Use separate jobs for building and publishing test releases. (#12671)

This follows the principle of least privilege. Our `poetry build` step
doesn't need, and shouldn't get, access to our GitHub OIDC capability.

This is the same structure as I used in the already-merged PR for
refactoring the regular PyPI release workflow: #12578.

.github/workflows/_release.yml

@@ -58,7 +58,7 @@ jobs:
         shell: bash
         working-directory: ${{ inputs.working-directory }}
         run: |
-          echo pkg-name="$(poetry version | cut -d' ' -f 1)" >> $GITHUB_OUTPUT
+          echo pkg-name="$(poetry version | cut -d ' ' -f 1)" >> $GITHUB_OUTPUT
           echo version="$(poetry version --short)" >> $GITHUB_OUTPUT
 
   test-pypi-publish:

.github/workflows/_test_release.yml

@@ -10,9 +10,60 @@ on:
 
 env:
   POETRY_VERSION: "1.6.1"
+  PYTHON_VERSION: "3.10"
 
 jobs:
-  publish_to_test_pypi:
+  build:
+    if: github.ref == 'refs/heads/master'
+    runs-on: ubuntu-latest
+
+    outputs:
+      pkg-name: ${{ steps.check-version.outputs.pkg-name }}
+      version: ${{ steps.check-version.outputs.version }}
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python + Poetry ${{ env.POETRY_VERSION }}
+        uses: "./.github/actions/poetry_setup"
+        with:
+          python-version: ${{ env.PYTHON_VERSION }}
+          poetry-version: ${{ env.POETRY_VERSION }}
+          working-directory: ${{ inputs.working-directory }}
+          cache-key: release
+
+      # We want to keep this build stage *separate* from the release stage,
+      # so that there's no sharing of permissions between them.
+      # The release stage has trusted publishing and GitHub repo contents write access,
+      # and we want to keep the scope of that access limited just to the release job.
+      # Otherwise, a malicious `build` step (e.g. via a compromised dependency)
+      # could get access to our GitHub or PyPI credentials.
+      #
+      # Per the trusted publishing GitHub Action:
+      # > It is strongly advised to separate jobs for building [...]
+      # > from the publish job.
+      # https://github.com/pypa/gh-action-pypi-publish#non-goals
+      - name: Build project for distribution
+        run: poetry build
+        working-directory: ${{ inputs.working-directory }}
+
+      - name: Upload build
+        uses: actions/upload-artifact@v3
+        with:
+          name: test-dist
+          path: ${{ inputs.working-directory }}/dist/
+
+      - name: Check Version
+        id: check-version
+        shell: bash
+        working-directory: ${{ inputs.working-directory }}
+        run: |
+          echo pkg-name="$(poetry version | cut -d ' ' -f 1)" >> $GITHUB_OUTPUT
+          echo version="$(poetry version --short)" >> $GITHUB_OUTPUT
+
+  publish:
+    needs:
+      - build
     runs-on: ubuntu-latest
     permissions:
       # This permission is used for trusted publishing:
@@ -21,30 +72,24 @@ jobs:
       # Trusted publishing has to also be configured on PyPI for each package:
       # https://docs.pypi.org/trusted-publishers/adding-a-publisher/
       id-token: write
-    defaults:
-      run:
-        working-directory: ${{ inputs.working-directory }}
+
     steps:
       - uses: actions/checkout@v4
 
-      - name: Set up Python + Poetry ${{ env.POETRY_VERSION }}
-        uses: "./.github/actions/poetry_setup"
+      - uses: actions/download-artifact@v3
         with:
-          python-version: "3.10"
-          poetry-version: ${{ env.POETRY_VERSION }}
-          working-directory: ${{ inputs.working-directory }}
-          cache-key: release
+          name: test-dist
+          path: ${{ inputs.working-directory }}/dist/
 
-      - name: Build project for distribution
-        run: poetry build
-      - name: Check Version
-        id: check-version
-        run: |
-          echo version=$(poetry version --short) >> $GITHUB_OUTPUT
-      - name: Publish package to TestPyPI
+      - name: Publish to test PyPI
         uses: pypa/gh-action-pypi-publish@release/v1
         with:
-          repository-url: https://test.pypi.org/legacy/
           packages-dir: ${{ inputs.working-directory }}/dist/
           verbose: true
           print-hash: true
+          repository-url: https://test.pypi.org/legacy/
+
+          # We overwrite any existing distributions with the same name and version.
+          # This is *only for CI use* and is *extremely dangerous* otherwise!
+          # https://github.com/pypa/gh-action-pypi-publish#tolerating-release-package-file-duplicates
+          skip-existing: true