fix: don't hang when an epilogue arrives after the closing boundary

lib/types/multipart.js

@@ -272,6 +272,14 @@ function Multipart (boy, cfg) {
 }
 
 Multipart.prototype.write = function (chunk, cb) {
+  // After the closing delimiter Dicer is no longer writable. A later
+  // chunk can still carry an RFC 2046 epilogue; ignore it so the write
+  // callback still runs instead of hanging on a destroyed parser.
+  if (!this.parser.writable) {
+    cb()
+    return
+  }
+
   const r = this.parser.write(chunk)
   if (r && !this._pause) {
     cb()

test/types-multipart-epilogue.test.js

@@ -0,0 +1,67 @@
+'use strict'
+
+const { test } = require('node:test')
+const { setImmediate: nextTick } = require('node:timers/promises')
+const Busboy = require('..')
+
+function multipartBody (boundary, extra) {
+  return '--' + boundary + '\r\n' +
+    'Content-Disposition: form-data; name="field"\r\n' +
+    '\r\n' +
+    'value\r\n' +
+    '--' + boundary + '--\r\n' +
+    (extra || '')
+}
+
+test('emits finish when an RFC 2046 epilogue arrives in a later write', async (t) => {
+  const boundary = 'boundary'
+  const busboy = new Busboy({
+    headers: { 'content-type': 'multipart/form-data; boundary=' + boundary }
+  })
+
+  const fields = []
+  busboy.on('field', (name, value) => fields.push([name, value]))
+
+  const finished = new Promise((resolve, reject) => {
+    const timer = setTimeout(() => {
+      reject(new Error('finish was never emitted'))
+    }, 1000)
+    busboy.once('finish', () => {
+      clearTimeout(timer)
+      resolve()
+    })
+    busboy.once('error', (err) => {
+      clearTimeout(timer)
+      reject(err)
+    })
+  })
+
+  busboy.write(Buffer.from(multipartBody(boundary), 'utf8'))
+  await nextTick()
+  busboy.write(Buffer.from('epilogue', 'utf8'))
+  busboy.end()
+
+  await finished
+  t.assert.deepStrictEqual(fields, [['field', 'value']])
+})
+
+test('still emits finish when the epilogue is in the same chunk as the closing delimiter', async (t) => {
+  const boundary = 'boundary'
+  const busboy = new Busboy({
+    headers: { 'content-type': 'multipart/form-data; boundary=' + boundary }
+  })
+
+  const fields = []
+  busboy.on('field', (name, value) => fields.push([name, value]))
+
+  const finished = new Promise((resolve, reject) => {
+    busboy.once('finish', resolve)
+    busboy.once('error', reject)
+  })
+
+  busboy.write(Buffer.from(multipartBody(boundary, 'epilogue'), 'utf8'))
+  busboy.end()
+
+  await finished
+  t.assert.deepStrictEqual(fields, [['field', 'value']])
+})