chore: add test sharding to unit tests

.github/workflows/lint.yml

@@ -12,7 +12,55 @@ name: lint
 permissions:
   contents: read
 
+env:
+  NOX_REUSE_EXISTING_VIRTUALENVS: "true"
+  NOX_ENVDIR: "/tmp/shared_nox_envs"
+  NOX_DEFAULT_VENV_BACKEND: "uv"
+  UV_VENV_SEED: "1"
+  # Run checks in parallel using 4 cores
+  PARALLEL_WORKERS: "4"
+
 jobs:
+  initialize:
+    runs-on: ubuntu-latest
+    outputs:
+      matrix: ${{ steps.set-matrix.outputs.matrix }}
+      is_full_run: ${{ steps.check-label.outputs.is_full_run }}
+    env:
+      MAX_SHARDS: 4
+    steps:
+    - name: Checkout
+      uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+      # Use a fetch-depth of 2 to avoid error `fatal: origin/main...HEAD: no merge base`
+      # See https://github.com/googleapis/google-cloud-python/issues/12013
+      # and https://github.com/actions/checkout#checkout-head.
+      with:
+        fetch-depth: 2
+        persist-credentials: false
+    - name: Check for unit_test:all_packages label
+      id: check-label
+      run: |
+        if [[ "${{ contains(github.event.pull_request.labels.*.name, 'unit_test:all_packages') }}" == "true" ]]; then
+          echo "is_full_run=true" >> $GITHUB_OUTPUT
+        else
+          echo "is_full_run=false" >> $GITHUB_OUTPUT
+        fi
+    - name: Setup Python
+      uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6
+      with:
+        python-version: "3.10"
+    - name: Get package shards
+      id: set-matrix
+      env:
+        BUILD_TYPE: presubmit
+        TARGET_BRANCH: ${{ github.base_ref || github.event.merge_group.base_ref }}
+        TEST_ALL_PACKAGES: ${{ steps.check-label.outputs.is_full_run }}
+      run: |
+        if [ -n "$TARGET_BRANCH" ]; then
+          git fetch origin "$TARGET_BRANCH" --depth=1 || true
+        fi
+        python3 ci/get_package_shards.py
+
   lint:
     runs-on: ubuntu-latest
     steps:
@@ -24,18 +72,27 @@ jobs:
       with:
         fetch-depth: 2
         persist-credentials: false
+    - name: Check for unit_test:all_packages label
+      id: check-label
+      run: |
+        if [[ "${{ contains(github.event.pull_request.labels.*.name, 'unit_test:all_packages') }}" == "true" ]]; then
+          echo "is_full_run=true" >> $GITHUB_OUTPUT
+        else
+          echo "is_full_run=false" >> $GITHUB_OUTPUT
+        fi
     - name: Setup Python
       uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6
       with:
         python-version: "3.14"
     - name: Install nox
       run: |
         python -m pip install --upgrade setuptools pip wheel
-        python -m pip install nox
+        python -m pip install nox uv
     - name: Run lint
       env:
         BUILD_TYPE: presubmit
         TARGET_BRANCH: ${{ github.base_ref || github.event.merge_group.base_ref }}
+        TEST_ALL_PACKAGES: ${{ steps.check-label.outputs.is_full_run }}
         TEST_TYPE: lint
         # TODO(https://github.com/googleapis/google-cloud-python/issues/13775): Specify `PY_VERSION` rather than relying on the default python version of the nox session.
         PY_VERSION: "unused"
@@ -45,13 +102,22 @@ jobs:
       env:
         BUILD_TYPE: presubmit
         TARGET_BRANCH: ${{ github.base_ref || github.event.merge_group.base_ref }}
+        TEST_ALL_PACKAGES: ${{ steps.check-label.outputs.is_full_run }}
         TEST_TYPE: lint_setup_py
         # TODO(https://github.com/googleapis/google-cloud-python/issues/13775): Specify `PY_VERSION` rather than relying on the default python version of the nox session.
         PY_VERSION: "unused"
       run: |
         ci/run_conditional_tests.sh
-  mypy:
+
+  mypy-shard:
+    needs: initialize
+    if: needs.initialize.outputs.matrix != '[]' && needs.initialize.outputs.matrix != ''
     runs-on: ubuntu-latest
+    strategy:
+      fail-fast: true
+      matrix:
+        package_shard: ${{ fromJson(needs.initialize.outputs.matrix) }}
+    name: ${{ matrix.package_shard.is_sharded && format('mypy ({0})', matrix.package_shard.name) || format('mypy ({0})', matrix.package_shard.description) }}
     steps:
     - name: Checkout
       uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
@@ -68,13 +134,28 @@ jobs:
     - name: Install nox
       run: |
         python -m pip install --upgrade setuptools pip wheel
-        python -m pip install nox
-    - name: Run mypy
+        python -m pip install nox uv
+    - name: Run mypy for ${{ matrix.package_shard.description }}
       env:
         BUILD_TYPE: presubmit
         TARGET_BRANCH: ${{ github.base_ref || github.event.merge_group.base_ref }}
         TEST_TYPE: mypy
+        PACKAGE_LIST: ${{ matrix.package_shard.packages }}
         # TODO(https://github.com/googleapis/google-cloud-python/issues/13775): Specify `PY_VERSION` rather than relying on the default python version of the nox session.
         PY_VERSION: "unused"
       run: |
         ci/run_conditional_tests.sh
+
+  mypy-status:
+    if: always()
+    needs: [initialize, mypy-shard]
+    runs-on: ubuntu-latest
+    name: mypy-status
+    steps:
+    - name: Check mypy job status
+      run: |
+        if [[ "${{ needs['mypy-shard'].result }}" != "success" && "${{ needs['mypy-shard'].result }}" != "skipped" ]]; then
+          echo "mypy failed with result: ${{ needs['mypy-shard'].result }}"
+          exit 1
+        fi
+        echo "All mypy shards passed successfully!"