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

[quirogas]

Part 3 of stacked disentanglement (#8928). Add automated presubmit bundle size regression testing for @google-cloud/firestore in serverless deployment environments.

Summary of Changes

• Automated esbuild Safeguard: Configures an automated verification task (test-bundle-size) using esbuild to compile and bundle the SDK under simulated serverless production conditions.
• Enforce Budget Limit: Enforces a maximum bundled output budget of ~2.19 MB (2,298,880 bytes). If structural regressions or unintended dependency bloat push the bundle above this budget, presubmit CI verification fails automatically.

Internal: b/531788771

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📚 Stack Navigation Index

1. Layer 1 (Base Disentanglement & Runtime Forwarding): feat(firestore): decouple handwritten sdk wrapper from embedded gapic clients #8928
2. Layer 2 (Complete Decoupling & Dead Code Cleanup): feat(firestore)!: complete gapic decoupling, adopt subpath imports, and remove legacy proto code (#9074) #9074
3. Layer 3 (Automated Presubmit Bundle Size Guard): chore(firestore): add automated presubmit serverless bundle size regression guard (#9075) #9075
4. Layer 4 (Call-Site Modernization & Eliminate Local Protos): refactor(firestore): modernize internal proto call sites and eliminate local declarations (#9076) #9076

[gemini-code-assist]

Code Review

This pull request introduces an automated bundle size verification check for Firestore. It adds a budget configuration file (bundle-budget.json), a script (check-bundle-size.js) that bundles and minifies the code using esbuild to verify its size, and a corresponding npm script. The review feedback suggests several improvements: using a unique, OS-level temporary directory via fs.mkdtempSync and os.tmpdir() to avoid concurrency conflicts; quoting paths in the execSync command to handle spaces and using npx --yes for non-interactive CI environments; and adding a precheck-bundle-size script to ensure the project is compiled before running the size check.

[gemini-code-assist]

To support creating a secure and collision-free temporary directory using fs.mkdtempSync in the OS temp directory, we should import the native os module.

Suggested change

	const fs = require('fs');
	const path = require('path');
	const { execSync } = require('child_process');
	const budget = require('../bundle-budget.json');
	const fs = require('fs');
	const path = require('path');
	const os = require('os');
	const { execSync } = require('child_process');
	const budget = require('../bundle-budget.json');

[gemini-code-assist]

Creating a static temporary directory .tmp-bundle-check inside the repository can cause conflicts during concurrent runs, pollute the git status if cleanup fails, and trigger watcher rebuilds.

Instead, use fs.mkdtempSync with os.tmpdir() to create a unique temporary directory outside the repository. To make this work seamlessly, resolve the absolute path to the build entrypoint and inject it into the generated file.

Suggested change

	const tmpDir = path.join(__dirname, '../.tmp-bundle-check');
	if (!fs.existsSync(tmpDir)) {
	fs.mkdirSync(tmpDir, { recursive: true });
	}
	const entrypoint = path.join(tmpDir, 'test_bundle_input.js');
	const outfile = path.join(tmpDir, 'test_bundle_out.min.js');
	fs.writeFileSync(entrypoint, `
	const { Firestore } = require('../build/src/index');
	const f = new Firestore();
	console.log(f);
	`, 'utf8');
	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');

[gemini-code-assist]

If the path to the temporary directory or repository contains spaces (common on Windows or certain user directories), the execSync command will fail because the paths are not quoted. Additionally, using npx --yes ensures that esbuild can be run non-interactively in CI environments without prompting for installation.

Suggested change

	execSync(`npx esbuild --bundle --platform=node --target=node18 --minify ${entrypoint} --outfile=${outfile}`, {
	execSync('npx --yes esbuild --bundle --platform=node --target=node18 --minify "' + entrypoint + '" --outfile="' + outfile + '"', {

[gemini-code-assist]

Running check-bundle-size requires the compiled JS files in the build/ directory to exist. Adding a precheck-bundle-size script ensures that npm run compile is automatically executed beforehand, preventing failures when running the check on a clean clone.

Suggested change

	"check-bundle-size": "node scripts/check-bundle-size.js",
	"precheck-bundle-size": "npm run compile",
	"check-bundle-size": "node scripts/check-bundle-size.js",