Is there a way to change private attribute of an Aspect progammatically? - bazel

Question

Say I have something like the following.
def _foo_aspect_impl(target, ctx):
# operations
return FooInfo(...)
foo_aspect = aspect(implementation = _foo_aspect_impl,
attr_aspects = ['deps'],
attrs = dict(
_tool = attr.Label(
# defs
),
)
)
def _foo_rule_impl(ctx):
for dep in ctx.attr.deps:
# do something with `dep[FooInfo]`
return DefaultInfo(...)
foo_rule = rule(
implementation = _foo_rule_impl,
attrs = dict(
"deps": attr.label_list(
aspects = [foo_aspect],
)
)
)
Is there a way to change the value of foo_aspect.attr._tool, either in WORKSPACE, or at the invocation of foo_rule? Former is much preferable.
The use case being that version and repository origin of _tool might change from project to project. When aspect resides in a repository shared by two projects, it does not make sense to create two branches for these two projects just for versioning of _tool.

Answer 1

After a lot of head scratching I found a rather complicated way of doing it.
Since the only thing that seems to be configuarable in WORKSPACE.bazel during loading phase is other workspaces / repositories, one could actually use target aliasing together with repository loading to mutiplex configuarable targets.
Here is how it works.
First, define a new repository rule new_virtual_repository, which creates repositories that does nothing but loading the BUILD.bazel and WORKSPACE.bazel files.
# repo.bzl
load("#bazel_tools//tools/build_defs/repo:utils.bzl", "workspace_and_buildfile")
def _new_virtual_repo_impl(ctx):
# Create build file
workspace_and_buildfile(ctx)
return ctx.attr
new_virtual_repository = repository_rule(
implementation = _new_virtual_repo_impl,
attrs = dict(
build_file = attr.label(allow_single_file = True),
build_file_content = attr.string(),
workspace_file = attr.label(allow_single_file = True),
workspace_file_content = attr.string(),
),
local = True,
)
Then, create an extension file config.bzl which implements a function that generates the BUILD.bazel file and load the virtual repository:
# config.bzl
load(":repo.bzl", "new_virtual_repository")
def config(tool):
build_file_content = """
alias(
name = "tool",
actual = "%s",
""" % (tool)
new_virtual_repository(
name = "config_repo",
build_file_content = build_file_content,
)
Now in the aspect specification:
# aspect.bzl
foo_aspect = aspect(
...
attrs = dict(
_tool = attr.Label("#config_repo//:tool"),
)
)
Finally, configure the actual tool in WORKSPACE.bazel:
# WORKSPACE.bazel
load("//:config.bzl", "config")
config(tool="<actual_tool_label>")