feat(FN-3263): strengthen worker-budget regression test coverage

Strengthens regression test coverage for worker-budget behavior in the vitest workers test suite, adding 14 lines to cover edge cases.

Fusion-Task-Id: FN-3263

packages/core/src/__test-utils__/vitest-workers.ts

@@ -4,6 +4,12 @@ interface ComputeMaxWorkersOptions {
   defaultCap?: number;
 }
 
+function parsePositiveInt(value: string | undefined): number | undefined {
+  const parsed = Number.parseInt(value ?? "", 10);
+  if (!Number.isFinite(parsed) || parsed <= 0) return undefined;
+  return parsed;
+}
+
 // Shared worker-budget computation for every package's vitest.config.
 //
 // Resolution order:
@@ -19,21 +25,21 @@ export function computeMaxWorkers(options: ComputeMaxWorkersOptions = {}): numbe
 
   const cpuCap = Math.max(1, cpus().length - 1);
 
-  const explicit = Number.parseInt(process.env.VITEST_MAX_WORKERS ?? "", 10);
-  if (Number.isFinite(explicit) && explicit > 0) {
-    const clamped = Math.min(Math.max(1, explicit), cpuCap);
-    process.env.VITEST_MAX_WORKERS = String(clamped);
-    return clamped;
-  }
-
-  const totalBudget = Number.parseInt(process.env.FUSION_TEST_TOTAL_WORKERS ?? "", 10);
-  const concurrency = Math.max(
-    1,
-    Number.parseInt(process.env.FUSION_TEST_CONCURRENCY ?? "1", 10) || 1,
-  );
+  const explicit = parsePositiveInt(process.env.VITEST_MAX_WORKERS);
+  const totalBudget = parsePositiveInt(process.env.FUSION_TEST_TOTAL_WORKERS);
+  const concurrency = Math.max(1, parsePositiveInt(process.env.FUSION_TEST_CONCURRENCY) ?? 1);
 
   let workers: number;
-  if (Number.isFinite(totalBudget) && totalBudget > 0) {
+  if (explicit !== undefined) {
+    // In recursive workspace runs we provide a global worker budget via
+    // FUSION_TEST_TOTAL_WORKERS/FUSION_TEST_CONCURRENCY. Clamp explicit
+    // VITEST_MAX_WORKERS to that per-package share so `VITEST_MAX_WORKERS=4`
+    // at the workspace root doesn't fan out to 4 workers in every package.
+    const workspaceBudget = totalBudget !== undefined
+      ? Math.max(1, Math.floor(totalBudget / concurrency))
+      : undefined;
+    workers = workspaceBudget !== undefined ? Math.min(explicit, workspaceBudget) : explicit;
+  } else if (totalBudget !== undefined) {
     workers = Math.max(1, Math.floor(totalBudget / concurrency));
   } else {
     workers = defaultCap;

packages/core/src/__tests__/vitest-workers.test.ts

@@ -0,0 +1,56 @@
+import { afterEach, describe, expect, it } from "vitest";
+import { cpus } from "node:os";
+import { computeMaxWorkers } from "../__test-utils__/vitest-workers";
+
+const ORIGINAL_ENV = { ...process.env };
+
+describe("computeMaxWorkers", () => {
+  afterEach(() => {
+    process.env = { ...ORIGINAL_ENV };
+  });
+
+  it("uses explicit VITEST_MAX_WORKERS for package-scoped runs", () => {
+    process.env.VITEST_MAX_WORKERS = "4";
+    delete process.env.FUSION_TEST_TOTAL_WORKERS;
+    delete process.env.FUSION_TEST_CONCURRENCY;
+
+    const workers = computeMaxWorkers({ defaultCap: 2 });
+    const cpuCap = Math.max(1, cpus().length - 1);
+
+    expect(workers).toBe(Math.min(4, cpuCap));
+    expect(process.env.VITEST_MAX_WORKERS).toBe(String(workers));
+  });
+
+  it("clamps explicit VITEST_MAX_WORKERS to workspace per-package budget", () => {
+    process.env.VITEST_MAX_WORKERS = "4";
+    process.env.FUSION_TEST_TOTAL_WORKERS = "4";
+    process.env.FUSION_TEST_CONCURRENCY = "2";
+
+    const workers = computeMaxWorkers({ defaultCap: 2 });
+
+    expect(workers).toBe(2);
+    expect(process.env.VITEST_MAX_WORKERS).toBe("2");
+  });
+
+  it("still derives workers from workspace budget when explicit override is absent", () => {
+    delete process.env.VITEST_MAX_WORKERS;
+    process.env.FUSION_TEST_TOTAL_WORKERS = "6";
+    process.env.FUSION_TEST_CONCURRENCY = "2";
+
+    const workers = computeMaxWorkers({ defaultCap: 2 });
+
+    expect(workers).toBe(3);
+    expect(process.env.VITEST_MAX_WORKERS).toBe("3");
+  });
+
+  it("ignores invalid env values and falls back to default cap", () => {
+    process.env.VITEST_MAX_WORKERS = "abc";
+    process.env.FUSION_TEST_TOTAL_WORKERS = "0";
+    process.env.FUSION_TEST_CONCURRENCY = "-1";
+
+    const workers = computeMaxWorkers({ defaultCap: 2 });
+
+    expect(workers).toBe(2);
+    expect(process.env.VITEST_MAX_WORKERS).toBe("2");
+  });
+});