test: nix flake checks with headless sway capture

nix/checks.ulu.nix

@@ -0,0 +1,68 @@
+{ ... }:
+{
+  perSystem =
+    { pkgs, self', ... }:
+    let
+      valw = self'.packages.default;
+
+      # Reuses the package's vendored dependencies, runs `cmd` instead of the build.
+      cargoCheck =
+        name: tools: cmd:
+        valw.overrideAttrs (old: {
+          pname = "valw-${name}";
+          nativeBuildInputs = old.nativeBuildInputs ++ tools;
+          buildPhase = cmd;
+          installPhase = "touch $out";
+        });
+    in
+    {
+      checks = {
+        clippy = cargoCheck "clippy" [ pkgs.clippy ] "cargo clippy --all-targets --offline -- -D warnings";
+        nextest = cargoCheck "nextest" [ pkgs.cargo-nextest ] "cargo nextest run --offline";
+
+        fmt = pkgs.runCommand "valw-fmt" { nativeBuildInputs = [ pkgs.rustfmt ]; } ''
+          rustfmt --check --edition 2024 ${valw.src}/src/main.rs
+          touch $out
+        '';
+
+        # Runs the real screencopy path against a headless sway. This is
+        # wlroots, not niri, but it catches capture and format bugs on every build.
+        headless =
+          pkgs.runCommand "valw-headless"
+            {
+              nativeBuildInputs = [
+                pkgs.file
+                pkgs.sway-unwrapped
+                valw
+              ];
+            }
+            ''
+              export HOME=$TMPDIR/home XDG_RUNTIME_DIR=$TMPDIR/run
+              mkdir -p $HOME
+              mkdir -m 700 $XDG_RUNTIME_DIR
+              export WLR_BACKENDS=headless WLR_RENDERER=pixman WLR_LIBINPUT_NO_DEVICES=1
+
+              echo 'output HEADLESS-1 resolution 1280x720' > sway.conf
+              sway -c sway.conf > sway.log 2>&1 &
+
+              for _ in $(seq 100); do
+                socket=$(ls $XDG_RUNTIME_DIR | grep -m1 '^wayland-[0-9]*$' || true)
+                [ -n "$socket" ] && break
+                sleep 0.1
+              done
+              [ -n "$socket" ] || { cat sway.log; exit 1; }
+              export WAYLAND_DISPLAY=$socket
+
+              valw doctor | tee doctor.txt
+              ! grep -q '^fail' doctor.txt
+
+              # Through a pipe: the clipboard child must not keep it open.
+              timeout 10 valw screen -o - | cat > shot.png
+              file shot.png | tee file.txt
+              grep -q 'PNG image data, 1280 x 720' file.txt
+
+              touch $out
+            '';
+      };
+    };
+}