commit f697c10164c23bf2e5993b563e9b290f37f39f70
author David 'Digit' Turner <digit@fuchsia.infra.roller.google.com> Tue Feb 18 23:48:25 2025 -0800
committer Copybara-Service <copybara-worker@google.com> Tue Feb 18 23:49:47 2025 -0800
tree 6a27affe7e008ffa17b691d51c12f68f9de70aba
parent 98813caf9340c15ef0ef56453c567a6194cf0719

[roll] Roll fuchsia [build][bazel] Avoid unnecessary Bazel invocations on args.gn updates.

This CL fixes the second issue described in the associated bug,
namely that any change to `args.json` (generated by GN after
parsing `args.gn`) will trigger Bazel rebuilds, even if the
changes are not used by the Bazel build graph.

The reason why was that the @fuchsia_build_info repository
used to treat `args.json` as an input, and filtered it based
on the content of //build/bazel/gn_build_args.txt and any
//vendor/*/build/bazel/gn_build_args.txt file, in order to
generate @fuchsia_build_info//:args.bzl and
@fuchsia_build_info//:vendor_*_args.bzl.

But since the filtering happened entirely in Bazel, Ninja could
not know which values in args.json are needed by Bazel, and would
just re-run any bazel_action() target when that file changes.

To solve this, the filtering logic is moved to `fx gen` time
instead. So a Bazel repository rule is removed, in favor of
generating the repository content through a Python function
invoked by build/regenerator.py.

- args.json and the gn_build_args.txt are all read at 'fx gen'
  time to perform the filtering and computing all results as
  a JSON manifest, which is compared to its last-generated
  value stored at:
  $BUILD_DIR/regenerator_outputs/fuchsia_build_info.generator-info.json

- If the manifest's content has changed, the directory at
  $BUILD_DIR/regenerator_outputs/fuchsia_build_info/ is
  entirely regenerated. Otherwise, it is kept untouched,
  leaving the timestamps of args.bzl and vendor_*_args.bzl
  there intact.

- Update toplevel.WORKSPACE.bazel to use local_repository()
  to point @fuchsia_build_info directly to the generated
  manifest.

The Python logic is added to workspace_utils.py, which
allows better unit-tests than a Bazel repository rule:

+ Add GeneratedWorkspaceFiles.update_if_needed() method
  to perform the conditional update of the output directory
  when the manifest changes only.

+ Add GnBuildArgs class to workspace_utils.py to generate
  the parsing of gn_build_args.txt files and the generation
  of args.bzl files out of it.

After this change, adding new lines to `args.gn` will not
re-trigger a Ninja `bazel_action()` target on the next
incremental `fx build`, unless the new values modify one
of the variables listed in the `gn_build_args.txt` files,
or the Ninja artifacts that serve as input to said
actions.

A simple way to test it is the following, after applying
this CL:

1. Do a regular minimal.x64 full build with `fx set minimal.x64 && fx build`.

2. Add `build_only_labels = [ "//build/bazel/scripts:tests" ]` to
   your args.gn, then run `fx build`, and verify that this does
   *not* trigger Bazel actions (it did before the CL).

3. Add `delegated_network_provisioning = true` to your args.gn,
   which changes one of the argument described by
  //build/bazel/gn_build_args.txt.

4. Verify that this will trigger a Bazel action with the following
   command that explains that this is due to the change in
   @fuchsia_build_info/args.bzl.

   ```sh
   $ fx build //products/microfuchsia:microfuchsia_eng.pkvm-x64_bazel_build_group -- -n -d explain 2>&1
   ninja: Entering directory `/work/fx-better-build/out/default'
   ninja explain: recorded mtime of obj/products/microfuchsia/microfuchsia_eng.pkvm-x64_bazel_build_group.path_mapping older than most recent input regenerator_outputs/fuchsia_build_info/args.bzl (1739892968744669159 vs 1739893047710016640)
   ```

Original-Bug: 396958139
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1207949
Original-Revision: 25b344279a1aa42916158cd2da2e89b1c32870d5
GitOrigin-RevId: 6b4031e672a89f994da3ce58f3bfb60257dd41e6
Change-Id: I156b131f7251ff942d8fcc42e29d7161d1e5e18c