Have all Bazel packages expose their documentation files (or any file with a given extension) - bazel

Question

Bazel has been working great for me recently, but I've stumbled upon a question for which I have yet to find a satisfactory answer:
How can one collect all files bearing a certain extension from the workspace?
Another way of phrasing the question: how could one obtain the functional equivalent of doing a glob() across a complete Bazel workspace?
Background
The goal in this particular case is to collect all markdown files to run some checks and generate a static site from them.
At first glance, glob() sounds like a good idea, but will stop as soon as it runs into a BUILD file.
Current Approaches
The current approach is to run the collection/generation logic outside of the sandbox, but this is a bit dirty, and I'm wondering if there is a way that is both "proper" and easy (ie, not requiring that each BUILD file explicitly exposes its markdown files.
Is there any way to specify, in the workspace, some default rules that will be added to all BUILD files?

Answer 1

You could write an aspect for this to aggregate markdown files in a bottom-up manner and create actions on those files. There is an example of a file_collector aspect here. I modified the aspect's extensions for your use case. This aspect aggregates all .md and .markdown files across targets on the deps attribute edges.
FileCollector = provider(
fields = {"files": "collected files"},
)
def _file_collector_aspect_impl(target, ctx):
# This function is executed for each dependency the aspect visits.
# Collect files from the srcs
direct = [
f
for f in ctx.rule.files.srcs
if ctx.attr.extension == f.extension
]
# Combine direct files with the files from the dependencies.
files = depset(
direct = direct,
transitive = [dep[FileCollector].files for dep in ctx.rule.attr.deps],
)
return [FileCollector(files = files)]
markdown_file_collector_aspect = aspect(
implementation = _file_collector_aspect_impl,
attr_aspects = ["deps"],
attrs = {
"extension": attr.string(values = ["md", "markdown"]),
},
)
Another way is to do a query on file targets (input and output files known to the Bazel action graph), and process these files separately. Here's an example querying for .bzl files in the rules_jvm_external repo:
$ bazel query //...:* | grep -e ".bzl$"
//migration:maven_jar_migrator_deps.bzl
//third_party/bazel_json/lib:json_parser.bzl
//settings:stamp_manifest.bzl
//private/rules:jvm_import.bzl
//private/rules:jetifier_maven_map.bzl
//private/rules:jetifier.bzl
//:specs.bzl
//:private/versions.bzl
//:private/proxy.bzl
//:private/dependency_tree_parser.bzl
//:private/coursier_utilities.bzl
//:coursier.bzl
//:defs.bzl