chore(firestore): add automated presubmit serverless bundle size regression guard (#9075)

handwritten/firestore/bundle-budget.json

@@ -0,0 +1,9 @@
+{
+  "serverless_minimal_bundle": {
+    "entrypoint": "./build/src/index.js",
+    "target_minified_bytes": 2293760,
+    "max_minified_bytes_deduped": 2500000,
+    "max_minified_bytes_undeduped": 5000000,
+    "description": "2.19 MB minified target budget for standard serverless Cloud Function deployment"
+  }
+}

handwritten/firestore/package.json

@@ -52,6 +52,8 @@
     "system-test:emulator": "concurrently -p \"[{name}]\" -n \"grpc,rest,enterprise-grpc,enterprise-rest\" -c \"cyan,magenta,blue,yellow\" \"npm:system-test:emulator:grpc\" \"npm:system-test:emulator:rest\" \"npm:system-test:enterprise:emulator:grpc\" \"npm:system-test:enterprise:emulator:rest\"",
     "presystem-test": "npm run compile",
     "conformance": "mocha build/conformance",
+    "precheck-bundle-size": "npm run compile",
+    "check-bundle-size": "node scripts/check-bundle-size.js",
     "test-only": "c8 mocha build/test",
     "pretest": "npm run compile",
     "test": "npm run test-only && npm run conformance",

handwritten/firestore/scripts/check-bundle-size.js

@@ -0,0 +1,34 @@
+const fs = require('fs');
+const path = require('path');
+const os = require('os');
+const { execSync } = require('child_process');
+const budget = require('../bundle-budget.json');
+
+const tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'firestore-bundle-'));
+const entrypoint = path.join(tmpDir, 'test_bundle_input.js');
+const outfile = path.join(tmpDir, 'test_bundle_out.min.js');
+const buildIndexPath = path.resolve(__dirname, '../build/src/index');
+
+fs.writeFileSync(
+  entrypoint,
+  '\nconst { Firestore } = require(' +
+    JSON.stringify(buildIndexPath) +
+    ');\nconst f = new Firestore();\nconsole.log(f);\n',
+  'utf8'
+);
+
+try {
+  execSync('npx --yes esbuild --bundle --platform=node --target=node18 --minify "' + entrypoint + '" --outfile="' + outfile + '"', {
+    stdio: 'inherit',
+  });
+  const stats = fs.statSync(outfile);
+  const size = stats.size;
+  console.log(`[test-bundle-size] Compiled serverless bundle size: ${size} bytes`);
+  if (size > budget.maxBundleSize) {
+    console.error(`[test-bundle-size] ERROR: Bundle size (${size} bytes) exceeds budget (${budget.maxBundleSize} bytes)`);
+    process.exit(1);
+  }
+  console.log('[test-bundle-size] SUCCESS: Bundle size is within budget.');
+} finally {
+  fs.rmSync(tmpDir, { recursive: true, force: true });
+}