I have basically the following structure:

• "lib_a" is a bazel module that depends on "lib_b" (remote, e.g., git repo)
• "lib_b is a bazel module that depends on "lib_c"
• "lib_c is a bazel module contained within a sub-folder of "lib_b", i.e., in "third_party/lib_c"

In order to build any target in lib_b that refers to@lib_c, I need to "use" that repo at the top level MODULE.bazel in "lib_a". I have tried to follow and adapt the examples that instruct to use a module extension for this purpose, but to no avail.

I have a repro here:mikael-s-persson/bazelbuild_examples@a908775

It has the following files (pretendlib_a andlib_b are two separately hosted packages (git/http)):

./lib_b/deps.bzl./lib_b/BUILD.bazel./lib_b/MODULE.bazel./lib_b/third_party/lib_c/BUILD.bazel./lib_b/third_party/lib_c/MODULE.bazel./lib_a/BUILD.bazel./lib_a/MODULE.bazel

In relevant part. In theory, thelib_b/MODULE.bazel could contain:

local_repository(    name = "lib_c",    path = "third_party/lib_c",)

But that only works to buildlib_b targets when it is the root module. Trying to build targets inlib_a that depend on targets fromlib_b fails becausethird_party/lib_c is not a subdirectory oflib_a, obviously.

I tried a bunch of different things (lost count), and some of which I noted in the code I linked to above. Nothing seems to work so far. The best I could manage was to create arelative_repository rule that is likelocal_repository but also takes a parent repository label (e.g.,@lib_b) and assumes the local path to be relative to where that repo is located from the workspace root, but that fails at thelib_a level because it can't find that directory, although I can definitely see that it exists where bazel it doesn't (in the build cache folder).

Long story short, how do I make this work?
How shouldlib_b uselib_c such thatlib_b is also buildable as a dependency, not just as the root module?
How canlib_a make@lib_c accessible too?