feat: do not return empty matrices if no package has to be built

Author: jfroche

.github/workflows/nix-build.yml

@@ -34,19 +34,21 @@ jobs:
       matrix: ${{ fromJSON(needs.nix-eval.outputs.matrix).aarch64_linux }}
     steps:
       - name: Checkout Repo
+        if: ${{ matrix.attr != '' }}
         uses: actions/checkout@v4
       - name: Install nix (ephemeral)
-        if: ${{ matrix.runs_on.group != 'self-hosted-runners-nix' }}
+        if: ${{ matrix.attr != '' && matrix.runs_on.group != 'self-hosted-runners-nix' }}
         uses: ./.github/actions/nix-install-ephemeral
         with:
           push-to-cache: 'true'
         env:
           DEV_AWS_ROLE: ${{ secrets.DEV_AWS_ROLE }}
           NIX_SIGN_SECRET_KEY: ${{ secrets.NIX_SIGN_SECRET_KEY }}
       - name: Install nix (self-hosted)
-        if: ${{ matrix.runs_on.group == 'self-hosted-runners-nix' }}
+        if: ${{ matrix.attr != '' && matrix.runs_on.group == 'self-hosted-runners-nix' }}
         uses: ./.github/actions/nix-install-self-hosted
       - name: nix build
+        if: ${{ matrix.attr != '' }}
         shell: bash
         run: nix build --accept-flake-config -L .#${{ matrix.attr }}
 
@@ -55,18 +57,21 @@ jobs:
       ${{ matrix.name }}${{ matrix.postgresql_version && format(' - Postgres {0}', matrix.postgresql_version) || '' }}
       (aarch64-darwin)
     needs: nix-eval
-    runs-on: ${{ matrix.runs_on.group && matrix.runs_on || matrix.runs_on.labels }}
+    runs-on: ${{ matrix.attr != '' && matrix.runs_on.group && matrix.runs_on || matrix.runs_on.labels }}
     if: ${{ fromJSON(needs.nix-eval.outputs.matrix).aarch64_darwin != null }}
     strategy:
       fail-fast: false
       max-parallel: 5
       matrix: ${{ fromJSON(needs.nix-eval.outputs.matrix).aarch64_darwin }}
     steps:
       - name: Checkout Repo
+        if: ${{ matrix.attr != '' }}
         uses: actions/checkout@v4
       - name: Install nix
+        if: ${{ matrix.attr != '' }}
         uses: ./.github/actions/nix-install-self-hosted
       - name: nix build
+        if: ${{ matrix.attr != '' }}
         shell: bash
         run: nix build --accept-flake-config -L .#${{ matrix.attr }}
 
@@ -75,23 +80,26 @@ jobs:
       ${{ matrix.name }}${{ matrix.postgresql_version && format(' - Postgres {0}', matrix.postgresql_version) || '' }}
       (x86_64-linux)
     needs: nix-eval
-    runs-on: ${{ matrix.runs_on.group && matrix.runs_on || matrix.runs_on.labels }}
+    runs-on: ${{ matrix.attr != '' && matrix.runs_on.group && matrix.runs_on || matrix.runs_on.labels }}
     if: ${{ fromJSON(needs.nix-eval.outputs.matrix).x86_64_linux != null }}
     strategy:
       fail-fast: false
       max-parallel: 5
       matrix: ${{ fromJSON(needs.nix-eval.outputs.matrix).x86_64_linux }}
     steps:
       - name: Checkout Repo
+        if: ${{ matrix.attr != '' }}
         uses: actions/checkout@v4
       - name: Install nix
+        if: ${{ matrix.attr != '' }}
         uses: ./.github/actions/nix-install-ephemeral
         with:
           push-to-cache: 'true'
         env:
           DEV_AWS_ROLE: ${{ secrets.DEV_AWS_ROLE }}
           NIX_SIGN_SECRET_KEY: ${{ secrets.NIX_SIGN_SECRET_KEY }}
       - name: nix build
+        if: ${{ matrix.attr != '' }}
         shell: bash
         run: nix build --accept-flake-config -L .#${{ matrix.attr }}
 

nix/packages/github-matrix/github_matrix.py

@@ -17,6 +17,7 @@
     Optional,
     Set,
     TypedDict,
+    get_args,
 )
 
 System = Literal["x86_64-linux", "aarch64-linux", "aarch64-darwin"]
@@ -252,10 +253,23 @@ def clean_package_for_output(pkg: NixEvalJobsOutput) -> GitHubActionPackage:
             grouped_by_system[pkg["system"]].append(clean_package_for_output(pkg))
 
     # Create output with system-specific matrices
+    # Ensure that we have at least one entry per system
     gh_output = {}
     for system, packages in grouped_by_system.items():
         gh_output[system.replace("-", "_")] = {"include": packages}
 
+    for system in get_args(System):
+        if system not in gh_output:
+            gh_output[system.replace("-", "_")] = {
+                "include": [
+                    {
+                        "attr": "",
+                        "name": "skipped",
+                        "system": system,
+                        "runs_on": {"labels": "ubuntu-latest"},
+                    }
+                ]
+            }
     print(
         f"debug: Generated GitHub Actions matrix: {json.dumps(gh_output, indent=2)}",
         file=sys.stderr,