behavior of the function: constant or view A function marked as a view promises not to modify any state.
The term constant is an alias for view that will be deprecated in a future release. At this time, the compiler does not enforce the view modifier, only producing a warning, but this is expected to become an enforced keyword in v0. It can only operate on arguments and return data, without reference to any stored data. Pure functions are intended to encourage declarative-style programming without side effects or state.
Functions not declared as payable will reject incoming payments. As you can see in our Faucet example, we have one payable function the fallback function , which is the only function that can receive incoming payments. Contract Constructor and selfdestruct There is a special function that is only used once. When a contract is created, it also runs the constructor function if one exists, to initialize the state of the contract.
The constructor is run in the same transaction as the contract creation. Constructors can be specified in two ways. Up to and including in Solidity v0. This can cause some pretty nasty, unexpected, and difficult-to-find bugs. Imagine for example if the constructor is setting the owner of the contract for purposes of control. If the function is not actually the constructor because of a naming error, not only will the owner be left unset at the time of contract creation, but the function may also be deployed as a permanent and "callable" part of the contract, like a normal function, allowing any third party to hijack the contract and become the "owner" after contract creation.
To address the potential problems with constructor functions being based on having an identical name as the contract, Solidity v0. Renaming the contract does not affect the constructor at all. Also, it is easier to identify which function is the constructor. If there is a constructor, it is executed as part of contract creation, to initialize the state of the contract as it is being created, and is then discarded.
In Solidity, this opcode is exposed as a high-level built-in function called selfdestruct, which takes one argument: the address to receive any ether balance remaining in the contract account. It looks like this: selfdestruct address recipient ; Note that you must explicitly add this command to your contract if you want it to be deletable—this is the only way a contract can be deleted, and it is not present by default.
It is an eternal contract that cannot be deleted. We probably want selfdestruct to be callable only by the EOA that originally created the contract. By convention, this is usually stored in an address variable called owner. Our constructor sets the owner variable, and the selfdestruct function will first check that the owner called it directly.
Our contract now has an address type variable named owner. The name "owner" is not special in any way. We could call this address variable "potato" and still use it the same way. The name owner simply makes its purpose clear. Next, our constructor, which runs as part of the contract creation transaction, assigns the address from msg.
We used msg. In the constructor, however, the msg. We know this is the case because this is a constructor function: it only runs once, during contract creation. Now we can add a function to destroy the contract. We need to make sure that only the owner can run this function, so we will use a require statement to control access. But if the same address stored in owner by the constructor calls it, the contract will self-destruct and send any remaining balance to the owner address.
Note that we did not use the unsafe tx. Function Modifiers Solidity offers a special type of function called a function modifier. You apply modifiers to functions by adding the modifier name in the function declaration. Modifiers are most often used to create conditions that apply to many functions within a contract. We have an access control statement already, in our destroy function.
This is the basic design pattern for access control, allowing only the owner of a contract to execute any function that has the onlyOwner modifier. This placeholder is replaced by the code of the function that is being modified. Essentially, the modifier is "wrapped around" the modified function, placing its code in the location identified by the underscore character. To apply a modifier, you add its name to the function declaration.
More than one modifier can be applied to a function; they are applied in the sequence they are declared, as a comma-separated list. Essentially, you can read this as "Only the owner can destroy this contract. Function modifiers are an extremely useful tool because they allow us to write preconditions for functions and apply them consistently, making the code easier to read and, as a result, easier to audit for security.
They are most often used for access control, but they are quite versatile and can be used for a variety of other purposes. Inside a modifier, you can access all the values variables and arguments visible to the modified function. In this case, we can access the owner variable, which is declared within the contract. We start with contracts that are simple and implement the most generic capabilities, then extend them by inheriting those capabilities in more specialized contracts.
In our Faucet contract, we introduced the constructor and destructor, together with access control for an owner, assigned on construction. Those capabilities are quite generic: many contracts will have them. We can define them as generic contracts, then use inheritance to extend them to the Faucet contract. It indirectly also uses the owner address variable and the constructor defined in owned. Inheritance makes each contract simpler and focused on its specific functionality, allowing us to manage the details in a modular way.
The functionality is the same as when those functions were within Faucet, but now we can reuse those functions in other contracts without writing them again. Code reuse and modularity make our code cleaner, easier to read, and easier to audit. Error Handling assert, require, revert A contract call can terminate and return an error. Error handling in Solidity is handled by four functions: assert, require, revert, and throw now deprecated. When a contract terminates with an error, all the state changes changes to variables, balances, etc.
This ensures that transactions are atomic, meaning they either complete successfully or have no effect on state and are reverted entirely. The assert and require functions operate in the same way, evaluating a condition and stopping execution with an error if the condition is false. By convention, assert is used when the outcome is expected to be true, meaning that we use assert to test internal conditions.
By comparison, require is used when testing inputs such as function arguments or transaction fields , setting our expectations for those conditions. As of Solidity v0. The error message is recorded in the transaction log. So, we can improve our code by adding an error message in our require function: require msg. The throw function is obsolete and will be removed in future versions of Solidity; you should use revert instead. The revert function can also take an error message as the only argument, which is recorded in the transaction log.
Certain conditions in a contract will generate errors regardless of whether we explicitly check for them. We can do that by adding a require statement before the transfer: require this. You will need to find the right balance between gas consumption and verbose error checking based on the expected use of your contract. The transaction receipt contains log entries that provide information about the actions that occurred during the execution of the transaction.
Events are the Solidity high-level objects that are used to construct these logs. Events are especially useful for light clients and DApp services, which can "watch" for specific events and report them to the user interface, or make a change in the state of the application to reflect an event in an underlying contract. Event objects take arguments that are serialized and recorded in the transaction logs, in the blockchain.
