scripts/clang/bisect-llvm-fuchsia-breakage

#!/bin/bash
# Copyright 2025 The Fuchsia Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

set -e
set -o pipefail

function usage {
  cat <<EOF
Usage: $(basename "$0") <toolchain_builder> <fuchsia_builder>

Description:
  This script is intended to be used with 'git bisect run' to find the
  LLVM commit that introduced a breakage in a Fuchsia build.
  It uses led to build a toolchain at the current LLVM commit, then
  uses led to launch a Fuchsia build using that toolchain, and reports
  the Fuchsia build status back to git bisect.

Arguments:
  <toolchain_builder>  (e.g., clang-linux-x64)
  <fuchsia_builder>    (e.g., core.x64)

Examples:
  # Run the script directly (e.g., for testing)
  $(basename "$0") clang-linux-x64 core.x64

  # Use with git bisect run
  git bisect start
  git bisect bad <commit-where-breakage-occurs>
  git bisect good <commit-before-breakage>
  git bisect run $(basename "$0") clang-linux-x64 core.x64
EOF
} # end usage

if [[ "$#" -ne 2 ]]; then
  echo "Error: Incorrect number of arguments." >&2
  usage
  exit 125
fi

readonly TOOLCHAIN_BUILDER=$1
readonly FUCHSIA_BUILDER=$2

# Get the current LLVM commit hash. This script is intended to be run
# from within the LLVM repository during a git bisect session.
readonly LLVM_COMMIT="$(git rev-parse HEAD)"
echo "Current LLVM commit: $LLVM_COMMIT"

echo "Building toolchain using $TOOLCHAIN_BUILDER at LLVM commit $LLVM_COMMIT..."

readonly DIRNAME="$(dirname "$0")"

# Launch the toolchain build at the specific LLVM commit and wait for its CAS digest.
# We replicate the led-try-head launch logic but pipe the output of led launch
# to led-wait-for-cas to get the digest.
TOOLCHAIN_DIGEST="$(
  led get-builder fuchsia/try:$TOOLCHAIN_BUILDER | \
    led edit -pa revision="$LLVM_COMMIT" | \
    led launch | \
    "$DIRNAME/led-wait-for-cas"
)"

# Check if we successfully obtained a digest. led-wait-for-cas exits non-zero
# on failure, which set -e will catch, but an explicit check is also good.
if [[ -z "$TOOLCHAIN_DIGEST" ]]; then
  echo "error: failed to get toolchain CAS digest from led-wait-for-cas"
  exit 125
fi

echo "Toolchain built successfully. CAS digest: $TOOLCHAIN_DIGEST"
echo "Launching Fuchsia build using $FUCHSIA_BUILDER with this toolchain..."

# TODO(https://fxbug.dev/447666217): differentiate between expected vs
# unexpected failures when checking the status of the fuchsia build.

# Run led-launch-fuchsia with the Fuchsia builder and the captured digest,
# and pipe the output (build ID) to led-wait-for-status to wait for the build
# and get its exit status for git bisect run.
"$DIRNAME/led-launch-fuchsia" "$FUCHSIA_BUILDER" "$TOOLCHAIN_DIGEST" \
  | "$DIRNAME/led-wait-for-status"