simplify formData body parsing (nodejs#2735)

* simplify formData body parsing

* perf: don't copy all headers

* fixup

lib/fetch/body.js

@@ -15,12 +15,12 @@ const { FormData } = require('./formdata')
 const { kState } = require('./symbols')
 const { webidl } = require('./webidl')
 const { Blob, File: NativeFile } = require('node:buffer')
-const { kBodyUsed } = require('../core/symbols')
 const assert = require('node:assert')
 const { isErrored } = require('../core/util')
-const { isUint8Array, isArrayBuffer } = require('util/types')
+const { isArrayBuffer } = require('util/types')
 const { File: UndiciFile } = require('./file')
 const { serializeAMimeType } = require('./dataURL')
+const { Readable } = require('node:stream')
 
 /** @type {globalThis['File']} */
 const File = NativeFile ?? UndiciFile
@@ -291,29 +291,6 @@ function cloneBody (body) {
   }
 }
 
-async function * consumeBody (body) {
-  if (body) {
-    if (isUint8Array(body)) {
-      yield body
-    } else {
-      const stream = body.stream
-
-      if (util.isDisturbed(stream)) {
-        throw new TypeError('The body has already been consumed.')
-      }
-
-      if (stream.locked) {
-        throw new TypeError('The stream is locked.')
-      }
-
-      // Compat.
-      stream[kBodyUsed] = true
-
-      yield * stream
-    }
-  }
-}
-
 function throwIfAborted (state) {
   if (state.aborted) {
     throw new DOMException('The operation was aborted.', 'AbortError')
@@ -328,7 +305,7 @@ function bodyMixinMethods (instance) {
       // given a byte sequence bytes: return a Blob whose
       // contents are bytes and whose type attribute is this’s
       // MIME type.
-      return specConsumeBody(this, (bytes) => {
+      return consumeBody(this, (bytes) => {
         let mimeType = bodyMimeType(this)
 
         if (mimeType === null) {
@@ -348,21 +325,21 @@ function bodyMixinMethods (instance) {
       // of running consume body with this and the following step
       // given a byte sequence bytes: return a new ArrayBuffer
       // whose contents are bytes.
-      return specConsumeBody(this, (bytes) => {
+      return consumeBody(this, (bytes) => {
         return new Uint8Array(bytes).buffer
       }, instance)
     },
 
     text () {
       // The text() method steps are to return the result of running
       // consume body with this and UTF-8 decode.
-      return specConsumeBody(this, utf8DecodeBytes, instance)
+      return consumeBody(this, utf8DecodeBytes, instance)
     },
 
     json () {
       // The json() method steps are to return the result of running
       // consume body with this and parse JSON from bytes.
-      return specConsumeBody(this, parseJSONFromBytes, instance)
+      return consumeBody(this, parseJSONFromBytes, instance)
     },
 
     async formData () {
@@ -375,16 +352,15 @@ function bodyMixinMethods (instance) {
 
       // If mimeType’s essence is "multipart/form-data", then:
       if (mimeType !== null && mimeType.essence === 'multipart/form-data') {
-        const headers = {}
-        for (const [key, value] of this.headers) headers[key] = value
-
         const responseFormData = new FormData()
 
         let busboy
 
         try {
           busboy = new Busboy({
-            headers,
+            headers: {
+              'content-type': serializeAMimeType(mimeType)
+            },
             preservePath: true
           })
         } catch (err) {
@@ -427,8 +403,10 @@ function bodyMixinMethods (instance) {
           busboy.on('error', (err) => reject(new TypeError(err)))
         })
 
-        if (this.body !== null) for await (const chunk of consumeBody(this[kState].body)) busboy.write(chunk)
-        busboy.end()
+        if (this.body !== null) {
+          Readable.from(this[kState].body.stream).pipe(busboy)
+        }
+
         await busboyResolve
 
         return responseFormData
@@ -442,20 +420,17 @@ function bodyMixinMethods (instance) {
           // application/x-www-form-urlencoded parser will keep the BOM.
           // https://url.spec.whatwg.org/#concept-urlencoded-parser
           // Note that streaming decoder is stateful and cannot be reused
-          const streamingDecoder = new TextDecoder('utf-8', { ignoreBOM: true })
+          const stream = this[kState].body.stream.pipeThrough(new TextDecoderStream('utf-8', { ignoreBOM: true }))
 
-          for await (const chunk of consumeBody(this[kState].body)) {
-            if (!isUint8Array(chunk)) {
-              throw new TypeError('Expected Uint8Array chunk')
-            }
-            text += streamingDecoder.decode(chunk, { stream: true })
+          for await (const chunk of stream) {
+            text += chunk
           }
-          text += streamingDecoder.decode()
+
           entries = new URLSearchParams(text)
         } catch (err) {
           // istanbul ignore next: Unclear when new URLSearchParams can fail on a string.
           // 2. If entries is failure, then throw a TypeError.
-          throw new TypeError(undefined, { cause: err })
+          throw new TypeError(err)
         }
 
         // 3. Return a new FormData object whose entries are entries.
@@ -493,7 +468,7 @@ function mixinBody (prototype) {
  * @param {(value: unknown) => unknown} convertBytesToJSValue
  * @param {Response|Request} instance
  */
-async function specConsumeBody (object, convertBytesToJSValue, instance) {
+async function consumeBody (object, convertBytesToJSValue, instance) {
   webidl.brandCheck(object, instance)
 
   throwIfAborted(object[kState])