It‘s sometimes necessary to bisect upstream Rust revisions in order to resolve problems we’re seeing in ChromeOS. To this end, cros-rustc.eclass has built-in support for building arbitrary Rust sources. The scripts in this directory try to make use of that support in order to make bisection easier.
NOTE: Rust bisection modifies cros-rustc.eclass. If you have local changes, please save them somehow before continuing.
Setting up for Rust bisection is relatively straightforward:
./setup_rust_for_bisection.sh../run_bisection_step.sh to reproduce the issue you're interested in.cd your/path/to/dev-lang/rust/files/rust.git bisect start --first-parent, being sure to provide old/new SHAs to git bisect. --first-parent is highly recommended due to the structure of some Rust merge commits; git bisect without --first-parent can end up checking out a Clippy repository instead of rustc (see first-parent comments), which is not what we want in general.git bisect run /your/path/to/run_bisection_step.sh.Wait for git bisect to discover where things started breaking.
As referenced, cros-rustc.eclass has support for using an arbitrary Rust source directory. This support is intended for local debugging only, so the only way to access it is by modifying the eclass itself. Once this support is enabled, we clone a full Rust repository to dev-lang/rust/files/rust, which is where cros-rustc.eclass expects Rust sources to be located. ./setup_rust_for_bisection.sh takes care of all of this.
To prepare this checkout to be built by emerge dev-lang/rust{,-host}, we run ./clean_and_sync_rust_root.sh to clean up any changes that‘ve been made to the source tree, and ensure that all of Rust’s submodules are synced appropriately, given the current HEAD of the main Rust checkout. Further, we run ./prepare_rust_for_offline_build.sh to download the correct bootstrap compiler, and update all files in Rust‘s vendor/ directory. These tools are intentionally kept separate, since it’s expected that users may be interested in making changes to a clean source tree before we run cargo vendor.
After all of the above setup is done, our Rust checkout at dev-lang/rust/files/rust can be successfully built offline, which is necessary to keep emerge happy. At this point, emerge dev-lang/rust{,-host} is run. This command installs the Rust version that was checked out by the user in dev-lang/rust/files/rust, rather than the version advertised by our Rust ebuild.
Since the above establishes an environment where emerge dev-lang/rust{,-host} can build Rust at arbitrary commits of a Git repository, bisection proceeds as one might expect -- you simply run something that boils down to:
git -C dev-lang/rust/files/rust bisect run bash -c '
../bisect-scripts/clean_and_sync_rust_root.sh || exit 128
../bisect-scripts/prepare_rust_for_offline_build.sh || exit 128
# `git bisect run` needs the working tree to be sufficiently clean. If
# e.g., portage applied patches, those need to be unapplied in many cases for
# bisection to continue.
trap '../bisect-scripts/clean_and_sync_rust_root.sh || :' EXIT
emerge dev-lang/rust{,-host} || exit 125
## Your test case goes here, setting $exit_status ##
'
For convenience, ./run_bisection_step.sh is available as a substitute for the bash -c '...' part of the above. It's written to be a stub that anyone can extend for their specific bisection needs; use of it is encouraged, since git bisect run has sharp corners when e.g., bisection setup commands unexpectedly fail.