ci: add docs-dispatch workflow (composer pings)

.github/workflows/docs-dispatch.yml

@@ -0,0 +1,83 @@
+name: Docs dispatch
+
+# Fires on push to main when content that affects the published docs site
+# changes (docs/, README, or CHANGELOG). Notifies makegov/docs via
+# repository_dispatch so the docs site rebuilds without waiting for someone
+# to push to the composer.
+#
+# tango-rust is a `coloc-source` repo: its docs/ folder is the authoritative
+# source for the Rust SDK pages on docs.makegov.com.
+#
+# Required secrets:
+#   DOCS_DISPATCH_TOKEN — GitHub token with contents:write on makegov/docs.
+#
+# Required variables (optional):
+#   DOCS_TARGET_REPO — override the dispatch target (default: makegov/docs).
+
+on:
+  push:
+    branches:
+      - main
+    paths:
+      - "docs/**"
+      - "README.md"
+      - "CHANGELOG.md"
+  workflow_dispatch:
+
+jobs:
+  dispatch:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+
+      # Every `${{ }}` value reaches the scripts through `env:`, never inline, so a file name or event field cannot inject shell.
+      - name: Detect changed paths
+        id: changes
+        env:
+          EVENT_NAME: ${{ github.event_name }}
+          BEFORE: ${{ github.event.before }}
+        run: |
+          set -euo pipefail
+          # The empty-tree object: diffing against it lists every tracked file.
+          empty_tree=4b825dc642cb6eb9a060e54bf8d69288fbee4904
+          # Without a usable `before` (a manual run, a new branch, or a force-push whose old tip is gone) the push range is unknown, so dispatch regardless and report the tip commit's paths.
+          force=false
+          if [ "$EVENT_NAME" = "push" ] && [ -n "$BEFORE" ] && ! echo "$BEFORE" | grep -qx '0*' && git cat-file -e "$BEFORE^{commit}" 2>/dev/null; then
+            base="$BEFORE"
+          else
+            force=true
+            if git rev-parse --verify -q HEAD~1 >/dev/null; then
+              base=HEAD~1
+            else
+              base="$empty_tree"
+            fi
+          fi
+          external=$(git diff --name-only "$base" HEAD -- 'docs' 'README.md' 'CHANGELOG.md' | paste -sd, -)
+          if [ -n "$external" ] || [ "$force" = true ]; then
+            has_external=true
+          else
+            has_external=false
+          fi
+          {
+            echo "external=$external"
+            echo "has_external=$has_external"
+          } >> "$GITHUB_OUTPUT"
+
+      - name: Dispatch to docs composer (makegov/docs)
+        if: steps.changes.outputs.has_external == 'true'
+        env:
+          GH_TOKEN: ${{ secrets.DOCS_DISPATCH_TOKEN }}
+          TARGET: ${{ vars.DOCS_TARGET_REPO || 'makegov/docs' }}
+          SOURCE_REPO: ${{ github.repository }}
+          SOURCE_REF: ${{ github.sha }}
+          CHANGED_PATHS: ${{ steps.changes.outputs.external }}
+        run: |
+          gh api "repos/$TARGET/dispatches" \
+            -f event_type=external_updated \
+            -f "client_payload[source_repo]=$SOURCE_REPO" \
+            -f "client_payload[source_ref]=$SOURCE_REF" \
+            -f "client_payload[changed_paths]=$CHANGED_PATHS"