GitHub Action动态计算Matrix输出无法被fromJSON解析问题

问题背景 #

需求是根据输入参数build-all动态生成GitHub Action的matrix：当build-all为false时，仅使用BUILD_MATRIX（来自GitHub Secrets的JSON对象数组）的第一个元素；为true时使用完整数组。当前实现的Workflow代码如下：

compute-matrix:
  name: Compute matrix
  runs-on: ubuntu-latest
  needs: [get-environment-info]
  environment: ${{ needs.get-environment-info.outputs.environment }}
  outputs:
    matrix: steps.matrix.outputs.matrix

  steps:
    - name: Matrix
      if: github.event.inputs.build-all == 'false'
      id: matrix
      run: |
        echo '${{ env.BUILD_MATRIX }}' > build-matrix.json
        MATRIX=$(cat build-matrix.json | jq '[.[0]] | tostring')
        echo "matrix=$MATRIX" >> $GITHUB_OUTPUT

    - name: Checkout
      uses: actions/checkout@v3
      with:
        fetch-depth: 0

    - name: Upload output
      uses: ./.github/actions/upload-artifact
      with:
        key: matrix
        value: ${{ steps.matrix.outcome == 'success' && steps.matrix.outputs.matrix || env.BUILD_MATRIX }}

    - name: Print
      run: |
        echo ${{ steps.matrix.outputs.matrix }}

another-job:
  name: Another Job
  runs-on: ubuntu-latest
  needs: [get-environment-info, compute-matrix]
  environment: ${{ needs.get-environment-info.outputs.environment }}
  strategy:
    matrix: 
      app: ${{ fromJSON(needs.compute-matrix.outputs.matrix) }}

报错信息 #

调试时发现artifact中的JSON是合法的，但后续job使用fromJSON读取输出时出现错误：

Error when evaluating 'strategy' for job 'another-job'. .github/workflows/wf.yaml (Line: 775, Col: 14): Error parsing fromJson,.github/workflows/wf.yaml (Line: 775, Col: 14): Error reading JToken from JsonReader. Path '', line 0, position 0.,.github/workflows/wf.yaml (Line: 775, Col: 14): Unexpected value ''

问题原因 #

核心问题在于**compute-matrix的output没有处理Matrix步骤被跳过的情况**：

• 当build-all为true时，if条件不满足，Matrix步骤会被跳过，此时steps.matrix.outputs.matrix是空字符串。
• 当前compute-matrix的output直接取这个空值，后续fromJSON解析空字符串必然报错。

解决方案 #

修改compute-matrix的output定义，复用上传artifact时的判断逻辑，确保无论Matrix步骤是否执行，都能输出合法的JSON：

compute-matrix:
  name: Compute matrix
  runs-on: ubuntu-latest
  needs: [get-environment-info]
  environment: ${{ needs.get-environment-info.outputs.environment }}
  outputs:
    # 关键修改：判断步骤是否成功，否则使用原始BUILD_MATRIX
    matrix: ${{ steps.matrix.outcome == 'success' && steps.matrix.outputs.matrix || env.BUILD_MATRIX }}

  steps:
    - name: Matrix
      if: github.event.inputs.build-all == 'false'
      id: matrix
      run: |
        echo '${{ env.BUILD_MATRIX }}' > build-matrix.json
        MATRIX=$(cat build-matrix.json | jq '[.[0]] | tostring')
        echo "matrix=$MATRIX" >> $GITHUB_OUTPUT

    # 其他步骤保持不变...

额外验证点 #

• 确认BUILD_MATRIX本身是合法的JSON数组字符串，无语法错误。
• 本地运行jq命令验证输出：将BUILD_MATRIX内容保存到文件，执行jq '[.[0]] | tostring'，确认输出是带引号的合法JSON字符串。

内容的提问来源于stack exchange，提问作者chenny