fix: ignore epilogue after closing multipart boundary

lib/types/multipart.js

@@ -272,6 +272,11 @@ function Multipart (boy, cfg) {
 }
 
 Multipart.prototype.write = function (chunk, cb) {
+  if (this.parser._finished) {
+    cb()
+    return
+  }
+
   const r = this.parser.write(chunk)
   if (r && !this._pause) {
     cb()

test/types-multipart-coverage.test.js

@@ -49,6 +49,52 @@ test('multipart parser accepts prototype property names', async (t) => {
   }
 })
 
+test('multipart parser ignores an epilogue written after the closing boundary', { timeout: 1000 }, async (t) => {
+  const boundary = 'epilogueboundary'
+  const body = buildBody([[
+    'Content-Disposition: form-data; name="field"',
+    '',
+    'value'
+  ]], boundary)
+  const busboy = new Busboy({
+    headers: { 'content-type': 'multipart/form-data; boundary=' + boundary }
+  })
+  const events = []
+  const fields = []
+  let finishes = 0
+  busboy.on('field', (key, value) => {
+    events.push('field')
+    fields.push([key, value])
+  })
+  const finished = new Promise((resolve, reject) => {
+    busboy.once('error', reject)
+    busboy.on('finish', () => {
+      events.push('finish')
+      finishes++
+      resolve()
+    })
+  })
+
+  busboy.write(Buffer.from(body, 'utf8'))
+  await new Promise(resolve => setImmediate(resolve))
+  await new Promise((resolve, reject) => {
+    busboy.write(Buffer.from('epilogue', 'utf8'), error => {
+      if (error) reject(error)
+      else {
+        events.push('epilogue write')
+        resolve()
+      }
+    })
+  })
+  busboy.end()
+
+  await finished
+  await new Promise(resolve => setImmediate(resolve))
+  t.assert.deepStrictEqual(fields, [['field', 'value']])
+  t.assert.deepStrictEqual(events, ['field', 'epilogue write', 'finish'])
+  t.assert.strictEqual(finishes, 1)
+})
+
 test('multipart parser rejects bare CR or LF in disposition parameters', async (t) => {
   const boundary = 'barecrlf'
   const dispositionValues = [