How to simulate generating a source-file in a Bazel action?

Question

Suppose I am writing a custom Bazel rule for foo-compiler.

The user provides a list of source-files to the rule:

foo_library(
  name = "hello",
  srcs = [ "A.foo", "B.foo" ],
)

To build this without Bazel, the steps would be:

Create a config file config.json that lists the sources:

{
  "srcs": [ "./A.foo", "./B.foo" ]
}

Place the config alongside the sources:

$ ls .
A.foo
B.foo
config.json

Call foo-compiler in that directory:

$ foo-compiler .

Now, in my Bazel rule implementation I can declare a file like this:

  config_file = ctx.actions.declare_file("config.json")

  ctx.actions.write(
    output = config_file,
    content = json_for_srcs(ctx.files.srcs),
  )

The file is created and it has the right content.

However, Bazel does not place config.json alongside the srcs.

Is there a way to tell Bazel where to place the file?

Or perhaps I need to copy each source-file alongside the config?

Can you tell the compiler what file to use for the config? If you can pass something like `--config=...` then it seems pretty easy to make this work. This is may also be important because if the compiler requires the file to be named exactly `config.json` by default, then as written `config_file = ctx.actions.declare_file("config.json")` means you can only ever have 1 `foo_library` per `BUILD` file, otherwise the outputs will conflict (bazel will throw an error like `file 'config.json is generated by these conflicting actions`). With the flag, you could have multiple config files and targets. — ahumesky, Jul 25 '22 at 16:26

score 0 · Answer 1 · answered Jul 24 '22 at 19:13

You can do this with ctx.actions.symlink e.g.

srcs = []

# Declare a symlink for each src files in the same directory as the declared
# config file.Then write that symlink.
for f in ctx.files.srcs:
    src = ctx.actions.declare_file(f.basename)
    srcs.append(src)
    ctx.actions.symlink(
        output = src,
        target_file = f,
    )

config_file = ctx.actions.declare_file("config.json")
ctx.actions.write(
    output = config_file,
    content = json_for_srcs(ctx.files.srcs),
)

# Run compiler
ctx.actions.run(
    inputs = srcs + [config_file],
    outputs = # TODO: Up to you,
    tools = [ctx.file.__compiler], #TODO: Update this to match your rule.
    command = ctx.file.__compiler.path,
    args = ["."],
    #...
)

Note that when you return your provider that you should only return the result of your compilation not the srcs. Otherwise, you'll likely run into problems with duplicate outputs.

How to simulate generating a source-file in a Bazel action?

1 Answers1