meta: add output-watcher GHA to help check output diff (#4868)

.github/workflows/output-watcher.yml

@@ -0,0 +1,87 @@
+name: Compare JS output
+
+on:
+  pull_request_target:
+    # We want all branches so we configure types to be the GH default again
+    types: [opened, synchronize, reopened]
+    paths:
+      - 'packages/@uppy/*/src/**/*'
+      - '.github/workflows/output-watcher.yml'
+
+permissions:
+  pull-requests: write
+env:
+  YARN_ENABLE_GLOBAL_CACHE: false
+
+jobs:
+  compare_diff:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v3
+        with:
+          fetch-depth: 2
+          ref: refs/pull/${{ github.event.pull_request.number }}/merge
+      - run: git reset HEAD^ --hard
+      - name: Get yarn cache directory path
+        id: yarn-cache-dir-path
+        run:
+          echo "dir=$(corepack yarn config get cacheFolder)" >> $GITHUB_OUTPUT
+
+      - uses: actions/cache@v3
+        id: yarn-cache # use this to check for `cache-hit` (`steps.yarn-cache.outputs.cache-hit != 'true'`)
+        with:
+          path: ${{ steps.yarn-cache-dir-path.outputs.dir }}
+          key: ${{ runner.os }}-yarn-${{ hashFiles('**/yarn.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-yarn-
+      - name: Install Node.js
+        uses: actions/setup-node@v3
+        with:
+          node-version: lts/*
+      - name: Install dependencies
+        run:
+          corepack yarn workspaces focus $(corepack yarn workspaces list --json
+          | jq -r .name | awk '/^@uppy-example/{ next } { if ($0!="uppy.io")
+          print $0 }')
+        env:
+          # https://docs.cypress.io/guides/references/advanced-installation#Skipping-installation
+          CYPRESS_INSTALL_BINARY: 0
+      - run: corepack yarn build:lib
+      - name: Store output file
+        run: tar cf /tmp/previousVersion.tar packages/@uppy/*/lib
+      - name: Fetch source from the PR
+        run: git checkout FETCH_HEAD -- packages
+      - run: corepack yarn build:lib
+      - name: Store output file
+        run: tar cf /tmp/newVersion.tar packages/@uppy/*/lib
+      - name: Setup git
+        run: |
+          git config --global user.email "actions@github.com"
+          git config --global user.name "GitHub Actions"
+          git init /tmp/uppy
+          echo '*.map' > /tmp/uppy/.gitignore
+      - name: Extract previous version
+        run: cd /tmp/uppy && tar xf /tmp/previousVersion.tar
+      - name: Commit previous version
+        run: cd /tmp/uppy && git add -A . && git commit -m 'previous version'
+      - name: Extract new version
+        run: cd /tmp/uppy && tar xf /tmp/newVersion.tar
+      - name: Build diff
+        id: diff
+        run: |
+          EOF=$(dd if=/dev/urandom bs=15 count=1 status=none | base64)
+          echo "OUTPUT_DIFF<<$EOF" >> "$GITHUB_OUTPUT"
+          cd /tmp/uppy && git --no-pager diff >> "$GITHUB_OUTPUT"
+          echo "$EOF" >> "$GITHUB_OUTPUT"
+      - name: Add/update comment
+        uses: marocchino/sticky-pull-request-comment@v2
+        with:
+          message: |
+            <details><summary>Diff output files</summary>
+
+            ```diff
+            ${{ steps.diff.outputs.OUTPUT_DIFF || 'No diff' }}
+            ```
+
+            </details>