debugging - SKILL.md Agent Skill

name: debugging description: How to debug tursodb using Bytecode comparison, logging, ThreadSanitizer, deterministic simulation, and corruption analysis tools

Debugging Guide

Bytecode Comparison Flow

Turso aims for SQLite compatibility. When behavior differs:

1. EXPLAIN query in sqlite3
2. EXPLAIN query in tursodb
3. Compare bytecode
   ├─ Different → bug in code generation
   └─ Same but results differ → bug in VM or storage layer

Example

# SQLite
sqlite3 :memory: "EXPLAIN SELECT 1 + 1;"

# Turso
cargo run --bin tursodb :memory: "EXPLAIN SELECT 1 + 1;"

Manual Query Inspection

cargo run --bin tursodb :memory: 'SELECT * FROM foo;'
cargo run --bin tursodb :memory: 'EXPLAIN SELECT * FROM foo;'

Logging

# Trace core during tests
RUST_LOG=none,turso_core=trace make test

# Output goes to testing/test.log
# Warning: can be megabytes per test run

Threading Issues

Use stress tests with ThreadSanitizer:

rustup toolchain install nightly
rustup override set nightly
cargo run -Zbuild-std --target x86_64-unknown-linux-gnu \
  -p turso_stress -- --vfs syscall --nr-threads 4 --nr-iterations 1000

Deterministic Simulation

Reproduce bugs with seed. Note: simulator uses legacy "limbo" naming.

# Simulator
RUST_LOG=limbo_sim=debug cargo run --bin limbo_sim -- -s <seed>

# Whopper (concurrent DST)
SEED=1234 ./testing/concurrent-simulator/bin/run

Architecture Reference

Parser → AST from SQL strings
Code generator → bytecode from AST
Virtual machine → executes SQLite-compatible bytecode
Storage layer → B-tree operations, paging

Corruption Debugging

For WAL corruption and database integrity issues, use the corruption debug tools in scripts.

See references/CORRUPTION-TOOLS.md for detailed usage.