fix(code-runtime): bound hostile output accounting

packages/code-runtime/code-runtime-worker/tests/output-json.spec.ts

@@ -1,10 +1,12 @@
-import { describe, expect, it } from 'vitest'
-import { truncateJsonStringBytes } from '../src/output-json.ts'
+import { describe, expect, it, vi } from 'vitest'
+import { jsonStringBytesUpTo, jsonValueBytesUpTo, truncateJsonStringBytes } from '../src/output-json.ts'
 
 describe('truncateJsonStringBytes', () => {
   it('returns a fitting string whole and rejects budgets without JSON quotes', () => {
     expect(truncateJsonStringBytes('fits', 6)).toBe('fits')
     expect(truncateJsonStringBytes('x', 1)).toBe('')
+    expect(jsonStringBytesUpTo('fits', 6)).toBe(6)
+    expect(jsonStringBytesUpTo('fits', 5)).toBeUndefined()
   })
 
   it('accounts every JSON escape and cuts only between complete code points', () => {
@@ -15,4 +17,43 @@ describe('truncateJsonStringBytes', () => {
     expect(truncateJsonStringBytes(text, budget)).toBe(prefix)
     expect(Buffer.byteLength(JSON.stringify(truncateJsonStringBytes(text, budget)), 'utf8')).toBe(budget)
   })
+
+  it('bounds hostile strings without materializing their complete escaped form', () => {
+    const stringify = vi.spyOn(JSON, 'stringify').mockImplementation(() => { throw new Error('must not stringify') })
+    try {
+      expect(jsonStringBytesUpTo('"'.repeat(10_000), 32)).toBeUndefined()
+      expect(truncateJsonStringBytes('"'.repeat(10_000), 32)).toBe('"'.repeat(15))
+    } finally {
+      stringify.mockRestore()
+    }
+  })
+})
+
+describe('jsonValueBytesUpTo', () => {
+  it('matches JSON serialization for every lossless value branch and stops at the cap', () => {
+    const value = {
+      empty: {},
+      nil: null,
+      yes: true,
+      no: false,
+      number: 1.5,
+      text: '"\n😀',
+      array: [1, 'x'],
+    }
+    const bytes = Buffer.byteLength(JSON.stringify(value), 'utf8')
+
+    expect(jsonValueBytesUpTo(value, bytes)).toBe(bytes)
+    expect(jsonValueBytesUpTo(value, bytes - 1)).toBeUndefined()
+    expect(jsonValueBytesUpTo({}, 1)).toBeUndefined()
+    expect(jsonValueBytesUpTo(null, 3)).toBeUndefined()
+    expect(jsonValueBytesUpTo(10, 1)).toBeUndefined()
+    expect(jsonValueBytesUpTo(false, 4)).toBeUndefined()
+    expect(jsonValueBytesUpTo(new Array<never>(1), 10)).toBeUndefined()
+    expect(jsonValueBytesUpTo([null], 5)).toBeUndefined()
+    expect(jsonValueBytesUpTo([0, 0], 3)).toBeUndefined()
+    expect(jsonValueBytesUpTo({ a: null, b: null }, 10)).toBeUndefined()
+    expect(jsonValueBytesUpTo({ long: null }, 2)).toBeUndefined()
+    expect(jsonValueBytesUpTo({ '': null }, 4)).toBeUndefined()
+    expect(jsonValueBytesUpTo({ a: null }, 9)).toBeUndefined()
+  })
 })

packages/code-runtime/code-runtime-worker/tests/runtime.spec.ts

@@ -401,6 +401,22 @@ describe('WorkerCodeRuntime — hostile programs (real workers)', () => {
     expect(Buffer.byteLength(JSON.stringify(result.logs), 'utf8')).toBeLessThan(200)
   })
 
+  it('bounds one oversized forged log while retaining its fitting escaped prefix', async () => {
+    const { runtime } = await setup({ maxOutputBytes: 96 })
+    const result = await runtime.run({
+      program: `
+        const { parentPort } = await import('node:worker_threads');
+        parentPort.postMessage({ type: 'log', text: '"'.repeat(1_000_000) });
+        for (;;) {}
+      `,
+      bindings: [],
+    })
+    expect(result.error).toEqual({ kind: 'output-limit', message: 'outer output exceeded 96 bytes' })
+    expect(result.logs).toHaveLength(1)
+    expect(result.logs[0]).toMatch(/^"+$/)
+    expect(Buffer.byteLength(JSON.stringify(result.logs), 'utf8') + Buffer.byteLength(JSON.stringify('outer output exceeded 96 bytes'), 'utf8')).toBeLessThanOrEqual(96)
+  })
+
   it('drops a malformed forged done carrying both value and error', async () => {
     const { runtime } = await setup()
     const result = await runtime.run({