fix(desktop): gate transport retries to idempotent or provably-unsent requests

Follow-up hardening on #92977 (issue #92976). The cherry-picked retry
wrapped every verb, so an ECONNRESET arriving after the backend had
already processed a POST (prompt submitted, session created) would
silently double-submit on retry.

- Extract the transport policy into electron/api-transport.ts so it is
  unit-testable without Electron: keep-alive agent pools, transient
  error classification, and a verb-gated withRetry.
- Retry rule: GET/HEAD/OPTIONS retry on any transient transport error;
  POST/PUT/PATCH/DELETE retry only when the request provably never
  reached the server (connect-phase failures like ECONNREFUSED /
  ENOTFOUND, or an error thrown before the body was flushed —
  requestState.bodySent === false). Ambiguous resets after the body
  went out surface to the caller; when in doubt, don't retry.
- Separate keep-alive pools for JSON calls vs streaming downloads so
  long downloads can't starve latency-sensitive JSON calls.
- Destroy pooled agents on app will-quit.
- Tests: shouldRetryRequest truth table, withRetry behavior, plus LIVE
  transport tests against real misbehaving node HTTP servers: a GET
  burst where the server resets keep-alive sockets (bare attempt fails,
  retried succeeds) and a POST whose socket is RST after server-side
  processing (hit counter stays 1 — no double submit).
This commit is contained in:
Teknium
2026-08-23 20:14:29 -07:00
parent 1cc76fce3a
commit 859010b94e
3 changed files with 521 additions and 39 deletions
+352
View File
@@ -0,0 +1,352 @@
/**
* Unit + live-transport tests for the Electron main process's Hermes REST
* retry policy (#92976 / PR #92977 salvage).
*
* The live tests run REAL node http servers that misbehave the way the
* reported backend does (closing sockets under burst keep-alive traffic) and
* prove two things end to end:
*
* - idempotent GETs that die with ECONNRESET are retried and succeed, where
* a single bare attempt (pre-PR behavior) surfaces the raw reset;
* - a POST whose socket is reset AFTER the server processed it is NOT
* retried: the server-side hit counter stays at 1 and the error surfaces.
*/
import http from 'node:http'
import type { AddressInfo } from 'node:net'
import { afterAll, describe, expect, it } from 'vitest'
import {
destroyKeepaliveAgents,
downloadAgentFor,
isIdempotentMethod,
isTransientTransportError,
jsonAgentFor,
shouldRetryRequest,
withRetry
} from './api-transport'
function errWithCode(code: string, message = code): NodeJS.ErrnoException {
const e: NodeJS.ErrnoException = new Error(message)
e.code = code
return e
}
afterAll(() => {
destroyKeepaliveAgents()
})
describe('isTransientTransportError', () => {
it('accepts transient socket-level codes and messages', () => {
for (const code of ['ECONNRESET', 'ECONNREFUSED', 'EPIPE', 'ETIMEDOUT', 'ENOTFOUND', 'EAI_AGAIN']) {
expect(isTransientTransportError(errWithCode(code))).toBe(true)
}
expect(isTransientTransportError(new Error('socket hang up'))).toBe(true)
expect(isTransientTransportError(new Error('read ECONNRESET'))).toBe(true)
})
it('rejects non-transport errors', () => {
expect(isTransientTransportError(new Error('404: not found'))).toBe(false)
expect(isTransientTransportError(new Error('Invalid JSON from http://x'))).toBe(false)
expect(isTransientTransportError(null)).toBe(false)
expect(isTransientTransportError(undefined)).toBe(false)
})
})
describe('isIdempotentMethod', () => {
it.each([
['GET', true],
['get', true],
['HEAD', true],
['OPTIONS', true],
['POST', false],
['PUT', false],
['PATCH', false],
['DELETE', false],
[undefined, true] // node http defaults omitted method to GET
])('%s -> %s', (method, expected) => {
expect(isIdempotentMethod(method)).toBe(expected)
})
})
describe('shouldRetryRequest truth table', () => {
const reset = () => errWithCode('ECONNRESET', 'read ECONNRESET')
const refused = () => errWithCode('ECONNREFUSED', 'connect ECONNREFUSED 127.0.0.1:1')
const hangUp = () => new Error('socket hang up')
it('GET: retries any transient error regardless of body state', () => {
expect(shouldRetryRequest(reset(), 'GET', { bodySent: true })).toBe(true)
