Add parallel testing demo

Author: chingc

.github/workflows/parallel_dir.yml

@@ -11,7 +11,7 @@ jobs:
     name: Collect
     runs-on: ubuntu-latest
     outputs:
-      TEST_DIRS: "${{ steps.generate.outputs.TEST_DIRS }}"
+      directories: "${{ steps.build.outputs.TEST_DIRS }}"
 
     steps:
       - uses: actions/checkout@v4
@@ -33,8 +33,8 @@ jobs:
           sed -e '$d' collected.tmp | sed -e '$d' | tee collected.txt
           echo '::endgroup::'
 
-      - name: Generate matrix
-        id: generate
+      - name: Build test matrix
+        id: build
         shell: uv run -- python {0}
         run: |
           from os import environ
@@ -55,7 +55,8 @@ jobs:
     needs: collect
 
     strategy:
-      matrix: ${{ fromJSON(needs.collect.outputs.TEST_DIRS) }}
+      matrix:
+        directory: ${{ fromJSON(needs.collect.outputs.directories) }}
 
     steps:
       - uses: actions/checkout@v4
@@ -78,4 +79,4 @@ jobs:
           echo '::notice ::Enabled pytest norecursedirs.'
 
       - name: Run test
-        run: uv run -- pytest '${{ matrix.test_dirs }}'
+        run: uv run -- pytest '${{ matrix.directory }}'

README.md

@@ -165,7 +165,7 @@ See the [workflow](.github/workflows/log_annotation.yml).
 
 <summary>Define a matrix of different job configurations.</summary>
 
-<br/>The matrix strategy lets easily target multiple operating systems or test multiple versions of a language.
+<br/>The matrix strategy helps you easily target multiple operating systems and language versions.
 
 - Uses `actions/setup-node`
 
@@ -187,6 +187,20 @@ See the [workflow](.github/workflows/mise.yml).
 
 </details>
 
+## parallel_*.yml
+
+<details>
+
+<summary>Parallel testing without any code changes or extra dependencies.</summary>
+
+<br/>The matrix strategy can be used in a particular way to enable parallel testing for free. This means no code changes and no extra dependencies. This isn't a single runner using multiple cores, it's multiple runners each running its own set of tests.
+
+See the workflows:
+- [Directory-level parallel testing](.github/workflows/parallel_dir.yml)
+- [File-level parallel testing](.github/workflows/parallel_file.yml)
+
+</details>
+
 ## References
 
 - [GitHub Actions](https://docs.github.com/en/actions)