Dry run release workflow for release pull requests.

Avoid failures of the release process by running everything except for
the release creation and artifact upload when making or merging a PR
that bumps the release version.

Author: ograff

.github/workflows/release.yaml

@@ -2,6 +2,11 @@
 name: Release
 on:
   workflow_dispatch:
+  pull_request:
+    branches:
+      - main
+    paths:
+      - version.bzl
   push:
     branches:
       - main
@@ -20,18 +25,6 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
-      # TODO: Unfortunately it's not obvious how to restrict `workflow_dispatch` to a particular branch
-      # so this step ensures releases are always done off of `main`.
-      - name: Ensure branch is 'main'
-        run: |
-          git fetch origin &> /dev/null
-          branch="$(git rev-parse --abbrev-ref HEAD)"
-          if [[ "${branch}" != "main" ]]; then
-            echo "The release branch must be main. Got '${branch}'' instead." >&2
-            exit 1
-          else
-            echo "Branch is '${branch}'"
-          fi
       - name: Ensure release does not already exist
         run: |
           git fetch origin &> /dev/null
@@ -168,6 +161,7 @@ jobs:
           | sed 's#{sha256_base64}#${{ env.ARCHIVE_SHA256_BASE64 }}#g' \
           > ${{ github.workspace }}/.github/release_notes.txt
       - name: Create release
+        if: startsWith(github.ref, 'refs/heads/main')
         uses: softprops/action-gh-release@v1
         id: rules_rust_release
         env:
@@ -179,6 +173,7 @@ jobs:
           target_commitish: ${{ github.base_ref }}
 
       - name: "Upload the rules archive"
+        if: startsWith(github.ref, 'refs/heads/main')
         uses: actions/upload-release-asset@v1
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
@@ -190,6 +185,7 @@ jobs:
 
       # There must be a upload action for each platform triple we create
       - name: "Upload aarch64-apple-darwin"
+        if: startsWith(github.ref, 'refs/heads/main')
         uses: actions/upload-release-asset@v1
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
@@ -199,6 +195,7 @@ jobs:
           asset_path: ${{ github.workspace }}/crate_universe/target/artifacts/aarch64-apple-darwin/cargo-bazel
           asset_content_type: application/octet-stream
       - name: "Upload aarch64-pc-windows-msvc"
+        if: startsWith(github.ref, 'refs/heads/main')
         uses: actions/upload-release-asset@v1
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
@@ -208,6 +205,7 @@ jobs:
           asset_path: ${{ github.workspace }}/crate_universe/target/artifacts/aarch64-pc-windows-msvc/cargo-bazel.exe
           asset_content_type: application/octet-stream
       - name: "Upload aarch64-unknown-linux-gnu"
+        if: startsWith(github.ref, 'refs/heads/main')
         uses: actions/upload-release-asset@v1
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
@@ -217,6 +215,7 @@ jobs:
           asset_path: ${{ github.workspace }}/crate_universe/target/artifacts/aarch64-unknown-linux-gnu/cargo-bazel
           asset_content_type: application/octet-stream
       - name: "Upload x86_64-apple-darwin"
+        if: startsWith(github.ref, 'refs/heads/main')
         uses: actions/upload-release-asset@v1
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
@@ -226,6 +225,7 @@ jobs:
           asset_path: ${{ github.workspace }}/crate_universe/target/artifacts/x86_64-apple-darwin/cargo-bazel
           asset_content_type: application/octet-stream
       - name: "Upload x86_64-pc-windows-gnu"
+        if: startsWith(github.ref, 'refs/heads/main')
         uses: actions/upload-release-asset@v1
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
@@ -235,6 +235,7 @@ jobs:
           asset_path: ${{ github.workspace }}/crate_universe/target/artifacts/x86_64-pc-windows-gnu/cargo-bazel.exe
           asset_content_type: application/octet-stream
       - name: "Upload x86_64-pc-windows-msvc"
+        if: startsWith(github.ref, 'refs/heads/main')
         uses: actions/upload-release-asset@v1
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
@@ -244,6 +245,7 @@ jobs:
           asset_path: ${{ github.workspace }}/crate_universe/target/artifacts/x86_64-pc-windows-msvc/cargo-bazel.exe
           asset_content_type: application/octet-stream
       - name: "Upload x86_64-unknown-linux-gnu"
+        if: startsWith(github.ref, 'refs/heads/main')
         uses: actions/upload-release-asset@v1
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
@@ -253,6 +255,7 @@ jobs:
           asset_path: ${{ github.workspace }}/crate_universe/target/artifacts/x86_64-unknown-linux-gnu/cargo-bazel
           asset_content_type: application/octet-stream
       - name: "Upload x86_64-unknown-linux-musl"
+        if: startsWith(github.ref, 'refs/heads/main')
         uses: actions/upload-release-asset@v1
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}