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Invalidation of repository rules

Remote repositories are the way to use dependencies from "outside" of the Bazel world in Bazel. Using them, you can download binaries from the internet or use some from your own host. You can even use Skylark to define your own repository rules to depend on a custom package manager or to implement auto-configuration rules.

This post explains when Skylark repositories are invalidated and hence when they are executed.

Dependencies

The implementation attribute of the repository_rule defines a function (the fetch operation) that is executed inside a Skyframe function. This function is executed when one of its dependencies change.

For repository that are declared local (set local = True in the call to the repository_rule function), the fetch operation is performed on every call of the Skyframe function.

Since a lot of dependencies can trigger this execution (if any part of the WORKSPACE file change for instance), a supplemental mechanism ensure that we re-execute the fetch operation only when stricly needed for non-local repository rules (see the design doc for more details).

After cr.bazel.build/8218 is released, Bazel will re-perform the fetch operation if and only if any of the following dependencies change:

  • Skylark files needed to define the repository rule.
  • Declaration of the repository rule in the WORKSPACE file.
  • Value of any environment variable declared with the environ attribute of the repository_rule function. The value of those environment variable can be enforced from the command line with the --action_env flag (but this flag will invalidate every action of the build).
  • Content of any file used and referred using a label (e.g., //mypkg:label.txt not mypkg/label.txt).

Good practices regarding refetching

Declare your repository as local very carefully

First and foremost, declaring a repository local should be done only for rule that needs to be eagerly invalidated and are fast to update. For native rule, this is used only for local_repository and new_local_repository.

Put all slow operation at the end, resolve dependencies first

Since a dependency might be unresolved when asked for, the function will be executed up to where the dependency is requested and all that part will be replayed if the dependency is not resolved. Put those file dependencies at the top, for instance prefer

def _impl(repository_ctx):
   repository_ctx.file("BUILD", repository_ctx.attr.build_file)
   repository_ctx.download("BIGFILE", sha256 = "...")

myrepo = repository_rule(_impl, attrs = {"build_file": attr.label()})

over

def _impl(repository_ctx):
   repository_ctx.download("BIGFILE")
   repository_ctx.file("BUILD", repository_ctx.attr.build_file)

myrepo = repository_rule(_impl, attrs = {"build_file": attr.label()})

(in the later example, the download operation will be re-executed if build_file is not resolved when executing the fetch operation).

Declare your environment variables

To avoid spurious refetch of repository rules (and the impossibility of tracking all usages of environmnent variables), only environment variables that have been declared through the environ attribute of the repository_rule function are invalidating the repositories.

Therefore, if you think you should re-run if an environment variable changes (like for auto-configuration rules), you should declare those dependencies, or your user will have to do bazel clean --expunge each time they change their environment.

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