Bisect-Aware Instrumentation

Overview

Instrument code to support efficient git bisect operations by producing deterministic pass/fail signals and concise runtime summaries. This skill helps create robust test scripts that work reliably with git bisect run, handling edge cases like flaky tests, build failures, and non-deterministic behavior.

Core Workflow

1. Understand the Regression

Before instrumenting, clarify:

• What behavior changed? (bug introduced, performance regression, test failure)
• What is the "good" commit? (known working state)
• What is the "bad" commit? (known broken state)
• How to reproduce the issue? (test command, manual steps)

2. Create Bisect Test Script

Generate a test script that returns proper exit codes for git bisect:

Exit Code Convention:

• 0: Good commit (test passes)
• 1-124, 126-127: Bad commit (test fails)
• 125: Skip commit (cannot test - build failure, missing dependencies)

Template:

#!/bin/bash
# bisect_test.sh - Test script for git bisect run

set -e  # Exit on error

# Build/setup phase
if ! make build 2>/dev/null; then
    echo "SKIP: Build failed"
    exit 125
fi

# Run test with timeout
timeout 30s ./run_test || TEST_RESULT=$?

# Interpret results
if [ $TEST_RESULT -eq 0 ]; then
    echo "GOOD: Test passed"
    exit 0
elif [ $TEST_RESULT -eq 124 ]; then
    echo "SKIP: Test timeout"
    exit 125
else
    echo "BAD: Test failed with code $TEST_RESULT"
    exit 1
fi

3. Add Determinism Safeguards

Handle non-deterministic behavior:

Retry Logic for Flaky Tests:

# Run test multiple times to confirm
PASS_COUNT=0
for i in {1..3}; do
    if ./run_test; then
        ((PASS_COUNT++))
    fi
done

if [ $PASS_COUNT -eq 3 ]; then
    echo "GOOD: All 3 runs passed"
    exit 0
elif [ $PASS_COUNT -eq 0 ]; then
    echo "BAD: All 3 runs failed"
    exit 1
else
    echo "SKIP: Flaky test ($PASS_COUNT/3 passed)"
    exit 125
fi

Environment Isolation:

# Clean state before each test
rm -rf /tmp/test_cache
export RANDOM_SEED=42
export TZ=UTC

4. Add Logging and Summaries

Generate concise output for each commit:

#!/bin/bash
COMMIT=$(git rev-parse --short HEAD)
LOG_FILE="bisect_log_${COMMIT}.txt"

echo "Testing commit: $COMMIT" | tee $LOG_FILE
echo "Timestamp: $(date -u +%Y-%m-%dT%H:%M:%SZ)" | tee -a $LOG_FILE

# Run test and capture output
if ./run_test > test_output.txt 2>&1; then
    echo "RESULT: GOOD" | tee -a $LOG_FILE
    exit 0
else
    echo "RESULT: BAD" | tee -a $LOG_FILE
    echo "Error output:" | tee -a $LOG_FILE
    tail -20 test_output.txt | tee -a $LOG_FILE
    exit 1
fi

5. Run Git Bisect

Execute the bisect workflow:

# Start bisect
git bisect start

# Mark known good and bad commits
git bisect bad HEAD
git bisect good v1.2.0

# Run automated bisect
chmod +x bisect_test.sh
git bisect run ./bisect_test.sh

# Review results
git bisect log

Instrumentation Patterns

Pattern 1: Performance Regression Detection

#!/bin/bash
# Detect when performance drops below threshold

THRESHOLD=1000  # milliseconds

# Run benchmark
DURATION=$(./benchmark | grep "Duration:" | awk '{print $2}')

if [ -z "$DURATION" ]; then
    echo "SKIP: Benchmark failed to run"
    exit 125
fi

if [ $DURATION -lt $THRESHOLD ]; then
    echo "GOOD: Performance $DURATION ms (< $THRESHOLD ms)"
    exit 0
else
    echo "BAD: Performance $DURATION ms (>= $THRESHOLD ms)"
    exit 1
fi

Pattern 2: Test Suite Bisection

#!/bin/bash
# Find commit that broke specific test

TEST_NAME="test_user_authentication"