expect(shouldRetryRequest(reset(), 'GET', { bodySent: false })).toBe(true)
expect(shouldRetryRequest(hangUp(), 'HEAD', { bodySent: true })).toBe(true)
})
it('GET: never retries non-transport errors (HTTP 4xx/5xx surfaced as Error)', () => {
expect(shouldRetryRequest(new Error('500: boom'), 'GET', { bodySent: true })).toBe(false)
})
it('POST: retries when the connection provably never happened', () => {
expect(shouldRetryRequest(refused(), 'POST', { bodySent: false })).toBe(true)
expect(shouldRetryRequest(refused(), 'POST', { bodySent: true })).toBe(true) // refused == nothing sent
expect(shouldRetryRequest(errWithCode('ENOTFOUND'), 'PUT', { bodySent: false })).toBe(true)
})
it('POST: retries transient errors thrown before the body was flushed', () => {
expect(shouldRetryRequest(reset(), 'POST', { bodySent: false })).toBe(true)
expect(shouldRetryRequest(hangUp(), 'DELETE', { bodySent: false })).toBe(true)
})
it('POST: does NOT retry ambiguous resets after the body went out', () => {
expect(shouldRetryRequest(reset(), 'POST', { bodySent: true })).toBe(false)
expect(shouldRetryRequest(hangUp(), 'POST', { bodySent: true })).toBe(false)
expect(shouldRetryRequest(errWithCode('EPIPE'), 'PUT', { bodySent: true })).toBe(false)
expect(shouldRetryRequest(errWithCode('ETIMEDOUT'), 'DELETE', { bodySent: true })).toBe(false)
})
it('POST: conservative when request state is unknown', () => {
// No bodySent flag at all — treat as "may have been sent", don't retry.
expect(shouldRetryRequest(reset(), 'POST', {})).toBe(false)
expect(shouldRetryRequest(reset(), 'POST')).toBe(false)
})
})
describe('withRetry', () => {
const noDelay = { delayFn: () => Promise.resolve() }
it('retries a GET through transient failures and resolves', async () => {
let attempts = 0
const result = await withRetry(
() => {
attempts += 1
if (attempts < 3) return Promise.reject(errWithCode('ECONNRESET'))
return Promise.resolve('ok')
},
{ method: 'GET', ...noDelay }
)
expect(result).toBe('ok')
expect(attempts).toBe(3)
})
it('gives each attempt a fresh requestState', async () => {
const seen: boolean[] = []
let attempts = 0
await withRetry(
(state: any) => {
seen.push(state.bodySent)
state.bodySent = true
attempts += 1
if (attempts < 2) return Promise.reject(errWithCode('ECONNREFUSED'))
return Promise.resolve(null)
},
{ method: 'POST', ...noDelay }
)
expect(seen).toEqual([false, false])
})
it('does not retry a POST that failed after the body was flushed', async () => {
let attempts = 0
await expect(
withRetry(
(state: any) => {
attempts += 1
state.bodySent = true
return Promise.reject(errWithCode('ECONNRESET', 'read ECONNRESET'))
},
{ method: 'POST', ...noDelay }
)
).rejects.toThrow('read ECONNRESET')
expect(attempts).toBe(1)
})
it('retries a POST on ECONNREFUSED (never reached the server)', async () => {
let attempts = 0
await expect(
withRetry(
() => {
attempts += 1
return Promise.reject(errWithCode('ECONNREFUSED'))
},
{ method: 'POST', maxRetries: 2, ...noDelay }
)
).rejects.toThrow('ECONNREFUSED')
expect(attempts).toBe(3)
})
it('bounds retries at maxRetries even for GET', async () => {
let attempts = 0
await expect(
withRetry(
() => {
attempts += 1
return Promise.reject(errWithCode('ECONNRESET'))
},
{ method: 'GET', maxRetries: 2, ...noDelay }
)
).rejects.toThrow()
expect(attempts).toBe(3)
})
it('never retries non-transient errors', async () => {
let attempts = 0
await expect(
withRetry(
() => {
attempts += 1
return Promise.reject(new Error('500: internal'))
},
{ method: 'GET', ...noDelay }
)
).rejects.toThrow('500')
expect(attempts).toBe(1)
})
})
describe('keep-alive agent pools', () => {
it('separates JSON and download pools per protocol', () => {
expect(jsonAgentFor('http:')).not.toBe(jsonAgentFor('https:'))
expect(jsonAgentFor('http:')).not.toBe(downloadAgentFor('http:'))
expect(jsonAgentFor('https:')).not.toBe(downloadAgentFor('https:'))
// Stable across calls (a real pool, not a factory).
expect(jsonAgentFor('http:')).toBe(jsonAgentFor('http:'))
})
})
// ---------------------------------------------------------------------------
// LIVE transport tests against real misbehaving HTTP servers.
