Github Actions MAtrix Strategy unexpected value ''

[tomstry]

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Workflow Configuration

Discussion Details

Hello, I have a question regarding passing a list of objects to matrix strategy.

outputs:
instances: ${{ steps.pick.outputs.instances }}
script_path: ${{ steps.pick.outputs.script_path }}

• name: Resolve instances + script path (jq, no status filter)
  id: pick
  shell: bash
  run: |
  set -euo pipefail
  case "${{ inputs.action }}" in
  pull) SCRIPT_PATH="/var/lib/cloud/app/pull_image.sh" ;;
  deploy) SCRIPT_PATH="/var/lib/cloud/app/deploy_docker.sh" ;;
  *) echo "Nieznana action"; exit 1 ;;
  esac
  echo "script_path=${SCRIPT_PATH}" >> "$GITHUB_OUTPUT"

      if [[ "${{ inputs.mig }}" == "eu" ]]; then
        MIG_NAME="${MIG_EU_NAME}"
        LOCATION="${MIG_EU_REGION}"
        SCOPE="region"
      else
        MIG_NAME="${MIG_US_NAME}"
        LOCATION="${MIG_US_REGION}"
        SCOPE="region"
      fi
  
      if [[ "${SCOPE}" == "region" ]]; then
        LIST_JSON="$(gcloud compute instance-groups managed list-instances "${MIG_NAME}" \
          --project="${PROJECT_ID}" \
          --region="${LOCATION}" \
          --format=json)"
      else
        LIST_JSON="$(gcloud compute instance-groups managed list-instances "${MIG_NAME}" \
          --project="${PROJECT_ID}" \
          --zone="${LOCATION}" \
          --format=json)"
      fi
      
      JSON="$(printf '%s' "${LIST_JSON}" \
        | jq -c 'map(.instance
                     | capture("zones/(?<zone>[^/]+)/instances/(?<name>[^/]+)")
                     | {name, zone})')"
  
       if [[ -z "${JSON}" || "${JSON}" == "[]" ]]; then
        echo "Brak instancji w MIG ${MIG_NAME} (${SCOPE}=${LOCATION})" >&2
        JSON="[]"
       fi
      echo "instances=${JSON}" >> $GITHUB_OUTPUT
  
      echo "Selected instances: ${JSON}"
  
  - name: Debug step outputs
    run: |
        echo "steps.pick.outputs.instances (raw below):"
        printf '%s\n' '${{ steps.pick.outputs.instances }}' | jq .
  

  exec:
  needs: resolve
  runs-on: ubuntu-latest
  strategy:
  fail-fast: false
  max-parallel: 1
  matrix: ${{ fromJSON(needs.resolve.outputs.instances) }}
  env:
  PROJECT_ID: ${{ secrets.PROJECT_ID }}
  SCRIPT_PATH: ${{ needs.resolve.outputs.script_path }}

Here I am passing to strategy list of VMs on which I want to execute a script.
Debug step is showing a correct json:
steps.pick.outputs.instances (raw below):
[

---

"name": "machine-eu-0rpr",
"zone": "europe-west3-b"

---

]

and first part of the pipeline ends up with success. But the second part throws an error:
Error when evaluating 'strategy' for job 'exec'. .github/workflows/pull_run_docker_image.yml (Line: 120, Col: 15): Error from function 'fromJSON': empty input, .github/workflows/pull_run_docker_image.yml (Line: 120, Col: 15): Unexpected value ''

Thank you in advance for your help!

[shaashvat01]

Hey there! I can see what's happening here. The issue is that your instances output from the resolve job isn't making it to the exec job, which is why you're getting the "empty input" error.

The problem is in how you're defining your matrix strategy. You're trying to use ${{ fromJSON(needs.resolve.outputs.instances) }} directly as the matrix value, but GitHub Actions expects a specific structure.

