Trying to write a custom rule - go

Question

I'm trying to write a custom rule for gqlgen. The idea is to run it to generate Go code from a GraphQL schema.
My intended usage is:
gqlgen(
name = "gql-gen-foo",
schemas = ["schemas/schema.graphql"],
visibility = ["//visibility:public"],
)
"name" is the name of the rule, on which I want other rules to depend; "schemas" is the set of input files.
So far I have:
load(
"#io_bazel_rules_go//go:def.bzl",
_go_context = "go_context",
_go_rule = "go_rule",
)
def _gqlgen_impl(ctx):
go = _go_context(ctx)
args = ["run github.com/99designs/gqlgen --config"] + [ctx.attr.config]
ctx.actions.run(
inputs = ctx.attr.schemas,
outputs = [ctx.actions.declare_file(ctx.attr.name)],
arguments = args,
progress_message = "Generating GraphQL models and runtime from %s" % ctx.attr.config,
executable = go.go,
)
_gqlgen = _go_rule(
implementation = _gqlgen_impl,
attrs = {
"config": attr.string(
default = "gqlgen.yml",
doc = "The gqlgen filename",
),
"schemas": attr.label_list(
allow_files = [".graphql"],
doc = "The schema file location",
),
},
executable = True,
)
def gqlgen(**kwargs):
tags = kwargs.get("tags", [])
if "manual" not in tags:
tags.append("manual")
kwargs["tags"] = tags
_gqlgen(**kwargs)
My immediate issue is that Bazel complains that the schemas are not Files:
expected type 'File' for 'inputs' element but got type 'Target' instead
What's the right approach to specify the input files?
Is this the right approach to generate a rule that executes a command?
Finally, is it okay to have the output file not exist in the filesystem, but rather be a label on which other rules can depend?

Answer 1

Instead of:
ctx.actions.run(
inputs = ctx.attr.schemas,
Use:
ctx.actions.run(
inputs = ctx.files.schemas,
Is this the right approach to generate a rule that executes a command?
This looks right, as long as gqlgen creates the file with the correct output name (outputs = [ctx.actions.declare_file(ctx.attr.name)]).
generated_go_file = ctx.actions.declare_file(ctx.attr.name + ".go")
# ..
ctx.actions.run(
outputs = [generated_go_file],
args = ["run", "...", "--output", generated_go_file.short_path],
# ..
)
Finally, is it okay to have the output file not exist in the filesystem, but rather be a label on which other rules can depend?
The output file needs to be created, and as long as it's returned at the end of the rule implementation in a DefaultInfo provider, other rules will be able to depend on the file label (e.g. //my/package:foo-gqlgen.go).