// ---------------------------------------------------------------------------
/** Minimal single-attempt GET mirroring the pre-PR fetchJson (no retry). */
function bareJsonGet(url: string): Promise<any> {
return new Promise((resolve, reject) => {
const req = http.request(new URL(url), { agent: jsonAgentFor('http:'), method: 'GET' }, res => {
const chunks: Buffer[] = []
res.on('error', reject)
res.on('data', c => chunks.push(c))
res.on('end', () => resolve(JSON.parse(Buffer.concat(chunks).toString('utf8'))))
})
req.on('error', reject)
req.end()
})
}
/** The head behavior: same request under the verb-gated retry policy. */
function retriedJsonGet(url: string): Promise<any> {
return withRetry(() => bareJsonGet(url), { method: 'GET', delayFn: () => Promise.resolve() })
}
function listen(server: http.Server): Promise<string> {
return new Promise(resolve => {
server.listen(0, '127.0.0.1', () => {
resolve(`http://127.0.0.1:${(server.address() as AddressInfo).port}`)
})
})
}
describe('live: GET burst against a server that resets keep-alive sockets', () => {
it('bare attempts fail with ECONNRESET/hang-up; retried GETs all succeed', async () => {
// Deterministic misbehavior: every other request gets its socket
// destroyed instead of a response — the observable client-side effect of
// a backend killing idle keep-alive sockets mid-burst.
let hits = 0
const server = http.createServer((req, res) => {
hits += 1
if (hits % 2 === 1) {
req.socket.destroy()
return
}
res.setHeader('content-type', 'application/json')
res.end(JSON.stringify({ n: hits }))
})
const base = await listen(server)
try {
// BASE (pre-PR, single attempt): the burst surfaces raw transport errors.
let baseFailures = 0
for (let i = 0; i < 6; i++) {
try {
await bareJsonGet(`${base}/api/sessions`)
} catch (error: any) {
baseFailures += 1
expect(isTransientTransportError(error)).toBe(true)
}
}
expect(baseFailures).toBeGreaterThan(0)
// HEAD (retry policy): the same burst fully succeeds. Sequential so the
// server's alternating destroy/respond pattern is deterministic per
// request (first attempt reset, retry served).
for (let i = 0; i < 6; i++) {
const r = await retriedJsonGet(`${base}/api/sessions`)
expect(r).toHaveProperty('n')
}
} finally {
server.close()
}
}, 20_000)
})
describe('live: POST reset after server-side processing', () => {
it('does not double-submit: server hit count stays 1, error surfaces', async () => {
// The server fully receives and "processes" the POST (counter increments),
// then RSTs the socket before responding — the dangerous ambiguous case.
let posts = 0
const server = http.createServer((req, res) => {
const chunks: Buffer[] = []
req.on('data', c => chunks.push(c))
req.on('end', () => {
posts += 1 // processed: prompt submitted / session created
req.socket.resetAndDestroy()
void res
})
})
const base = await listen(server)
const postOnce = () =>
withRetry(
(state: any) =>
new Promise((resolve, reject) => {
const body = Buffer.from(JSON.stringify({ prompt: 'hello' }))
const req = http.request(
new URL(`${base}/api/prompt`),
{
agent: jsonAgentFor('http:'),
method: 'POST',
headers: { 'content-type': 'application/json', 'content-length': String(body.length) }
},
res => {
res.resume()
res.on('end', () => resolve(null))
}
)
req.on('error', reject)
state.bodySent = true
req.write(body)
req.end()
}),
{ method: 'POST', delayFn: () => Promise.resolve() }
)
try {
await expect(postOnce()).rejects.toSatisfy((error: any) => isTransientTransportError(error))
expect(posts).toBe(1) // exactly one server-side submission — no retry
} finally {
server.close()
}
}, 20_000)
it('sanity: an identical GET-shaped retry WOULD have re-hit the server', async () => {
// Companion proof that the verb gate (not luck) is what kept posts === 1:
// the same reset-after-processing server sees multiple hits under GET.
