Status: Experimental. We may make breaking changes to the API, but we will announce them and help you update your code.
An external repository is a rule that can be used only in the WORKSPACE file and enable non-hermetic operation at the loading phase of Bazel. Each external repository rule creates its own workspace, with its own BUILD files and artifacts. They can be used to depend on third-party libraries (such as Maven packaged libraries) but also to generate BUILD files specific to the host Bazel is running on.
In a Skylark extension, use the repository_rule function to create a new repository rule and store it in a global variable.
A custom repository rule can be used just like a native repository rule. It has a mandatory name attribute and every target present in its build files can be refered as @<name>//package:target where <name> is the value of the name attribute.
The rule is loaded when you explictly build it, or if it is a dependency of the build. In this case, Bazel will execute its implementation function. This function describe how to creates the repository, its content and BUILD files.
An attribute is a rule argument, such as url or sha256. You must list the attributes and their types when you define a repository rule.
local_repository = repository_rule( implementation=_impl, local=True, attrs={"path": attr.string(mandatory=True)})
name attributes are implicitely defined for all repository_rules. To access an attribute, use repository_ctx.attr.<attribute_name>. The name of a repository rule is accessible with repository_ctx.name.
If an attribute name starts with _ it is private and users cannot set it.
Every repository rule requires an implementation function. It contains the actual logic of the rule and is executed strictly in the Loading Phase. The function has exactly one input parameter, repository_ctx, and should always returns None. The input parameter repository_ctx can be used to access attribute values, and non-hermetic functions (finding a binary, exuting a binary, creating a file in the repository or downloading a file from the Internet). See the library for more context. Example:
def _impl(repository_ctx): repository_ctx.symlink(repository_ctx.attr.path, "") local_repository = repository_rule( implementation=_impl, ...)
For now, we only have one full example of usage of the repository_rule: C++ auto-configured toolchain.
This example uses a Skylark repository rule to automatically create the C++ configuration files for Bazel by looking for the local C++ compiler, the environment and the flags the C++ compiler supports.