Bazel Change Detector

This repo contains a toy implementation of a target change detector using Bazel. The goal is to accurately detect which targets need to be rebuilt when files change.

The basic approach is:

• Dump dependency information using bazel cquery
• Build a hash for each bazel target. For targets that represent files, the hash is the hash of the files' contents. For rules, it is the attributes of the rule and the hashes of its dependencies.
• Repeat at the second repo state
• Diff the hashes of bazel targets to determine changed targets

Naive approaches to this problem perform bazel queries based on a file list. This approach has the following advantages:

• Accurately detects effects of changes to BUILD files. Naive approaches will assume that when BUILD files change, all dependent targets must be rebuilt. This approach is resilient to BUILD files due to comment changes, attribute changes, etc. and is granular to only the affected targets.
• Accurately detects effects of changes to WORKSPACE file, for the same reasons as above
• Doesn't take a list of changed files as input (although it does need to run at two repo states, e.g. across a git checkout operation).

Try it

bazel run //:bazeldeps -- --diff_changes=local - diff local changes against last commit on current branch. Performs git stash operations to save/restore local changes.

bazel run //:bazeldeps -- --diff_changes=last_commits - diff changes between most recent commit on current branch and its predecessor. Performs git checkout operations to save/restore current checkout.

TODO: Some files/rules are ignored. Run with --alsologtostderr to see which files/rules these are. Rationale is they (probably) don't affect the build one way or the other.

This repo contains a hierarchy of toy C++ dependencies to experiment with.

File changes

1. Change math/add.h
2. bazel run //:bazeldeps -> changes //math:add.h, //math:add, //calculator

Rule changes

1. Change math/BUILD.bazel - add copts = ["-02"] attribute to add
2. bazel run //:bazeldeps -> changes //math:add, //calculator (but not //math:mul)

BUILD file comment change

1. Change math/BUILD.bazel - add comment
2. bazel run //:bazeldeps -> no changes

WORKSPACE changes

1. Change WORKSPACE - change pflag git commit to 972238283c0625cf3e881de7699ba8f2524c340a
2. bazel run //:bazeldeps -> changes to pflag files/targets, //:bazeldeps, //:go_default_library (but not other Go targets)

Starlark rule changes

TODO: This is currently unsupported (although very possible)