# Run specific test
if pytest tests/${TEST_NAME}.py -v; then
    echo "GOOD: $TEST_NAME passed"
    exit 0
else
    echo "BAD: $TEST_NAME failed"
    exit 1
fi

Pattern 3: Build Failure Detection

#!/bin/bash
# Find commit that broke the build

if make clean && make all; then
    echo "GOOD: Build succeeded"
    exit 0
else
    echo "BAD: Build failed"
    exit 1
fi

Pattern 4: Output Validation

#!/bin/bash
# Find commit that changed program output

EXPECTED_OUTPUT="Success: 42"

ACTUAL_OUTPUT=$(./program 2>&1)

if [ "$ACTUAL_OUTPUT" = "$EXPECTED_OUTPUT" ]; then
    echo "GOOD: Output matches expected"
    exit 0
else
    echo "BAD: Output mismatch"
    echo "  Expected: $EXPECTED_OUTPUT"
    echo "  Actual: $ACTUAL_OUTPUT"
    exit 1
fi

Advanced Techniques

Handling Complex Build Systems

#!/bin/bash
# Handle projects with complex dependencies

# Check if dependencies are available
if ! command -v node &> /dev/null; then
    echo "SKIP: Node.js not available in this commit"
    exit 125
fi

# Install dependencies (with caching)
if [ -f package.json ]; then
    npm ci --silent || {
        echo "SKIP: Dependency installation failed"
        exit 125
    }
fi

# Run test
npm test

Parallel Test Execution

#!/bin/bash
# Run multiple tests in parallel for faster bisection

# Run tests in parallel
parallel --halt soon,fail=1 ::: \
    "pytest tests/unit/" \
    "pytest tests/integration/" \
    "npm run lint"

if [ $? -eq 0 ]; then
    echo "GOOD: All tests passed"
    exit 0
else
    echo "BAD: At least one test failed"
    exit 1
fi

State Preservation

#!/bin/bash
# Preserve state between bisect steps

STATE_DIR=".bisect_state"
mkdir -p $STATE_DIR

# Save current commit info
git rev-parse HEAD > $STATE_DIR/current_commit

# Run test
./run_test
RESULT=$?

# Log result
echo "$(git rev-parse --short HEAD): $RESULT" >> $STATE_DIR/results.log

exit $RESULT

Troubleshooting

Issue: Bisect Marks Wrong Commit

Cause: Test script has incorrect exit codes or flaky behavior

Solution: Add verbose logging and retry logic

set -x  # Enable debug output
# Add retry logic as shown in section 3

Issue: Too Many Commits Skipped

Cause: Build failures or missing dependencies across history

Solution: Use broader skip conditions

# Skip commits with known issues
if git log -1 --format=%s | grep -q "WIP\|broken"; then
    echo "SKIP: Known broken commit"
    exit 125
fi

Issue: Bisect Takes Too Long

Cause: Slow test execution

Solution: Optimize test or use binary search hints

# Use timeout to fail fast
timeout 10s ./run_test || exit 125

# Or provide bisect hints
git bisect skip $(git rev-list --grep="refactor" HEAD~50..HEAD)

Best Practices

1. Make tests deterministic: Fix random seeds, timestamps, and external dependencies
2. Use timeouts: Prevent hanging tests from blocking bisect
3. Log everything: Save detailed logs for each tested commit
4. Handle build failures gracefully: Use exit code 125 to skip untestable commits
5. Test the test script: Verify it works on known good and bad commits before bisecting
6. Keep it fast: Optimize tests to run quickly (bisect tests O(log n) commits)

Quick Reference

Start bisect:

git bisect start
git bisect bad <bad-commit>
git bisect good <good-commit>

Run automated bisect:

git bisect run ./bisect_test.sh

Manual bisect:

git bisect good  # Current commit is good
git bisect bad   # Current commit is bad
git bisect skip  # Cannot test current commit

End bisect:

git bisect reset

Resources

• references/git_bisect_guide.md: Comprehensive git bisect documentation
• references/exit_codes.md: Exit code conventions and best practices
• scripts/bisect_template.sh: Template bisect test script
• scripts/bisect_wrapper.py: Python wrapper for complex bisect logic