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Frequently Asked Questions (FAQ)

Table of Contents

Why use Gazelle and Go?

The Gazelle framework provides lots of great features for generating Bazel build and Starlark files. Right now, the best way to leverage the framework is to write the plugin in Go.

In addition, adoption of the Gazelle ecosystem has started to take off. There are lots of useful plugins for other languages. Letting Gazelle generate and maintain Bazel build files is a real game changer for developer productivity.

Why split the implementation between Go and Starlark?

As mentioned previously, the easiest way to implement a Gazelle plugin is to write it in Go. This works great for generating build files in the primary workspace. However, there is a chicken-and-egg problem when it comes time to generate build files in a repository rule. The repository rule needs to generate files during the loading phase. The Go toolchain and the Gazelle framework defined in the workspace are not available to the repository rule during this phase. So, one needs to either perform some gymnastics to build the Gazelle plugin (see below) or use a language/runtime that is guaranteed to be available during the loading phase. Since Starlark is available during the loading phase, the build file generation logic for the repository rules is implemented in Starlark.

How does the Gazelle plugin for Go handle this?

In short, they assume that if you are using the Gazelle plugin for Go, then you must have a Go toolchain installed on the host system. In essence, they shell out and run Go from the system.

Is the same build file generation logic used for the Go/Gazelle and Starlark implementations?

No. The Gazelle plugin inspects the Swift source files and the directory structure to determine the placement and content of the Bazel build files. The repository rules leverage information about the Swift packages (e.g., dump and describe JSON). However, both implementations use the module_index.json to resolve module references to Bazel targets for the external dependencies.

Does this replace rules_spm?

Yes. There are some limitations with the rules_spm implementation. After receiving feedback and suggestions from the community, we opted to create a clean sheet implementation which includes new features and improvements:

  • Bazel build file generation for the primary workspace.
  • Build the external dependencies with rules_swift.
  • Pin the exact versions for the direct and transitive dependencies.

Can I migrate from rules_spm to rules_swift_package_manager?

Absolutely. A migration guide from rules_spm is on the roadmap.

Can I just manage my external Swift packages and not generate Bazel build files for my project?

Yes. Just omit the //:update_build_files target that is mentioned in the quickstart.

After running //:swift_update_pkgs, I see a .build directory. What is it? Do I need it?

The //:swift_update_pkgs target runs the Gazelle plugin in update-repos mode. This mode resolves the external dependencies listed in your Package.swift by running Swift package manager commands. These commands result in a .build directory being created. The directory is a side effect of running the Swift package manager commands. It can be ignored and should not be checked into source control. It is not used by the Gazelle plugin or the Starlark repository rules.

Does the Gazelle plugin run Swift package manager with every execution?

No. The Gazelle plugin only executes the Swift package manager when run in update-repos mode. This mode only needs to be run when modifying your external dependencies (e.g., add/remove a dependency, update the version selection for a dependency). The update mode for the Gazelle plugin generates Bazel build files for your project. It uses information written to the swift_deps_index.json and the source files that exist in your project to generate the Bazel build files.

Can I store the Swift dependency files in a sub-package (i.e., not in the root of the workspace)?

Yes. The vapor example demonstrates storing the Swift dependency files in a sub-package called swift.

Why does this happen?

This can happen with some Swift packages (e.g. firebase-ios-sdk). rules_xcodeproj removes the sandboxed spawn strategy in their default build configuration due to slow performance of the MacOS sandbox. The above bazelrc stanza adds it back. An issue exists tracking the work to allow these Swift packages to be built using the local spawn strategy.

How do I handle the error Unable to resolve byName reference XXX in @swiftpkg_yyy.?

tl;dr A transitive dependency uses a by-name product reference format that was deprecated in Swift 5.2. You need to patch the Swift package to use a more explicit product reference.

How do I fix this issue?

Patch the Swift package to use a fully-qualified product reference.

Let's look at an example. At the time of this writing, the pusher-websocket-swift Package.swift refers to the TweetNacl product in the tweetnacl-swiftwrap package using just the name of the product.

// Parts of the definition have been omitted for brevity.
let package = Package(
    name: "PusherSwift",
    dependencies: [
        .package(url: "https://github.com/pusher/NWWebSocket.git", .upToNextMajor(from: "0.5.4")),
        .package(url: "https://github.com/bitmark-inc/tweetnacl-swiftwrap", .upToNextMajor(from: "1.0.0")),
    ],
    targets: [
        .target(
            name: "PusherSwift",
            dependencies: [
                "NWWebSocket",
                "TweetNacl",   // <=== byName reference to product in tweetnacl-swiftwrap
            ],
            path: "Sources"
        ),
    ],
)

To make this work with rules_swift_package_manager, you need to patch this file so that the reference is the following:

        .target(
            name: "PusherSwift",
            dependencies: [
                "NWWebSocket",
                .product(name: "TweetNacl", package: "tweetnacl-swiftwrap"),  // <=== Explicit ref
            ],
            path: "Sources"
        ),

What is the cause of the error? Why can't rules_swift_package_manager handle this situation?

This specific case is the reason that previous versions of rules_swift_package_manager pre-processed the transitive Swift dependencies using the Gazelle plugin’s update-repos action and stored the results in a JSON file that was checked into source control.

Here is a quick explainer of why rules_swift_package_manager cannot resolve this dependency with the new architecture:

  • Each Swift package is declared as an external Bazel repository.
  • During the loading phase, there is no mechanism for one external repository to peek into another external repository. In other words, a Swift package cannot query another Swift package to determine all of the available products.
  • So, when rules_swift_package_manager generates a BUILD.bazel file for a Swift package external repo, it only has the information that is returned by calling swift package description and swift package dump-package. The output from these calls only provides the identity and the type of the Swift package (e.g., remote, file system).
  • As a result, byName lookups can only be resolved to the targets/products in the current package and to products in Swift packages where the identity exactly matches the product name.