Document that the error event on the thread stream is fatal

Signed-off-by: Matteo Collina <[email protected]>

docs/api.md

@@ -1307,6 +1307,9 @@ module.exports = function build () {
 }
 ```
 
+Note that _any `'error'`_ event emitted by the transport must be consider a fatal error and the process must be terminated.
+It's not-recoverable.
+
 For more on transports, how they work, and how to create them see the [`Transports documentation`](/docs/transports.md).
 
 * See [`Transports`](/docs/transports.md)

test/fixtures/crashing-transport.js

@@ -0,0 +1,13 @@
+const { Writable } = require('node:stream')
+
+module.exports = () =>
+  new Writable({
+    autoDestroy: true,
+    write (chunk, enc, cb) {
+      setImmediate(() => {
+        /* eslint-disable no-empty */
+        for (let i = 0; i < 1e3; i++) {}
+        process.exit(0)
+      })
+    }
+  })

test/transport/core.test.js

@@ -3,6 +3,7 @@
 const os = require('node:os')
 const { join } = require('node:path')
 const { once } = require('node:events')
+const { setImmediate: immediate } = require('node:timers/promises')
 const { readFile, writeFile } = require('node:fs').promises
 const { watchFileCreated, watchForWrite, file } = require('../helper')
 const { test } = require('tap')
@@ -12,10 +13,8 @@ const strip = require('strip-ansi')
 const execa = require('execa')
 const writer = require('flush-write-stream')
 const rimraf = require('rimraf')
-const { promisify } = require('node:util')
 const { tmpdir } = os
 
-const immediate = promisify(setImmediate)
 const pid = process.pid
 const hostname = os.hostname()
 

test/transport/crash.test.js

@@ -0,0 +1,34 @@
+'use strict'
+
+const { join } = require('node:path')
+const { once } = require('node:events')
+const { setImmediate: immediate } = require('node:timers/promises')
+const { test } = require('tap')
+const pino = require('../../')
+
+test('pino.transport emits error if the worker exits with 0 unexpectably', async ({ same, teardown, equal }) => {
+  // This test will take 10s, because flushSync waits for 10s
+  const transport = pino.transport({
+    target: join(__dirname, '..', 'fixtures', 'crashing-transport.js'),
+    sync: true
+  })
+  teardown(transport.end.bind(transport))
+
+  await once(transport, 'ready')
+
+  let maybeError
+  transport.on('error', (err) => {
+    maybeError = err
+  })
+
+  const logger = pino(transport)
+  for (let i = 0; i < 100000; i++) {
+    logger.info('hello')
+  }
+
+  await once(transport.worker, 'exit')
+
+  await immediate()
+
+  same(maybeError.message, 'the worker has exited')
+})