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penpot/.serena/memories/render-wasm/core.md
Andrey Antukh 5ef70c7284
✨ Serialize render-wasm builds per checkout (#11903) 
Protect shared setup, compilation, artifact copy, and target cleanup with
one flock lock per checkout. Route watch builds and frontend cleanup
through the protected scripts.

Document the lock contract and normalize the frontend and exporter
build:wasm commands.

Closes #11901

AI-assisted-by: Space Bunny Free
2026-09-25 09:42:27 +02:00

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render-wasm Architecture and Workflow

render-wasm/: Rust crate compiled to WebAssembly via Emscripten/Skia; frontend loads generated JS/WASM renderer. FFI/memory/tile behavior: mem:render-wasm/ffi-rendering-subtleties.

Stable Architecture

  • Exported functions live around src/main.rs / src/wapi.rs and are called from ClojureScript bridge namespaces under frontend/src/app/render_wasm*.
  • Updates are two-phase: ClojureScript calls exported setters to push shape data, then render_frame() performs Skia drawing.
  • Rendering is tile-based and shape data is stored separately from hierarchy.

Source Areas

  • src/state*: renderer state structures.
  • src/render/ and src/render.rs: tile/surface render pipeline.
  • src/shapes/ and src/shapes.rs: shape data and Skia drawing.
  • src/wasm/, src/wasm.rs, src/mem.rs: JS/WASM memory and interop helpers.
  • src/math/ and src/view.rs: geometry and viewport helpers.

Build Environment

./build sources _build_env, which sets the Emscripten paths and EMCC_CFLAGS. The WASM heap starts at 256 MB and uses geometric growth.

  • Linux builds require flock (util-linux).
  • build, target cleanup, and each watch rebuild share render-wasm/.render-wasm-build.lock per checkout. The dispatcher for both targets does not hold the lock in its parent process.
  • The lock covers setup, Cargo build, and artifact copy. The watch process releases it while waiting for changes. The operating system releases it on success, error, or signal; the file remains.
  • The lock only coordinates repository scripts. Manual Cargo commands do not take it.
  • Set RENDER_WASM_LOCK_FILE to the same path only when separate checkouts intentionally share a CARGO_TARGET_DIR.

Commands

From render-wasm/:

  • Build/copy frontend artifacts: ./build [frontend|export]; no target builds frontend, then export.
  • Clean one target: ./clean [frontend|export]; never run root cargo clean on the shared target/.
  • Watch rebuild: ./watch [frontend|export]; its initial and change-triggered builds use ./build.
  • Rust tests: ./test or cargo test <name>.
  • Cross-cutting testing principles and anti-patterns: mem:testing.
  • Lint: ./lint.
  • Format check: cargo fmt --check.

Do not change exported WASM function signatures without updating the corresponding frontend bridge and verifying the frontend renderer path.