Using Enrico's patch for the action to report errors properly

.github/workflows/nix-action-master.yml

@@ -36,12 +36,19 @@ jobs:
         authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
         extraPullNames: coq, math-comp
         name: coq-community
+    - id: stepGetDerivation
+      name: Getting derivation for current job (coq)
+      run: "NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link \\\n   --argstr bundle
+        \"master\" --argstr job \"coq\" \\\n   --dry-run 2> err > out || (touch fail;
+        true)\n"
+    - name: Error reporting
+      run: "echo \"out=\"; cat out\necho \"err=\"; cat err\n"
+    - name: Failure check
+      run: if [ -e fail ]; then exit 1; else exit 0; fi;
     - id: stepCheck
-      name: Checking presence of CI target coq
-      run: "nb_dry_run=$(NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link \\\n   --argstr
-        bundle \"master\" --argstr job \"coq\" \\\n   --dry-run 2>&1 > /dev/null)\n
-        echo $nb_dry_run\necho status=$(echo $nb_dry_run | grep \"built:\" | sed \"\
-        s/.*/built/\") >> $GITHUB_OUTPUT\n"
+      name: Checking presence of CI target for current job
+      run: "(echo -n status=; cat out | grep \\\"built:\\\" | sed \\\"s/.*/built/\\\
+        \") >> $GITHUB_OUTPUT\n"
     - if: steps.stepCheck.outputs.status == 'built'
       name: Building/fetching current CI target
       run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "master"
@@ -84,12 +91,19 @@ jobs:
         authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
         extraPullNames: coq, math-comp
         name: coq-community
+    - id: stepGetDerivation
+      name: Getting derivation for current job (coq-shell)
+      run: "NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link \\\n   --argstr bundle
+        \"master\" --argstr job \"coq-shell\" \\\n   --dry-run 2> err > out || (touch
+        fail; true)\n"
+    - name: Error reporting
+      run: "echo \"out=\"; cat out\necho \"err=\"; cat err\n"
+    - name: Failure check
+      run: if [ -e fail ]; then exit 1; else exit 0; fi;
     - id: stepCheck
-      name: Checking presence of CI target coq-shell
-      run: "nb_dry_run=$(NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link \\\n   --argstr
-        bundle \"master\" --argstr job \"coq-shell\" \\\n   --dry-run 2>&1 > /dev/null)\n
-        echo $nb_dry_run\necho status=$(echo $nb_dry_run | grep \"built:\" | sed \"\
-        s/.*/built/\") >> $GITHUB_OUTPUT\n"
+      name: Checking presence of CI target for current job
+      run: "(echo -n status=; cat out | grep \\\"built:\\\" | sed \\\"s/.*/built/\\\
+        \") >> $GITHUB_OUTPUT\n"
     - if: steps.stepCheck.outputs.status == 'built'
       name: 'Building/fetching previous CI target: coq'
       run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "master"

action.nix

@@ -76,16 +76,35 @@ with builtins; with lib; let
         extraPullNames = map (v: v.name) (tail reordered);
       })) ];
 
-  stepCheck = { job, bundles ? [] }:
+  stepGetDerivation = { job, bundles ? [] }:
     let bundlestr = if isList bundles then "\${{ matrix.bundle }}" else bundles; in {
-    name = "Checking presence of CI target ${job}";
-    id = "stepCheck";
+    name = "Getting derivation for current job (${job})";
+    id = "stepGetDerivation";
     run = ''
-      nb_dry_run=$(NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link \
+      NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link \
          --argstr bundle "${bundlestr}" --argstr job "${job}" \
-         --dry-run 2>&1 > /dev/null)
-      echo $nb_dry_run
-      echo status=$(echo $nb_dry_run | grep "built:" | sed "s/.*/built/") >> $GITHUB_OUTPUT
+         --dry-run 2> err > out || (touch fail; true)
+    '';
+  };
+
+  stepErrorReporting = {
+    name = "Error reporting";
+    run = ''
+        echo "out="; cat out
+        echo "err="; cat err
+    '';
+  };
+
+  stepFailureCheck = {
+    name = "Failure check";
+    run = "if [ -e fail ]; then exit 1; else exit 0; fi;";
+  };
+
+  stepCheck = {
+    name = "Checking presence of CI target for current job";
+    id = "stepCheck";
+    run = ''
+      (echo -n status=; cat out | grep \"built:\" | sed \"s/.*/built/\") >> $GITHUB_OUTPUT
     '';
   };
 
@@ -108,7 +127,8 @@ with builtins; with lib; let
       steps = [ stepCommitToInitiallyCheckout stepCheckout1
                 stepCommitToTest stepCheckout2 stepCachixInstall ]
               ++ (stepCachixUseAll cachix)
-              ++ [ (stepCheck { inherit job bundles; }) ]
+              ++ [ (stepGetDerivation { inherit job bundles; }) 
+                    stepErrorReporting stepFailureCheck stepCheck ]
               ++ (map (job: stepBuild { inherit job bundles; }) jdeps)
               ++ [ (stepBuild { inherit job bundles; current = true; }) ];
     } // (optionalAttrs (isList bundles) {strategy.matrix.bundle = bundles;});