let gets = 0
const server = http.createServer(req => {
gets += 1
req.socket.resetAndDestroy()
})
const base = await listen(server)
try {
await expect(retriedJsonGet(`${base}/api/thing`)).rejects.toThrow()
expect(gets).toBeGreaterThan(1) // retried — proves the machinery fires
} finally {
server.close()
}
}, 20_000)
})
+144
View File
@@ -0,0 +1,144 @@
/**
* Shared HTTP transport policy for the Electron main process's Hermes REST
* helpers (fetchJson / fetchPublicJson / downloadViaTokenToFile).
*
* Two concerns live here so they can be unit-tested without Electron:
*
* 1. Connection-pooled keep-alive agents. Opening a fresh TCP socket per call
* is what produced the burst-traffic ECONNRESET storms (#92976): the
* backend closes idle keep-alive sockets and the next write on a reused
* raw socket dies with 'socket hang up'. JSON calls and streaming
* downloads get SEPARATE pools so a handful of long-lived download
* streams can never starve the small, latency-sensitive JSON calls out of
* the socket pool.
*
* 2. A retry policy that is safe for non-idempotent verbs. A transient
* transport error does NOT mean the server didn't process the request —
* an ECONNRESET can arrive after the backend already handled a POST
* (created the session, submitted the prompt) and merely lost the socket
* before the response was read. Blindly retrying every verb double-submits.
*
* The rule implemented by shouldRetryRequest():
* - Idempotent verbs (GET / HEAD / OPTIONS) retry on any transient
* transport error — replaying them is harmless by definition.
* - Non-idempotent verbs (POST / PUT / PATCH / DELETE) retry ONLY when
* the request provably never reached the server:
* a) connection-establishment failures (ECONNREFUSED, ENOTFOUND,
* EAI_AGAIN, EHOSTUNREACH, ENETUNREACH) — no connection means no
* request; or
* b) a transient error thrown before we started flushing the
* request (requestState.bodySent === false).
* Anything ambiguous — ECONNRESET / EPIPE / 'socket hang up' after
* the body went out — is NOT retried; the error surfaces to the
* caller. When in doubt, don't retry a non-idempotent request.
*/
import http from 'node:http'
import https from 'node:https'
// JSON pool: many small concurrent calls (session lists, config, prompts).
const HTTP_JSON_AGENT = new http.Agent({ keepAlive: true, maxSockets: 50 })
const HTTPS_JSON_AGENT = new https.Agent({ keepAlive: true, maxSockets: 50 })
// Download pool: few long-lived streaming bodies. Isolated from the JSON pool
// so saturating it with large file downloads can't block interactive calls.
const HTTP_DOWNLOAD_AGENT = new http.Agent({ keepAlive: true, maxSockets: 8 })
const HTTPS_DOWNLOAD_AGENT = new https.Agent({ keepAlive: true, maxSockets: 8 })
function jsonAgentFor(protocol) {
return protocol === 'https:' ? HTTPS_JSON_AGENT : HTTP_JSON_AGENT
}
function downloadAgentFor(protocol) {
return protocol === 'https:' ? HTTPS_DOWNLOAD_AGENT : HTTP_DOWNLOAD_AGENT
}
// Close pooled sockets so lingering keep-alive connections can't hold the
// process open (or leak FDs) across quit. Wired to app 'will-quit' in main.ts.
function destroyKeepaliveAgents() {
for (const agent of [HTTP_JSON_AGENT, HTTPS_JSON_AGENT, HTTP_DOWNLOAD_AGENT, HTTPS_DOWNLOAD_AGENT]) {
agent.destroy()
}
}
// Transient transport errors: retry MAY be safe (subject to verb gating).
const TRANSIENT_CODES = new Set(['ECONNRESET', 'ECONNREFUSED', 'EPIPE', 'ETIMEDOUT', 'EAI_AGAIN', 'ENOTFOUND', 'EHOSTUNREACH', 'ENETUNREACH'])
// Errors that prove the request never reached the server: the TCP connection
// (or name resolution) failed outright, so nothing was submitted.
