Hashing

Features like caching or smart test execution require a way to determine whether a target has changed. Tuist calculates a hash for each target in the dependency graph to determine if a target has changed. The hash is calculated based on the following attributes:

• The target's attributes (e.g., name, platform, product, etc.)
• The target's files
• The hash of the target's dependencies

Cache attributes

Additionally, when calculating the hash for caching, we also hash the following attributes.

Swift version

We hash the Swift version obtained from running the command /usr/bin/xcrun swift --version to prevent compilation errors due to Swift version mismatches between the targets and the binaries.

MODULE STABILITY

Previous versions of binary caching relied on the BUILD_LIBRARY_FOR_DISTRIBUTION build setting to enable module stability and enable using binaries with any compiler version. However, it caused compilation issues in projects with targets that don't support module stability. Generated binaries are bound to the Swift version used to compile them, and the Swift version must match the one used to compile the project.

Configuration

The idea behind this flag was to ensure debug binaries were not used in release builds and viceversa. However, we are still missing a mechanism to remove the other configurations from the projects to prevent them from being used.

Debugging

If you notice non-deterministic behaviors when using the caching across environments or invocations, it might be related to differences across the environments or a bug in the hashing logic. We recommend following these steps to debug the issue:

1. Ensure the same configuration and Swift version is used across environments.
2. Check if there are differences between the Xcode projects generated by two consecutive invocations of tuist generate or across environments. You can use the diff command to compare the projects. The generated projects might include absolute paths causing the hashing logic to be non-deterministic.

BETTER DEBUGGING EXPERIENCE PLANNED

Improving our debugging experience is in our roadmap. The print-hashes command, which lacks the context to understand the differences, will be replaced by a more user-friendly command that uses a tree-like structure to show the differences between the hashes.