Feature: Add `extra-args` To Pixi-build-rattler-build

Nov 29, 2025 by Alex Johnson 54 views

Feature: Add `extra-args` Configuration Option to pixi-build-rattler-build Backend

This article delves into the proposed addition of the extra-args configuration option to the pixi-build-rattler-build backend. This enhancement aims to provide users with greater flexibility and control over the build process by allowing them to pass arbitrary flags to the underlying rattler-build invocation. We'll explore the motivation behind this feature, its use cases, the current limitations, the proposed solution, and alternative approaches considered.

Summary

The pixi-build-rattler-build backend, a crucial component of the pixi build system, currently offers limited configuration options, specifically debug-dir and extra-input-globs. To enhance its versatility and cater to a broader range of build scenarios, the introduction of an extra-args option is proposed. This option, mirroring the functionality available in pixi-build-cmake, would enable users to supply arbitrary flags directly to the rattler-build command. This is particularly useful for accessing experimental features, skipping tests, or preserving build directories, among other things. By adding this functionality, users can have more control over the build process and tailor it to their specific needs.

Use Case: Multi-Output Conda Packages with `cache:`

Consider a scenario where you're constructing a multi-output conda package leveraging the cache: feature within your recipe.yaml file. This feature, designed to optimize build times by caching and reusing build artifacts, necessitates the --experimental flag when invoking rattler-build. This particular use case highlights the need for the extra-args option. The cache: feature is especially beneficial for large projects, as it can significantly reduce build times by avoiding redundant computations. For instance, in a large project like LLVM, the ability to cache intermediate build results can save hours of build time. Without the --experimental flag, the cache: feature will not work, negating its benefits. This means longer build times and increased development cycles. Therefore, having a way to pass this flag through pixi is essential for efficient development.

# recipe.yaml
schema_version: 1

cache:
 source:
 path: ../repo
 # ... shared build configuration

outputs:
 - package:
 name: my-package-libs
 # ...
 - package:
 name: my-package-devel
 # ...

When executing rattler-build build --experimental -r recipe.yaml directly, the build process functions as expected. However, the challenge arises when attempting to integrate this into a pixi build workflow. Currently, pixi lacks a mechanism to directly pass the --experimental flag to the underlying rattler-build command. This limitation hinders the utilization of experimental features and restricts the flexibility of the build process within the pixi ecosystem. This discrepancy between the direct rattler-build invocation and the pixi build process creates a significant hurdle for developers who rely on these advanced features. The absence of a direct way to pass flags means developers are forced to find workarounds, which can be cumbersome and may not always be feasible. This highlights the importance of bridging this gap and providing a seamless way to configure rattler-build through pixi.

Current Behavior and Limitations

The current configuration options for the pixi-build-rattler-build backend are limited. When attempting to use the extra-args option, an error is thrown, indicating that the field is not recognized. This restriction prevents users from leveraging the full potential of rattler-build within the pixi environment. The error message clearly indicates the limitation, stating that extra-args is an unknown field and that the configuration only expects debug-dir or extra-input-globs. This lack of support for extra-args creates a significant gap in functionality, especially for users who need to use specific rattler-build flags for their projects. This can lead to frustration and may force users to choose between using pixi's dependency management features and leveraging rattler-build's advanced options. The inability to pass flags limits the ability to customize the build process and can hinder the adoption of new features or experimental functionalities.

# pixi.toml
[package.build]
backend = { name = "pixi-build-rattler-build", version = "*" }

[package.build.config]
extra-args = ["--experimental"] # ❌ Not supported

Results in:

Error: unknown field `extra-args`, expected one of `debug-dir`, `debug_dir`, `extra-input-globs`

This error message underscores the need for the proposed solution, which would seamlessly integrate the ability to pass arbitrary flags to rattler-build. The inability to specify extra-args severely restricts the flexibility of the build process. For example, if a developer wants to enable a specific feature or debug a build, they need to resort to workarounds or avoid using pixi altogether. This limits pixi's usefulness as a comprehensive build tool and hinders its adoption in more complex projects. The clarity of the error message is helpful, but it also highlights the current deficiency in the pixi-build-rattler-build backend. Addressing this limitation would significantly enhance the user experience and make pixi a more powerful tool for managing and building conda packages.

Proposed Solution: Introducing `extra-args`

To address the limitations discussed, the proposed solution is to introduce an extra-args configuration option to the pixi-build-rattler-build backend. This approach aligns with the existing functionality in pixi-build-cmake, providing a consistent and intuitive way to pass command-line arguments. The implementation would involve modifying the backend to recognize and process the extra-args field in the pixi.toml file. This would then translate these arguments into the corresponding flags when invoking rattler-build. By mirroring the implementation in pixi-build-cmake, the development effort can be reduced and the overall consistency of the pixi build system can be maintained. This consistency is important for users who work with both backends, as it reduces the learning curve and makes the system more predictable.