const NEVER_SENT_CODES = new Set(['ECONNREFUSED', 'ENOTFOUND', 'EAI_AGAIN', 'EHOSTUNREACH', 'ENETUNREACH'])
const IDEMPOTENT_METHODS = new Set(['GET', 'HEAD', 'OPTIONS'])
function isIdempotentMethod(method) {
return IDEMPOTENT_METHODS.has(String(method || 'GET').toUpperCase())
}
function isTransientTransportError(error) {
if (!error) return false
if (TRANSIENT_CODES.has(error.code)) return true
const msg = String(error.message || '')
return msg.includes('socket hang up') || msg.includes('read ECONNRESET')
}
/**
* The verb-gated retry decision.
*
* @param error the transport error from the failed attempt
* @param method HTTP verb of the request ('GET', 'POST', ...)
* @param requestState per-attempt state; requestState.bodySent is set true by
* the caller just BEFORE the first byte of the request is
* flushed, so a `false` here proves nothing went out.
*/
function shouldRetryRequest(error, method, requestState: any = {}) {
if (!isTransientTransportError(error)) return false
if (isIdempotentMethod(method)) return true
// Non-idempotent: only when the request provably never reached the server.
if (NEVER_SENT_CODES.has(error && error.code)) return true
if (requestState.bodySent === false) return true
// Ambiguous (reset/hang-up after the body was flushed): the server may have
// processed it. Surface the error rather than risk a double submit.
return false
}
/**
* Run `makeAttempt` with bounded retries under the policy above.
*
* `makeAttempt(requestState)` must return a Promise and should set
* `requestState.bodySent = true` immediately before flushing the request
* (before the first req.write()/req.end()). Each attempt gets a fresh state
* object initialized to { bodySent: false }.
*/
async function withRetry(makeAttempt, options: any = {}) {
const method = String(options.method || 'GET').toUpperCase()
const maxRetries = Number.isInteger(options.maxRetries) ? options.maxRetries : 2
const delayFn = options.delayFn || (attempt => new Promise(r => setTimeout(r, Math.min(200 * Math.pow(2, attempt), 2000))))
let lastError
for (let attempt = 0; attempt <= maxRetries; attempt++) {
const requestState = { bodySent: false }
try {
return await makeAttempt(requestState)
} catch (error) {
lastError = error
if (attempt < maxRetries && shouldRetryRequest(error, method, requestState)) {
await delayFn(attempt)
continue
}
throw error
}
}
throw lastError
}
export {
destroyKeepaliveAgents,
downloadAgentFor,
isIdempotentMethod,
isTransientTransportError,
jsonAgentFor,
shouldRetryRequest,
withRetry
}
+25 -39
View File
@@ -168,6 +168,7 @@ import {
pumpStreamToFile
} from './gateway-file-download'
import { probeGatewayWebSocket } from './gateway-ws-probe'
import { destroyKeepaliveAgents, downloadAgentFor, jsonAgentFor, withRetry } from './api-transport'
import { registerGitIpc } from './git-ipc'
import { clearStaleGitLocks } from './gitlock'
import { readAndConsumeHandoffResult } from './handoff-result'
@@ -4856,40 +4857,11 @@ function multipartBody(upload) {
return { body, contentType: `multipart/form-data; boundary=${boundary}` }
}
// Connection-pooled HTTP agents to avoid ECONNRESET from per-call TCP sockets.
// Each fetchJson / fetchPublicJson / downloadViaTokenToFile call shares these
// instead of opening a fresh connection every time.
const HTTP_KEEPALIVE_AGENT = new http.Agent({ keepAlive: true, maxSockets: 50 })
const HTTPS_KEEPALIVE_AGENT = new https.Agent({ keepAlive: true, maxSockets: 50 })
const RETRYABLE_CODES = new Set(['ECONNRESET', 'ECONNREFUSED', 'EPIPE', 'ERR_NETWORK'])
function isRetryableError(error) {
if (!error) return false
if (RETRYABLE_CODES.has(error.code)) return true
const msg = String(error.message || '')
return msg.includes('socket hang up') || msg.includes('read ECONNRESET')
}
async function withRetry(fn, maxRetries = 2) {
let lastError
for (let attempt = 0; attempt <= maxRetries; attempt++) {
try {
return await fn()
} catch (error) {
lastError = error
if (attempt < maxRetries && isRetryableError(error)) {
await new Promise(r => setTimeout(r, Math.min(200 * Math.pow(2, attempt), 2000)))
continue
}
throw error
}
}
throw lastError
}
function fetchJson(url, token, options: any = {}) {
return withRetry(() => new Promise((resolve, reject) => {
// Retry policy lives in api-transport.ts: idempotent verbs retry on any
// transient transport error; POST/PUT/DELETE only when the request provably
// never reached the server (see shouldRetryRequest) — never double-submit.