Here's what you need to change:

exec:
  needs: resolve
  runs-on: ubuntu-latest
  strategy:
    fail-fast: false
    max-parallel: 1
    matrix:
      instance: ${{ fromJSON(needs.resolve.outputs.instances) }}
  env:
    PROJECT_ID: ${{ secrets.PROJECT_ID }}
    SCRIPT_PATH: ${{ needs.resolve.outputs.script_path }}

Notice the key difference: matrix: needs a named property (I used instance here, but you can call it whatever makes sense). You can't just pass the JSON array directly to matrix:.

Then in your job steps, you'd reference the matrix values like:

• ${{ matrix.instance.name }}
• ${{ matrix.instance.zone }}

This will create one job execution for each object in your instances array, and each job will have access to that specific instance's name and zone via the matrix.instance context.

Hope this helps!

[tomstry]

Hello @shaashvat01, thank you very much for your help, but unfortunaletly after the change I am still facing the same error. Here I paste my full workflow file:

name: Run app action on all VMs in MIG

on:
  workflow_dispatch:
    inputs:
      action:
        description: "Define action"
        type: choice
        options: [pull, deploy]
        required: true
        default: pull
      mig:
        description: "Select MIG"
        type: choice
        options: [eu, us]
        required: true
        default: eu
      use_iap:
        description: "Łącz przez IAP?"
        type: choice
        options: ["false", "true"]
        required: true
        default: "true"

jobs:
  resolve:
    runs-on: ubuntu-latest
    outputs:
      instances: ${{ steps.pick.outputs.instances }}
      script_path: ${{ steps.pick.outputs.script_path }}
    env:
      PROJECT_ID: ${{ secrets.PROJECT_ID }}

      # ===== REGIONALNE MIG-i (zmień jeśli masz zonalne) =====
      MIG_EU_NAME: empattie-mig-eu
      MIG_EU_REGION: europe-west3
      MIG_US_NAME: empattie-mig-us
      MIG_US_REGION: us-east4

      # --- Dla MIG-ów ZONALNYCH (zamiast REGION ustaw SCOPE="zone" i użyj poniższych):
      # MIG_EU_ZONE: europe-west3-b
      # MIG_US_ZONE: us-east4-b

    steps:
      - uses: actions/checkout@v4

      - name: Auth to Google Cloud
        uses: google-github-actions/auth@v2
        with:
          credentials_json: ${{ secrets.GCP_SA_KEY }}

      - name: Set up gcloud
        uses: google-github-actions/setup-gcloud@v2
        with:
          project_id: ${{ secrets.PROJECT_ID }}

      - name: Install jq
        run: sudo apt-get update && sudo apt-get install -y jq

      - name: Resolve instances + script path (jq, no status filter)
        id: pick
        shell: bash
        run: |
          set -euo pipefail
          # 1) Mapowanie action -> script path
          case "${{ inputs.action }}" in
            pull)   SCRIPT_PATH="/var/lib/cloud/app/pull_image.sh" ;;
            deploy) SCRIPT_PATH="/var/lib/cloud/app/deploy_docker.sh" ;;
            *) echo "Nieznana action"; exit 1 ;;
          esac
          echo "script_path=${SCRIPT_PATH}" >> "$GITHUB_OUTPUT"
      
          if [[ "${{ inputs.mig }}" == "eu" ]]; then
            MIG_NAME="${MIG_EU_NAME}"
            LOCATION="${MIG_EU_REGION}"
            SCOPE="region"
          else
            MIG_NAME="${MIG_US_NAME}"
            LOCATION="${MIG_US_REGION}"
            SCOPE="region"
          fi
      
          if [[ "${SCOPE}" == "region" ]]; then
            LIST_JSON="$(gcloud compute instance-groups managed list-instances "${MIG_NAME}" \
              --project="${PROJECT_ID}" \
              --region="${LOCATION}" \
              --format=json)"
          else
            LIST_JSON="$(gcloud compute instance-groups managed list-instances "${MIG_NAME}" \
              --project="${PROJECT_ID}" \
              --zone="${LOCATION}" \
              --format=json)"
          fi
          
