Hashing
Features like caching or selective 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 the flag -configuration
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:
- Run
tuist hash cache
ortuist hash test
(hashes for binary caching or selective testing), copy the hashes, rename the project directory, and run the command again. The hashes should match. - If the hashes don't match, it's likely that the generated project depends on the environment. Run
tuist graph --json
in both cases and compare the graphs. Alternatively, generate the projects and compare theirproject.pbxproj
files with a diff tool such as Diffchecker. - If the hashes are the same but differ across environments (for example, CI and local), make sure the same configuration and Swift version are used everywhere. The Swift version is tied to the Xcode version, so confirm the Xcode versions match.
If the hashes are still non-deterministic, let us know and we can help with the debugging.
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.