Emulate http_file but for a file from another repository

Question

I'm including a repository that has an extra_deps rule of the form:

maybe(
    http_file,
    name = "external_dependency",
    downloaded_file_path = "foo.h",
    sha256 = "<some_sha>",
    urls = ["https://example.com/foo.h"],
)

If I have an existing repository, foo_repo, that provides foo.h, how can I substitute the target for it in place of external_dependency? http_file apparently provides @external_dependency//file, so I can't simply define an alias.

Answer 1

Using https://github.com/bazelbuild/bazel/blob/master/tools/build_defs/repo/http.bzl as a reference, you can define a custom repository rule that provides @external_dependency//file. For example:

def _repository_file(ctx):
    ctx.file("WORKSPACE", "workspace(name = \"{name}\")".format(name = ctx.name))
    ctx.file("file/BUILD.bazel", """
filegroup(
    name = "file",
    srcs = ["{}"],
    visibility = ["//visibility:public"],
)
""".format(ctx.attr.source))

repository_file = repository_rule(
    attrs = {"source": attr.label(mandatory = True, allow_single_file = True)},
    implementation = _repository_file,
    doc = """Analogue of http_file, but for a file in another repository.

Usage:
repository_file(
    name = "special_file"
    source = "@other_repo//path/to:special_file.txt",
)
    """,
)

Now use:

repository_file(
    name = "external_dependency",
    source = "@foo_repo//path/to:foo.h",
)