test(release): pin the release-pr main-ref guard (RIG-3813)

tools/toolchain/moon.yml

@@ -39,12 +39,14 @@ tasks:
     # edit to one must invalidate this task — otherwise it stays cached and the
     # assertion silently stops re-running against the thing it constrains.
     # ci.yml + pgtest.go: a test asserts their two Postgres image digests are
-    # equal. flake.nix + devenv.nix: version-guard-core.test.ts lifts the real
+    # equal. release.yml: a test pins the release-pr main-ref guard.
+    # flake.nix + devenv.nix: version-guard-core.test.ts lifts the real
     # version.txt guards out of both and asserts they are still extractable.
     inputs:
       - '*.ts'
       - '/bun.lock'
       - '/.github/workflows/ci.yml'
+      - '/.github/workflows/release.yml'
       - '/go/internal/pgtest/pgtest.go'
       - '/flake.nix'
       - '/devenv.nix'

tools/toolchain/release-guard-core.test.ts

@@ -0,0 +1,53 @@
+// This test is CAPABLE OF FAILING: an absent or unparseable job block must fail
+// rather than turn an uncheckable release guard into a pass.
+
+import { describe, expect, test } from "bun:test";
+
+const releasePrBlock = (workflow: string): string => {
+	const matches = workflow.match(
+		/^ {2}release-pr:\n([\s\S]*?)(?=^ {2}\S|(?![\s\S]))/m,
+	);
+	if (matches === null)
+		throw new Error(
+			"release-pr block could not be located; the privileged contents-write tag-minting job is unchecked",
+		);
+	expect(
+		matches,
+		"release-pr must be present so the privileged contents-write tag-minting job stays guarded",
+	).toHaveLength(2);
+	const [, block] = matches;
+	if (block === undefined)
+		throw new Error(
+			"release-pr block could not be extracted; the privileged contents-write tag-minting job is unchecked",
+		);
+	expect(
+		block.length,
+		"release-pr must contain a substantial job block so the privileged contents-write tag-minting job is actually checked",
+	).toBeGreaterThan(100);
+	return block;
+};
+
+const readReleasePrBlock = async (): Promise<string> => {
+	const root = new URL("../../", import.meta.url).pathname;
+	return releasePrBlock(
+		await Bun.file(`${root}.github/workflows/release.yml`).text(),
+	);
+};
+
+describe("the release-pr main-ref guard", () => {
+	test("keeps the privileged tag-minting job restricted to main", async () => {
+		const block = await readReleasePrBlock();
+		expect(
+			block,
+			"release-pr must require main because it holds contents: write and mints release tags",
+		).toMatch(/^ {4}if: github\.ref == 'refs\/heads\/main'$/m);
+	});
+
+	test("pins release-please to the main target branch", async () => {
+		const block = await readReleasePrBlock();
+		expect(
+			block,
+			"release-pr must pass target-branch: main because it holds contents: write and mints release tags",
+		).toMatch(/^ {10}target-branch: main$/m);
+	});
+});