# Rust shell reveal stream: host checkpoint

Source: `nix/rust-shell-client/src/protocol.rs`, `src/main.rs`, and `src/render.rs` in this commit. This is an opt-in Rust client checkpoint; it is not an installed session or physical touch result.

The client accepts the compositor's bounded newline JSON v1 `begin`, `update`, `finish`, and `cancel` messages on the private 0600 `k230-shell-rust.sock`. A complete `finish` followed by EOF retains the settled-open drawer or shade; an incomplete stream settles closed. Sequence and route must remain consistent, malformed or oversized lines abort, and a five-second idle peer timeout bounds abandoned contacts. The compositor continues to own gesture classification and live app or deck pixels. The client renders a transparent full-output overlay with no input region during tracking; the panel receives input only after the open settle. Cairo/Pango scene shaping is cached across progress updates, which copy/translate the prepared scene into bounded SHM buffers. This says nothing yet about presented frame rate on the board.

Host proof from the source tree:

```sh
cargo fmt --manifest-path nix/rust-shell-client/Cargo.toml --check
cargo test --manifest-path nix/rust-shell-client/Cargo.toml --locked
cargo clippy --manifest-path nix/rust-shell-client/Cargo.toml --locked --all-targets -- -D warnings
git diff --check
```

All completed successfully on 2026-09-23. The Rust unit tests include split socket writes, mid-drag reversal, cancellation, premature EOF, completed-finish EOF, input progress, renderer cache reuse, and geometry bounds.

The exact committed source `a8fa5d3283e679724a554d9db52203b98f90ce0e` was built with:

```sh
nix build "git+file://$PWD?rev=a8fa5d3283e679724a554d9db52203b98f90ce0e#handheld-shell-rust" --max-jobs 1 --cores 4 --no-link --print-out-paths
```

The target derivation was `/nix/store/f7kn0zqldgh9bkj48k2pjx2b966wd3k4-k230-shell-rust-riscv64-unknown-linux-gnu-0.1.0.drv`; output `/nix/store/kn3x90rplwfrf83z1nk0jjpa0whigrk1-k230-shell-rust-riscv64-unknown-linux-gnu-0.1.0`. `file` reports ELF64 RISC-V LP64D. `nix path-info -S` reports an 86,450,072-byte recursive closure. This is package proof only. Paired QEMU pixels/input routing and physical board observations remain open.

Review found that buffered progress lines incurred one 16 ms event-loop pause each. Commit `692daa9a0dc788c081af03d27af4691b93de7b5b` drains at most 64 records per pass, coalesces contiguous updates of the same gesture while preserving begin/terminal ordering, and reads `K230_SETTINGS_REDUCED_MOTION=1` for the shorter settle. Exact source extracted with `git archive 692daa9a` passed 13 host tests (11 library, 2 route server) and Clippy with `-D warnings`. Its immutable build passed with:

```sh
nix build "git+file://$PWD?rev=692daa9a0dc788c081af03d27af4691b93de7b5b#handheld-shell-rust" --max-jobs 1 --cores 4 --no-link --print-out-paths
```

Derivation `/nix/store/kaa33n1b2l1vlyjzsd42si2x229qar0v-k230-shell-rust-riscv64-unknown-linux-gnu-0.1.0.drv` produced `/nix/store/h56a3xk2nmjh29vi1yd1g0q5iks4rpg3-k230-shell-rust-riscv64-unknown-linux-gnu-0.1.0`. This revised binary has not been physically tested. The paired QEMU result for earlier `a8fa5d32` is recorded separately by the compositor owner and must not be attributed to the revised build.