          JSON="$(printf '%s' "${LIST_JSON}" \
            | jq -c 'map(.instance
                         | capture("zones/(?<zone>[^/]+)/instances/(?<name>[^/]+)")
                         | {name, zone})')"
      
           if [[ -z "${JSON}" || "${JSON}" == "[]" ]]; then
            echo "Brak instancji w MIG ${MIG_NAME} (${SCOPE}=${LOCATION})" >&2
            # Ustaw pustą listę jako domyślną, jeśli nie ma instancji
            JSON="[]"
           fi
          echo "instances=${JSON}" >> $GITHUB_OUTPUT

          echo "Wybrane instancje (do matrix): ${JSON}"

      - name: Debug step outputs
        run: |
            echo "steps.pick.outputs.instances (raw below):"
            printf '%s\n' '${{ steps.pick.outputs.instances }}' | jq .

  exec:
    needs: resolve
    runs-on: ubuntu-latest
    strategy:
      fail-fast: false
      max-parallel: 1
      matrix: 
        instance: ${{ fromJSON(needs.resolve.outputs.instances) }}
    env:
      PROJECT_ID: ${{ secrets.PROJECT_ID }}
      SCRIPT_PATH: ${{ needs.resolve.outputs.script_path }}

    steps:
      - name: Auth to Google Cloud
        uses: google-github-actions/auth@v2
        with:
          credentials_json: ${{ secrets.GCP_SA_KEY }}

      - name: Set up gcloud
        uses: google-github-actions/setup-gcloud@v2
        with:
          project_id: ${{ secrets.PROJECT_ID }}

      - name: SSH run on ${{ matrix.instance.name }} (${{ matrix.instance.zone }})
        shell: bash
        run: |
          set -euo pipefail
          BASE=(gcloud compute ssh "${{ matrix.instance.name }}" --project "${PROJECT_ID}" --zone "${{ matrix.instance.zone }}")
          if [[ "${{ inputs.use_iap }}" == "true" ]]; then BASE+=(--tunnel-through-iap); fi
          "${BASE[@]}" --command="sudo bash '${SCRIPT_PATH}'"

Debug step is showing output as below:
steps.pick.outputs.instances (raw below):
[

---

"name": "emp-0rpr",
"zone": "europe-west3-b"

---

]

Error:
Error when evaluating 'strategy' for job 'exec'. .github/workflows/pull_run_docker_image.yml (Line: 121, Col: 19): Error from function 'fromJSON': empty input, .github/workflows/pull_run_docker_image.yml (Line: 121, Col: 19): Unexpected value ''

[shaashvat01]

Hey, thanks for the details!

the issue isn’t with matrix: syntax but with how the JSON output is written from your resolve job. GitHub truncates it because it contains special characters.

replace
echo "instances=${JSON}" >> $GITHUB_OUTPUT

with:
{ echo "instances<<EOF" echo "${JSON}" echo "EOF" } >> "$GITHUB_OUTPUT"

Also this:
matrix: instance: ${{ fromJSON(needs.resolve.outputs.instances || '[]') }}

That should fix the Unexpected value '' error

[tomstry]

After the change I got an error:
/home/runner/work/_temp/d4c8e99b-3182-4101-9ac5-8b1ef074605d.sh: line 44: syntax error: unexpected end of file
Error: Process completed with exit code 2.

after changing line by adding semicolons:
{ echo "instances<<EOF"; echo "${JSON}"; echo "EOF"; } >> "$GITHUB_OUTPUT"

I have similar error:
Error when evaluating 'strategy' for job 'exec'. .github/workflows/pull_run_docker_image.yml (Line: 119, Col: 19): Matrix vector 'instance' does not contain any values