Prebuilts of APIs from platform and other Mainline modules that the ART Module
needs.

These prebuilts should reflect the APIs in Android 12 (version 31), which is the
earliest release that may take an update of the ART Module. Hence updating the
prebuilts to newer versions needs to be done with care. Prebuilts that are part
of the NDK are generally safe since they are annotated to provide a version 31
compatible ABI.

To update:

1. Submit the changes that need to go into the prebuilts.

2. Wait for new builds on branches `aosp-main` and/or `aosp-main-throttled`,
   depending on which prebuilts are required. See `update.py` for details about
   which branches and targets are used for what.

   In general `mainline_modules_sdks-trunk_staging-userdebug` is the slowest
   build to complete, so look for that to be on the safe side.

3. Edit the list returned by `install_entries` in `update.py` in this directory
   to only cover the updates you need (this change shouldn't be submitted).

4. Run:

   ```
   prebuilts/runtime/mainline/update.py \
     --aosp-main-build <build id> --aosp-main-throttled-build <build id>
   ```

   where the two `<build id>` are the builds from step #2.

   The script will download, extract and put the right prebuilts in the right
   places in `prebuilts/runtime`, `prebuilts/module_sdk/conscrypt`, and/or
   `prebuilts/module_sdk/StatsD`. However, some prebuilts may not have
   corresponding CI builds so they may not be complete - see `update.py` for
   details.

5. Make sure that riscv64 still works.

   That architecture is currently not included in the SDK builds in CI (cf.
   b/286551985). Either revert riscv64-related changes, or update them as well
   following the process in README_riscv64.md.

6. Try to further minimize the updates in the created CLs and ensure they are
   safe. This usually means reverting some prebuilts back to their old contents,
   and reverting specific hunks in the `Android.bp` files. Also, please improve
   the autogenerated CL descriptions to make them more accurate and describe the
   reason for the update. Please avoid doing any other changes - in particular
   there must be no local change in any prebuilt file (not counting
   `Android.bp`'s). [email protected] is happy to help with reviews.

Note that the script takes a `--local-dist` argument that you can use to test
with prebuilts from a local build in another tree. However, CLs created that way
should not be submitted.

Unlike other architectures, riscv64 is not supported by mainline yet
(see b/286551985#comment6 for reasons why). Therefore we have to follow a
semi-manual update process described below.

This is not a fully correct process, as the prebuilts are taken from an
arbitrary local build, and only for riscv64. Common architecture-independent
parts of the SDKs such as headers are not updated, as that would affect
architectures other than riscv64. One can only hope they did not diverge too
much from the previous update.

For conscrypt and statsd, we (the ART team) don't have the ownership to commit
the updates in their proper repositories, prebuilts/module_sdk/conscrypt and
prebuilts/module_sdk/StatsD. Therefore as a temporary workaround these prebuilts
are committed in prebuilts/runtime/mainline/local_riscv64. Before the build
starts, they are copied to their proper places by the art/tools/buildbot-*.sh
scripts.

To update:

1. On aosp/main (full manifest):

   1. Cherry pick https://r.android.com/2647900 (this is necessary for the SDK
      build in step 4).

   2. Build implementation libs:
      ```
      $ lunch aosp_riscv64-trunk_staging-userdebug
      $ m dist
      ```

   3. Build apexes:
      ```
      $ banchan \
          com.android.runtime \
          com.android.tzdata \
          com.android.os.statsd \
          com.android.conscrypt \
          com.android.i18n \
          riscv64
      $ m DIST_DIR=out/dist/mainline_modules_riscv64 dist
      ```

   4. Build module SDKs:
      ```
      $ packages/modules/common/build/mainline_modules_sdks.sh \
          --build-release latest \
          --build-platform-sdks-for-mainline
      ```

2. On aosp/master-art branch (reduced manifest):

   1. Run:
      ```
      $ prebuilts/runtime/mainline/update.py \
          --local-dist-riscv64 <root-of-aosp-tree>/out/dist
      ```

   2. Manually inspect the updated Android.bp files and revert any sections not
      related to riscv64 (it may be necessary to revert the entire file). Then try
      to build with `lunch riscv64 && art/tools/buildbot-build.sh`, you may find
      that further tweaking is needed. Make sure that the build succeeds.

   3. Update the commit (use `lunch riscv64 && art/tools/buildbot-build.sh` as a
      validation step in commit message). Be sure to set mast@ as reviewer.