Chain expand_template and run in one bazel rule - bazel

Question

I am trying to write a custom rule, where I first generate a file from a template, then pass this file to a script to generate some c++ headers that are the output of my rule.
def _msg_library_impl(ctx):
# For each target in deps, print its label and files.
for source in enumerate(ctx.attr.srcs):
print("File = " + str(source))
out_header = ctx.actions.declare_file("some_header.hpp")
out_arguments = ctx.actions.declare_file("arguments.json")
ctx.actions.expand_template(
template = ctx.file._arguments_file,
output = out_arguments,
substitutions = {
"{output_dir}": out_header.dirname,
"{idl_tuples}": out_header.path,
},
)
args = ctx.actions.args()
args.add("--arguments-file")
args.add(out_arguments)
ctx.actions.run(
outputs = [out_header],
progress_message = "Generating headers '{}'".format(out_header.short_path),
executable = ctx.executable._generator,
arguments = [args],
)
return [
CcInfo(compilation_context=cc_common.create_compilation_context(
includes=depset([out_header.dirname]),
headers=depset([out_header])))
]
msg_library = rule(
implementation = _msg_library_impl,
output_to_genfiles = True,
attrs = {
"srcs": attr.label_list(allow_files = True),
"outs": attr.output_list(),
"_arguments_file": attr.label(
allow_single_file = [".json"],
default = Label("//examples/generation_rule:arguments_template.json"),
),
"_generator": attr.label(
default = Label("//examples/generation_rule:generator"),
executable = True,
cfg = "exec"
),
},
)
Here, generator is a python library that, given an input file provided to srcs and an arguments file generates headers.
The issue that I am facing is that it seems that the expand_template doesn't actually run before run is called, so the generated file is nowhere to be found. What am I doing wrong here? Did I misunderstand how things work?

Answer 1

You need to indicate the file is an input to the action, in addition to passing its path in the arguments. Change the ctx.actions.run to:
ctx.actions.run(
outputs = [out_header],
inputs = [out_arguments],
progress_message = "Generating headers '{}'".format(out_header.short_path),
executable = ctx.executable._generator,
arguments = [args],
)