Check on CI demo sanity on Mac and Linux

Author: vrom911

.github/workflows/demo-test.yaml

@@ -0,0 +1,57 @@
+name: "Demo Sanity Check"
+
+on:
+  merge_group:
+  push:
+    branches:
+      - master
+      - release
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  demo-test:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+    runs-on: ${{ matrix.os }}
+    name: "Test demo on ${{ matrix.os }}"
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          submodules: true
+
+      - name: ❄ Setup Nix/Cachix
+        uses: ./.github/actions/nix-cachix-setup
+        with:
+          authToken: '${{ secrets.CACHIX_CARDANO_SCALING_AUTH_TOKEN }}'
+
+      - name: Modify demo to run headlessly
+        run: |
+          sed -i.bak 's/is_foreground = true/is_foreground = false/g' nix/hydra/demo.nix
+
+      - name: Run demo in background
+        run: |
+          nix run .#demo &
+          echo $! > process-compose.pid
+
+      - name: Wait for hydra nodes to be ready
+        run: |
+          set -e
+          echo "Waiting for hydra nodes to start..."
+          nix shell nixpkgs#coreutils -c timeout 300s bash -c '
+            until [ $(grep -c "hydra-node-.* is ready" devnet/logs/process-compose.log 2>/dev/null || echo 0) -ge 3 ]; do
+              echo -n "."
+              sleep 5
+              if ! ps -p $(cat process-compose.pid) > /dev/null; then
+                echo "Process-compose exited prematurely."
+                cat devnet/logs/process-compose.log
+                exit 1
+              fi
+            done
+            echo "All hydra nodes are ready!"
+          '
+      - name: Stop demo
+        if: always()
+        run: |
+          kill $(cat process-compose.pid)