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Architecture

ADR 0010: no GPL code; clean-room implementation from papers

Accepted · 2026-10-31 · racing-drone/docs/adr/0010-no-gpl-clean-room-policy.md

Status: Accepted Date: 2026-10-31 Source: the stack’s no-GPL policy, set in cortex’s ADR 0002, which cmake/deps.cmake already cites Related: ADR 0009

Context

Section titled “Context”

cortex’s ADR 0002 rules out GPL code anywhere in cortex, for any purpose. The reason applies to racing-drone unchanged: partners redistribute the stack inside their own certified products, and any GPL obligation, even one confined to a test, is something their legal reviews would have to clear.

In drone racing the pull towards GPL code is strong. Much of the published research code is GPL-licensed, and so are the flight stacks racing quadrotors usually fly:

  • UZH’s racing and agile-flight code: rpg_mpc and rpg_quadrotor_mpc (MPC), rpg_time_optimal (time-optimal planning) and agile_autonomy (“Learning high-speed flight in the wild”), all GPL-3.0
  • Betaflight, INAV and Cleanflight (GPL-3.0)
  • ArduPilot (GPL-3.0)
  • Paparazzi (GPL-2.0)

Permissive references exist too: PX4 (BSD-3-Clause), and UZH’s agile_flight and deep_drone_acrobatics (MIT). The baselines’ designs (MPCC, INDI, the gate-relative estimator, ADRs 0002 to 0004) come from papers.

Decision

Section titled “Decision”

No GPL-licensed code anywhere in racing-drone, for any purpose. Not as a dependency, not in a test target, not as a build-time tool in what racing-drone builds, and not as a reference oracle. The scope is ADR 0009’s: what racing-drone ships, the code that builds it and the tests, and not the docs site’s toolchain.

  • Everything is implemented clean-room from papers, textbooks and specifications, plus permissively licensed (MIT / BSD / Apache-2.0) references.
  • Check a reference’s license before opening its code. Reading GPL source and then writing the same thing “from scratch” is not clean-room. A GPL project’s structure may be consulted, meaning its directory layout and how it splits the problem into modules, and nothing else: never its code, its algorithms’ details, or its data, which includes tuning tables, gains, parameters and trained weights.
  • Test oracles are in-house or permissive: racing-drone’s own simulator and courses, recorded flights it made itself, results published in papers, and permissive code such as PX4. Never compare against the output of a GPL program (the GPL-3.0 UZH code above, Betaflight, ArduPilot, …), its simulator or its logs.
  • Conventions are not code. Following a convention that a GPL project also uses (a frame, a rotor order, a unit) is fine, when it is taken from a permissive source or a specification.
  • LGPL, MPL, AGPL and SSPL code is rejected, as a dependency and as a reference, as ADR 0009 rejects it. An exception would need an ADR of its own that supersedes this one.

Consequences

Section titled “Consequences”
  • Each component costs more to build: the design comes from the literature, and validation uses racing-drone’s own simulation and permissive references.
  • A paper’s results can be compared against; its GPL code cannot be read or run.
  • Partners can redistribute racing-drone under MIT terms with no copyleft review.
  • Current state: racing-drone’s baselines, worlds and scorers are written in-house from papers. No GPL code or oracle is used, and no learned policy or third-party dataset is in the repo yet.