flake: add bench run step

flake.nix

@@ -49,6 +49,44 @@
         zig fmt --check .
       '';
 
+      # nix run .#bench
+      apps.bench = with env.pkgs; env.app ([git coreutils gnutar] ++ optionals (stdenv.isLinux) [poop]) ''
+        set -e
+
+        tmpdir="$(mktemp -d)"
+        trap 'rm -rf "$tmpdir"' EXIT
+
+        [[ "$(uname)" == "Linux" ]] && zig build -Daio:posix=disable -p "$tmpdir/new-uring"
+        zig build -Daio:posix=force -p "$tmpdir/new-posix"
+
+        git archive "''${COMMIT:-HEAD~1}" | tar x -C "$tmpdir"
+        cd "$tmpdir"
+
+        [[ "$(uname)" == "Linux" ]] && zig build -Daio:posix=disable -p "$tmpdir/old-uring"
+        zig build -Daio:posix=force -p "$tmpdir/old-posix"
+
+        all=(old-posix/bench/*)
+        for bench in "''${@:-''${all[@]}}"; do
+          base="$(basename "$bench")"
+          [[ -f "new-posix/bench/$base" ]] || continue
+          if [[ "''${POOP:-0}" == 1 ]]; then
+            poop "old-uring/bench/$base" "new-uring/bench/$base"
+            poop "old-posix/bench/$base" "new-posix/bench/$base"
+          else
+            if [[ "$(uname)" == "Linux" ]]; then
+              echo "$base: old-uring"
+              "old-uring/bench/$base"
+              echo "$base: new-uring"
+              "new-uring/bench/$base"
+            fi
+            echo "$base: old-posix"
+            "old-posix/bench/$base"
+            echo "$base: new-posix"
+            "new-posix/bench/$base"
+          fi
+        done
+      '';
+
       # nix run .#deps
       apps.deps = env.showExternalDeps;