test(landing): guard _redirects against catalog drift (#575)

Exports a pure buildRedirects() from generate-redirects.mjs and adds a unit test asserting the committed apps/landing/public/_redirects matches it, so tool additions can't silently leave the generated redirects stale (see #573).

apps/landing/scripts/generate-redirects.mjs

@@ -1,16 +1,37 @@
 // Generates public/_redirects (Cloudflare Pages) mapping old flat tool URLs
 // (/tools/<id>) to the new section-nested URLs (/tools/<section>/<id>/).
 // Both slash variants are emitted because Cloudflare matches paths exactly.
+//
+// buildRedirects() is exported (side-effect-free) so a drift test can assert the
+// committed public/_redirects still matches the catalog; the file is only
+// written when this script runs directly (the landing prebuild).
 import { writeFileSync } from "node:fs";
-import { fileURLToPath } from "node:url";
+import { fileURLToPath, pathToFileURL } from "node:url";
 import { TOOLS, toolSection } from "@snapotter/shared";
 
-const lines = ["# Generated by scripts/generate-redirects.mjs -- do not edit by hand"];
-for (const tool of TOOLS) {
-  const to = `/tools/${toolSection(tool)}/${tool.id}/`;
-  lines.push(`/tools/${tool.id}/ ${to} 301`);
-  lines.push(`/tools/${tool.id} ${to} 301`);
+/** Build the full _redirects file contents from the shared TOOLS catalog. */
+export function buildRedirects() {
+  const lines = ["# Generated by scripts/generate-redirects.mjs -- do not edit by hand"];
+  for (const tool of TOOLS) {
+    const to = `/tools/${toolSection(tool)}/${tool.id}/`;
+    lines.push(`/tools/${tool.id}/ ${to} 301`);
+    lines.push(`/tools/${tool.id} ${to} 301`);
+  }
+  return `${lines.join("\n")}\n`;
+}
+
+/** Absolute path to the generated file. */
+export const REDIRECTS_PATH = fileURLToPath(new URL("../public/_redirects", import.meta.url));
+
+function writeRedirects() {
+  const content = buildRedirects();
+  writeFileSync(REDIRECTS_PATH, content);
+  const redirectLines = content.trimEnd().split("\n").length - 1;
+  console.log(`wrote ${REDIRECTS_PATH} (${TOOLS.length} tools, ${redirectLines} redirect lines)`);
+}
+
+// Write only when executed directly (prebuild); importing for the drift test
+// stays side-effect-free.
+if (process.argv[1] && import.meta.url === pathToFileURL(process.argv[1]).href) {
+  writeRedirects();
 }
-const out = fileURLToPath(new URL("../public/_redirects", import.meta.url));
-writeFileSync(out, `${lines.join("\n")}\n`);
-console.log(`wrote ${out} (${TOOLS.length} tools, ${lines.length - 1} redirect lines)`);

tests/unit/landing/redirects-drift.test.ts

@@ -0,0 +1,24 @@
+import { readFileSync } from "node:fs";
+import { describe, expect, it } from "vitest";
+import {
+  buildRedirects,
+  REDIRECTS_PATH,
+} from "../../../apps/landing/scripts/generate-redirects.mjs";
+
+/**
+ * Drift guard: apps/landing/public/_redirects is generated from the shared
+ * TOOLS catalog by scripts/generate-redirects.mjs (runs in the landing
+ * prebuild) but is committed, so it silently goes stale whenever a tool is
+ * added without regenerating it (that is how remove-gif-background's redirects
+ * went missing, PR #573). Adding a tool touches @snapotter/shared and runs this
+ * suite, so the drift fails here at PR time instead of shipping.
+ */
+describe("landing _redirects drift", () => {
+  it("committed public/_redirects matches the generator output", () => {
+    const committed = readFileSync(REDIRECTS_PATH, "utf8");
+    expect(
+      committed,
+      "apps/landing/public/_redirects is stale. Regenerate it with `pnpm --filter @snapotter/landing prebuild` and commit the file.",
+    ).toBe(buildRedirects());
+  });
+});