scrypto-test

A library for testing Scrypto blueprints, from the Radix DLT project.

lib.rs:

This crate is an implementation of for Scrypto-Test, a unit testing framework for Scrypto that follows an invocation-based approach instead of a transaction-based approach to testing, allowing Scrypto developers to write tests that look and feel like Scrypto. Scrypto-Test is not a replacement for transaction-based tests offered by the LedgerSimulator, it would just be an addition and another way for Scrypto developers to test their code where Scrypto-Test could be classified as a unit-testing framework while the LedgerSimulator could be classified as an integration-testing framework.

Why

We already have a way to test Scrypto blueprints in the from of the scrypto_test::prelude::LedgerSimulator which is essentially an in-memory ledger that we can run transactions against, get back transaction receipts, and determine-based on the TransactionReceipt-if the behavior of the blueprint or component is as we expect or not. This approach is tried and tested and has been proven to work as evident by the hundreds and thousands of tests in the radix_engine_tests crate that make it clear that the transaction-based approach works. However, it has a number of issues, especially when we think about the target audience of our testing framework: DeFi developers.

In the current (transaction-based) model, there is a lot of boilerplate code involved to write what should be a straightforward test. As an example, to test that a contribution of X and Y resources to Radiswap results in Z pool units minted the test author needs to:

• Create these two resources to use for testing.
• Decide on whether these resources should just be mintable on demand or if the supply of these resources should be stored in some account that will be used in the test to withdraw from.
• If an account will hold those resources, then the author needs to create that account.
• Split out the instructions into multiple manifests as needed such as in cases where one instruction depends on the output of a previous instruction.
• Ensure that the execution of the previous transactions did indeed succeed and extract out the information required from receipts either through the worktop changes, balance changes, or by other means.
• Manage and ensure that the worktop by the end of the transaction is empty of all resources and that the accounts that the resources will be deposited into sign the transaction.

Most if not all of the items listed above are not core to what the developer wishes to test, recall that they wished to test whether a contribution of X and Y resources returns Z pool units. However, they spent a majority of their time thinking about completely different problems. Thus, there is not only a large amount of boilerplate code, but there is also large mental overhead for a test should be conceptually easy and simple to write. As you can see from the description above, most of the time that is spent writing tests is not spent writing tests, but spend initializing and creating the environment and managing side effects just to then write a simple test in the form of a method call to some node id.

Scrypto-Test Model

This model differs from the transaction based model of writing tests in that we do not have transaction instructions, processor, and worktop at all. In fact, nothing related to transactions exists in this model. Instead, there exists a TestEnvironment struct that each test can instantiate instances of. Each TestEnvironment instance has a substate store, track, and kernel. On top of that, TestEnvironment implements the SystemApi trait. The TestEnvironment can be looked at as self-contained Radix Engine that’s exposed through the SystemApi and that has some other high-level helper methods as it contains all of the layers of the engine. This means that:

• A TestEnvironment instance is a self-contained instance of the Radix Engine and Kernel which are exposed through the SystemApi.
• Since TestEnvironment implements the SystemApi it can be used as a substitute to ScryptoEnv from Scrypto and the SystemService from native. This means that the simple interface seen in the radix_native_sdk crate can be used within tests.
• If each test has it’s own TestEnvironment instance (they instantiate that themselves if they need it), then tests have no shared dependencies and are isolated.
• The biggest struggle with the transaction-based model was around dealing with transient nodes. More specifically, if we wanted to make sure that bucket X returned from some invocation contained Y resources, how would we do that? Unfortunately, there was no easy way to do it. In this model, if we make an invocation and get a Bucket back, there is no worktop for the bucket to go into, we have a proper Bucket object that we can call amount() on and assert against. Thus, this approach makes it easier to have assertions around transient nodes.

Once the TestEnvironment has been instantiated we would get a Kernel with two Call Frames:

1. The Root Call Frame: We have a root Call Frame to be consistent with how other parts of the stack use the kernel where there is always a root Call Frame. After the instantiation is complete, the root callframe is pushed onto the stack of previous Call Frames the kernel has.
2. The Test Call Frame: This is the Call Frame that is used for all of the invocation that will be made throughout the test. This Call Frame functions exactly like any other Call Frame, it can own nodes, get messages from other Call Frames, and so on. As an example, say we invoke a method on some node that returns a Bucket, this Bucket is now owned and visible to this Call Frame. We are now able to call methods such as resource_address() and amount() on this Bucket since it’s a node we own and the TestEnvironment has a Heap.