You can supply the keyword indexed before an argument, to make the value part of an indexed table hash table that can be searched or filtered by an application. We will add two events, one to log any withdrawals and one to log any deposits. We will call these events Withdrawal and Deposit, respectively. Example 3. How do we see the results of a transaction and "catch" the events? The web3. Within those we can see the events generated by the transaction. Follow the instructions in [truffle] to set up a project directory and compile the Faucet code.
The first log entry logs[0] contains an event name in logs[0]. By showing these on the console, we can see the emitted event name and the event arguments. Events are a very useful mechanism, not only for intra-contract communication, but also for debugging during development. Calling Other Contracts send, call, callcode, delegatecall Calling other contracts from within your contract is a very useful but potentially dangerous operation. In short, the risks arise from the fact that you may not know much about a contract you are calling into or that is calling into your contract.
When writing smart contracts, you must keep in mind that while you may mostly expect to be dealing with EOAs, there is nothing to stop arbitrarily complex and perhaps malign contracts from calling into and being called by your code. Creating a new instance The safest way to call another contract is if you create that other contract yourself.
That way, you are certain of its interfaces and behavior. To do this, you can simply instantiate it, using the keyword new, as in other object-oriented languages. In Solidity, the keyword new will create the contract on the blockchain and return an object that you can use to reference it. The contract Faucet must be defined within the scope of Token, which you can do with an import statement if the definition is in another file: import "Faucet.
In this example, we call the destroy function of Faucet from within the destroy function of Token: import "Faucet. Addressing an existing instance Another way you can call a contract is by casting the address of an existing instance of the contract. With this method, you apply a known interface to an existing instance.
It is therefore critically important that you know, for sure, that the instance you are addressing is in fact of the type you assume. For all we know, the withdraw function at this address could execute something completely different from what we expect, even if it is named the same. Using addresses passed as input and casting them into specific objects is therefore much more dangerous than creating the contract yourself. Raw call, delegatecall Solidity offers some even more "low-level" functions for calling other contracts.
These correspond directly to EVM opcodes of the same name and allow us to construct a contract-to-contract call manually. As such, they represent the most flexible and the most dangerous mechanisms for calling other contracts. The callcode method will be deprecated soon, so it should not be used. As mentioned in address object , a delegatecall is different from a call in that the msg context does not change.
For example, whereas a call changes the value of msg. Essentially, delegatecall runs the code of another contract inside the context of the execution of the current contract. It is most often used to invoke code from a library. It also allows you to draw on the pattern of using library functions stored elsewhere, but have that code work with the storage data of your contract. The delegate call should be used with great caution.
It can have some unexpected effects, especially if the contract you call was not designed as a library. In CallExamples. Example 4. Both the called library and the contract have identical calledFunction functions, which emit an event calledEvent. The event calledEvent logs three pieces of data: msg. Each time calledFunction is called it may have a different execution context with different values for potentially all the context variables , depending on whether it is called directly or through delegatecall.
In caller, we first call the contract and library directly, by invoking calledFunction in each. Then, we explicitly use the low-level functions call and delegatecall to call calledContract. This way we can see how the various calling mechanisms behave. The event emitted is: sender: '0x8ffcb9aaaefad8e9d7b9c8b9fb90f', origin: '0xabab3a6eebc60c78a8bef57', from: '0xca3eaaf5dcaeed9b3e10' As you can see, msg.
The tx. The event was emitted by calledContract, as we can see from the last argument in the event. Instead, it is the address of our account, and is the same as the transaction origin. So, when the calledLibrary code was running, it inherited the execution context of caller, as if its code was running inside caller.
The variable this shown as from in the event emitted is the address of caller, even though it is accessed from within calledLibrary. The next two calls, using the low-level call and delegatecall, verify our expectations, emitting events that mirror what we just saw. Gas Considerations Gas, described in more in detail in [gas] , is an incredibly important consideration in smart contract programming.
Gas is a resource constraining the maximum amount of computation that Ethereum will allow a transaction to consume. If the gas limit is exceeded during computation, the following series of events occurs: An "out of gas" exception is thrown. The state of the contract prior to execution is restored reverted. All ether used to pay for the gas is taken as a transaction fee; it is not refunded. View accumulated earnings on any staked ether. In this test setup, the application is running both the client side code and server side code in the browser.
Automated Testing Automated tests can be run with npm test command. The overall goal in writing these tests was to test each smart contract function in the context in which the front end Dapp will be interacting with the smart contract. These tests are specifically designed to test the smart contract. Not the front end code. For a production application, tests would be conducted using the front end code in addition to web3 direct manipulation.
Verify that both StakePool and StakeContract are starting with 0 ether balance. Verify that ether can be deposited by multiple users, including owner account account[0]. Verify that StakePool correctly recorded deposits for all users.
Compare that total ether balance for StakePool is equal to total deposits. Verify that each user including owner account can request deposited ether to be staked Verify that contract state shows correct accounting of ether as requested to be staked. Verify that deposits are set to zero when balance is requested to be staked. Verify that StakePool can send ether to StakeContract for staking. Verify that staked balances are recorded correctly. Verify that StakeContract contains all the ether in this test.
Verify that there is on ether left in StakePool. Verify that StakePool can calculate a correct profit distribution. This test takes multiple steps: First deposit ether from external account to simulate earnings being created in StakeContract. Verify that earnings are distributed correctly. Verify that all accounts can request ether be unstaked. Verify that all ether is unstaked. Verify that StakeContract is now empty.
Verify that StakePool contains all expected ether. Verify that ether can not be withdrawn from by an account who has not deposited ether. Verify that user can not withdraw more than their own balance of ether.
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