[package.build.config]
extra-args = ["--experimental"]

This addition would empower users to pass a wide range of rattler-build CLI flags through pixi, including:

--experimental: Enables access to experimental features like cache:. This is crucial for users who want to leverage the latest features and optimizations in rattler-build.
--no-test: Skips the execution of tests during the build process. This can be useful in situations where tests are not needed or are causing issues.
--keep-build: Preserves the build directory after the build is complete. This is helpful for debugging and examining the build artifacts.
Any future flags: Ensures compatibility with future versions of rattler-build and allows users to take advantage of new features and options as they are introduced.

The inclusion of extra-args provides a versatile solution that addresses current limitations and anticipates future needs. This flexibility is essential for a build system that aims to support a wide range of projects and build scenarios. By allowing users to pass arbitrary flags, pixi can accommodate the diverse requirements of different projects and provide a powerful and customizable build experience. This also reduces the need for workarounds and ensures that users can leverage the full capabilities of rattler-build within the pixi ecosystem. The ability to pass any future flags is particularly important, as it ensures that pixi can adapt to changes in rattler-build and continue to provide a seamless build experience.

Alternatives Considered and Why `extra-args` is the Best Approach

Several alternative solutions were considered before arriving at the extra-args approach. Each alternative was evaluated based on its flexibility, ease of implementation, and overall impact on the user experience. While some alternatives addressed specific use cases, they lacked the comprehensive solution provided by extra-args. The following alternatives were considered:

Add only an experimental = true option: This would address the immediate need to enable the cache: feature. However, this approach is limited in scope and wouldn't provide a general solution for passing arbitrary flags. It would also require adding new options for each new flag, leading to a cluttered and inflexible configuration. The lack of a general solution makes this approach less desirable in the long run, as it does not scale well with the evolving features of rattler-build.
Restructure the recipe to avoid cache:: This would involve redesigning the recipe to avoid the use of the cache: feature. This approach can be complex and may not always be feasible, especially for large projects. It can also lead to increased build times and reduced efficiency. For example, in large projects like LLVM, restructuring the recipe to avoid caching would significantly increase build times, making it an impractical solution. This alternative also does not address the underlying issue of needing to pass flags to rattler-build, and it would not be a viable solution for other use cases.
Use rattler-build directly instead of pixi build: This would bypass pixi's dependency management and workspace features, negating the benefits of using pixi in the first place. This approach is a step backwards, as it requires users to manage dependencies and build processes manually, which is precisely what pixi aims to automate. It also loses the benefits of pixi's workspace features, which allow for managing multiple related projects in a cohesive manner. While this approach would allow users to pass any flag to rattler-build, it comes at the cost of losing the advantages of pixi's integrated environment.

The extra-args approach emerges as the most versatile and future-proof solution. It provides a general mechanism for passing any flag to rattler-build, ensuring that users have the flexibility they need to tailor the build process to their specific requirements. This approach also aligns with the existing pixi-build-cmake backend, providing a consistent user experience across different build systems. By adopting extra-args, pixi can continue to provide a powerful and flexible build environment that meets the evolving needs of its users. The ability to pass any flag ensures that users can leverage new features and options in rattler-build as they are introduced, without requiring changes to pixi itself. This forward compatibility is a key advantage of the extra-args approach.

Additional Context and Supporting Information

The CMake backend already supports the extra-args configuration option, demonstrating the feasibility and utility of this approach. This existing implementation serves as a valuable reference and reduces the complexity of adding the feature to the pixi-build-rattler-build backend. The consistency between the CMake and rattler-build backends enhances the overall user experience and makes pixi easier to learn and use. The following link provides more information on the CMake backend's support for extra-args: pixi-build-cmake documentation.

Furthermore, the related pixi PR that added [package.build.configuration] laid the groundwork for this feature. This PR established the necessary infrastructure for configuring build backends, making the addition of extra-args a natural extension of the existing system. The following link provides more details on the PR that added [package.build.configuration]: pixi PR #3115. This context highlights the evolutionary nature of pixi's development and the commitment to providing a flexible and configurable build environment.

In conclusion, the addition of the extra-args configuration option to the pixi-build-rattler-build backend represents a significant enhancement to the pixi build system. It addresses a critical limitation by allowing users to pass arbitrary flags to rattler-build, providing greater flexibility and control over the build process. This feature is particularly important for users who need to leverage experimental features, skip tests, or preserve build directories. The extra-args approach is the most versatile and future-proof solution, aligning with the existing functionality in pixi-build-cmake and ensuring a consistent user experience. By adopting this approach, pixi can continue to provide a powerful and customizable build environment that meets the evolving needs of its users.

For more information on rattler-build and its features, visit the rattler-build documentation. This external resource provides comprehensive details on rattler-build's capabilities and can further enhance your understanding of its integration with pixi.