return withRetry((requestState: any) => new Promise((resolve, reject) => {
const { body, contentType } = options.upload
? multipartBody(options.upload)
: {
@@ -4899,7 +4871,7 @@ function fetchJson(url, token, options: any = {}) {
const parsed = new URL(url)
const client = parsed.protocol === 'https:' ? https : http
const agent = parsed.protocol === 'https:' ? HTTPS_KEEPALIVE_AGENT : HTTP_KEEPALIVE_AGENT
const agent = jsonAgentFor(parsed.protocol)
const timeoutMs = resolveTimeoutMs(options.timeoutMs, DEFAULT_FETCH_TIMEOUT_MS)
if (parsed.protocol !== 'http:' && parsed.protocol !== 'https:') {
@@ -4978,12 +4950,17 @@ function fetchJson(url, token, options: any = {}) {
req.destroy(new Error(`Timed out connecting to Hermes backend after ${timeoutMs}ms`))
})
// From here the request goes on the wire: a later transport error can no
// longer prove the server didn't process it, so non-idempotent verbs must
// not be retried past this point.
requestState.bodySent = true
if (body) {
req.write(body)
}
req.end()
}))
}), { method: options.method || 'GET' })
}
// Token-auth download that streams the response body straight to a
@@ -5010,7 +4987,7 @@ function downloadViaTokenToFile(url, token, ctx, options: any = {}) {
}
const client = parsed.protocol === 'https:' ? https : http
const agent = parsed.protocol === 'https:' ? HTTPS_KEEPALIVE_AGENT : HTTP_KEEPALIVE_AGENT
const agent = downloadAgentFor(parsed.protocol)
const timeoutMs = resolveTimeoutMs(options.timeoutMs, DEFAULT_FETCH_TIMEOUT_MS)
const req = client.request(
@@ -5051,7 +5028,7 @@ function fetchPublicJson(url, options: any = {}) {
// NO ``X-Hermes-Session-Token`` header — used by the auth-mode probe before
// any credentials exist, and any time we must not leak a token to an
// endpoint that doesn't need one.
return withRetry(() => new Promise((resolve, reject) => {
return withRetry((requestState: any) => new Promise((resolve, reject) => {
const body = options.body === undefined ? undefined : Buffer.from(JSON.stringify(options.body))
let parsed
@@ -5064,7 +5041,7 @@ function fetchPublicJson(url, options: any = {}) {
}
const client = parsed.protocol === 'https:' ? https : http
const agent = parsed.protocol === 'https:' ? HTTPS_KEEPALIVE_AGENT : HTTP_KEEPALIVE_AGENT
const agent = jsonAgentFor(parsed.protocol)
const timeoutMs = resolveTimeoutMs(options.timeoutMs, DEFAULT_FETCH_TIMEOUT_MS)
if (parsed.protocol !== 'http:' && parsed.protocol !== 'https:') {
@@ -5131,12 +5108,15 @@ function fetchPublicJson(url, options: any = {}) {
req.destroy(new Error(`Timed out connecting to Hermes backend after ${timeoutMs}ms`))
})
// Past this point the request is on the wire — see fetchJson.
requestState.bodySent = true
if (body) {
req.write(body)
}
req.end()
}))
}), { method: options.method || 'GET' })
}
function mimeTypeForPath(filePath) {
@@ -14520,6 +14500,12 @@ app.on('before-quit', () => {
}
})
// Close the pooled keep-alive sockets on quit so lingering connections can't
// hold the event loop open or leak FDs past app teardown.
app.on('will-quit', () => {
destroyKeepaliveAgents()
})
// Answered synchronously so preload can publish the verdict before the
// renderer's first script — see the note there on why it cannot decide this
// itself. Registered at module scope, which runs long before any window.