Building Makefile using bazel

Question

I am trying to build Makefile of a submodule in a bazel project. I see that bazel does provide genrule to execute bash command. I am facing two issues currently -

1. How to cd into a directory before executing command, something like -

genrule(
    name = "hello-make",
    srcs = ["hello"] + glob(["hello/**"]),
    outs = ["hello/main"],
    cmd = "(cd $(location :hello) && make)",
)

2. How do I update update submodule before executing genrule?

Adding cmd = "(git submodule init && git submodule update)" gives fatal: Not a git repository (or any of the parent directories): .git

Step to reproduce:

1. git clone [email protected]:bazelbuild/examples.git
2. cd examples/ && git submodule add [email protected]:mucsi96/cpp-hello-world.git cpp-tutorial/stage1/main/hello
3. cd cpp-tutorial/stage1/ && bazel build //main:hello-world

After this step I want to add a rule that allows me init, update and make the hello submodule.

Is there a better way to build [email protected]:mucsi96/cpp-hello-world.git than creating it as a submodule of git clone [email protected]:bazelbuild/examples.git?

The actual projects are more complex and creating a BUILD file for cpp-hello-world.git is not feasible.

Answer 1

Instead of rolling your own genrule to build a (presumably C++) project that uses a Makefile, check out rules_foreign_cc instead. rules_foreign_cc is used by envoy and other various large C++ programs to build external CMake and Make based dependencies.

See the simple_make example. In it, you would first make a filegroup to collect all the sources and files related to the project to be built, including the Makefile itself:

filegroup(
    name = "sources",
    srcs = glob(["**"]),
    visibility = ["//simple_make:__subpackages__"],
)

Then, call rules_foreign_cc's make rule, which also comes with other make-specific attributes like prefix, make_env_vars, and even a way to override the entire make command with make_commands. In this example, we're just using the lib_source and static_libraries attributes:

load("@rules_foreign_cc//tools/build_defs:make.bzl", "make")

make(
    name = "make_lib",
    lib_source = "//simple_make/code:sources",
    static_libraries = ["liba.a"],
)

Finally, run bazel build //package/to:make_lib to invoke the make rule.

How do I update update submodule before executing genrule?

Try not to do this especially if it updates the sources of the project in-place. genrules and other rules should not modify the state of your sources during a build, but instead only build output files. Consider running a separate script to update submodules prior to the build.