actions: add renovate

Signed-off-by: Johan Haals <johan.haals@gmail.com>

.github/renovate.json5

@@ -0,0 +1,3 @@
+{
+  extends: ['config:base', ':disableDependencyDashboard', ':gitSignOff'],
+}

.github/workflows/sync_renovate-changesets.yml

@@ -0,0 +1,97 @@
+name: Sync Renovate changeset
+on:
+  pull_request_target:
+    paths:
+      - '.github/workflows/sync_renovate-changesets.yml'
+      - '**/yarn.lock'
+
+jobs:
+  generate-changeset:
+    runs-on: ubuntu-latest
+    if: github.actor == 'renovate[bot]' && github.repository == 'backstage/backstage'
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 2
+          ref: ${{ github.head_ref }}
+          token: ${{ secrets.GH_SERVICE_ACCOUNT_TOKEN }}
+      - name: Configure Git
+        run: |
+          git config --global user.email noreply@backstage.io
+          git config --global user.name 'Github changeset workflow'
+      - name: Generate changeset
+        uses: actions/github-script@v5
+        with:
+          script: |
+            const { promises: fs } = require("fs");
+            // Parses package.json files and returns the package names
+            async function getPackagesNames(files) {
+              const names = [];
+              for (const file of files) {
+                const data = JSON.parse(await fs.readFile(file, "utf8"));
+                if (!data.private) {
+                  names.push(data.name);
+                }
+              }
+              return names;
+            }
+
+            async function createChangeset(fileName, packageBumps, packages) {
+              let message = "";
+              for (const [pkg, bump] of packageBumps) {
+                message = message + `Updated dependency \`${pkg}\` to \`${bump}\`.\n`;
+              }
+
+              const pkgs = packages.map((pkg) => `'${pkg}': patch`).join("\n");
+              const body = `---\n${pkgs}\n---\n\n${message}\n`;
+              await fs.writeFile(fileName, body);
+            }
+
+            async function getBumps(files) {
+              const bumps = new Map();
+              for (const file of files) {
+                const { stdout: changes } = await exec.getExecOutput("git", [
+                  "show",
+                  file,
+                ]);
+                for (const change of changes.split("\n")) {
+                  if (!change.startsWith("+ ")) {
+                    continue;
+                  }
+                  const match = change.match(/"(.*?)"/g);
+                  bumps.set(match[0].replace(/"/g, ""), match[1].replace(/"/g, ""));
+                }
+              }
+              return bumps;
+            }
+
+            const branch = await exec.getExecOutput("git branch --show-current");
+            if (!branch.stdout.startsWith("renovate/")) {
+              console.log("Not a renovate branch, skipping");
+              return;
+            }
+            const diffOutput = await exec.getExecOutput("git diff --name-only HEAD~1");
+            const diffFiles = diffOutput.stdout.split("\n");
+            if (diffFiles.find((f) => f.startsWith(".changeset"))) {
+              console.log("Changeset already exists, skipping");
+              return;
+            }
+            const files = diffFiles
+              .filter((file) => file !== "package.json") // skip root package.json
+              .filter((file) => file.includes("package.json"));
+            const packageNames = await getPackagesNames(files);
+            if (!packageNames.length) {
+              console.log("No package.json changes to published packages, skipping");
+              return;
+            }
+            const { stdout: shortHash } = await exec.getExecOutput(
+              "git rev-parse --short HEAD"
+            );
+            const fileName = `.changeset/renovate-${shortHash.trim()}.md`;
+
+            const packageBumps = await getBumps(files);
+            await createChangeset(fileName, packageBumps, packageNames);
+            await exec.exec("git", ["add", fileName]);
+            await exec.exec("git commit -C HEAD --amend --no-edit");
+            await exec.exec("git push --force");