Support relative paths in problemMatchers to support monorepos

Background

I'm working on CI/CD automation for a repository that includes multiple components, each in a subdirectory. Each component directory contains a Dockerfile, and in the course of building images errors are emitted with paths relative to the subdirectory of each component.

Example repository tree:

components/project1/Dockerfile
components/project1/src/example.fake
components/project2/Dockerfile
components/project2/src/example2.fake

Example error emitted during project1 build:

src/example.fake: syntax error

Describe the enhancement

To support monorepos, problem matchers should provide some mechanism for setting a static prefix to be applied to all errors emitted in a region of job output. I was not able to determine how to use the fromPath in Actions' problem matchers for my use case, as the build process being executed has no awareness of its fully qualified path and thus can't include it in error output.

Possible implementations

One approach might be to ask integrators to embed this information in the problem matcher configuration itself, possibly via a field similar to VS Code Tasks fileLocation property. This would require problem matchers to be imported into each repository and include a fileLocation field.

components/project1/.github/tsc.json

{
    "problemMatcher": [
        {
            "owner": "tsc",
            "fileLocation": ["relative", "components/project1"],
            "pattern": [
                {
                    "regexp": "^(?:\\s+\\d+\\>)?([^\\s].*)\\((\\d+|\\d+,\\d+|\\d+,\\d+,\\d+,\\d+)\\)\\s*:\\s+(error|warning|info)\\s+(\\w{1,2}\\d+)\\s*:\\s*(.*)$",
                    "file": 1,
                    "location": 2,
                    "severity": 3,
                    "code": 4,
                    "message": 5
                }
            ]
        }
    ]
}

Another approach that used an action command could retain the use of shared matchers, like the one from setup-node above, and be integrated with existing matrix builds like so:

strategy:
  matrix:
    component:
    - project1
    - project2
steps:
- uses: actions/checkout@v2
- uses: actions/setup-node@v1
- run: |
    # Tell the tsc matcher configured by setup-node to assume errors are relative to each component
    echo "::set-matcher-prefix owner=tsc:components/${{ matrix.component }}:"
- name: Build and push Docker image
  uses: docker/build-push-action@v1
  with:
    path: components/${{ matrix.component }}
    ...
- run: |
    # Clear prefix for subsequent steps
    echo "::set-matcher-prefix owner=tsc::"

If there's an alternative approach or workaround I'm possibly missing here, please let me know! Thanks for your time. 😄

Start by reviewing the problem matcher configuration in components/project1/.github/tsc.json and the proposed ::set-matcher-prefix command flow. Define how a relative path prefix should be configured or applied per component, including clearing it for later steps, and validate the behavior against the project1/project2 matrix example.