feat: add transformRequest option for custom request streams

README.md

@@ -114,6 +114,26 @@ try {
 
 ```
 
+You can provide a custom stream using the `transformRequest` option. This is useful for environments like Google Cloud Functions where the request body has already been consumed:
+
+```js
+const { Readable } = require('node:stream')
+
+// In environments like Google Cloud Functions/Firebase, `request.rawBody`
+// contains the full raw request body buffer/string.
+fastify.register(require('@fastify/multipart'), {
+  transformRequest: (request) => Readable.from(request.rawBody)
+})
+
+// In a standard Fastify/Node.js server, `request.raw` is already a readable
+// stream for the incoming HTTP request, so you can usually omit
+// `transformRequest` entirely or simply return `request.raw`:
+//
+// fastify.register(require('@fastify/multipart'), {
+//   transformRequest: (request) => request.raw
+// })
+```
+
 Additionally, you can pass per-request options to the  `req.file`, `req.files`, `req.saveRequestFiles` or `req.parts` function.
 
 ```js

index.js

@@ -10,7 +10,7 @@ const { generateId } = require('./lib/generateId')
 const createError = require('@fastify/error')
 const streamToNull = require('./lib/stream-consumer')
 const deepmergeAll = require('@fastify/deepmerge')({ all: true })
-const { PassThrough, Readable } = require('node:stream')
+const { PassThrough, Readable, pipeline } = require('node:stream')
 const { pipeline: pump } = require('node:stream/promises')
 const secureJSON = require('secure-json-parse')
 
@@ -26,6 +26,7 @@ const InvalidMultipartContentTypeError = createError('FST_INVALID_MULTIPART_CONT
 const InvalidJSONFieldError = createError('FST_INVALID_JSON_FIELD_ERROR', 'a request field is not a valid JSON as declared by its Content-Type', 406)
 const FileBufferNotFoundError = createError('FST_FILE_BUFFER_NOT_FOUND', 'the file buffer was not found', 500)
 const NoFormData = createError('FST_NO_FORM_DATA', 'FormData is not available', 500)
+const InvalidTransformRequestError = createError('FST_INVALID_TRANSFORM_REQUEST', 'transformRequest must return a readable stream', 500)
 
 function setMultipart (req, _payload, done) {
   req[kMultipart] = true
@@ -171,6 +172,8 @@ function fastifyMultipart (fastify, options, done) {
     ? options.throwFileSizeLimit
     : true
 
+  const transformRequest = typeof options.transformRequest === 'function' ? options.transformRequest : (request) => request
+
   fastify.decorate('multipartErrors', {
     PartsLimitError,
     FilesLimitError,
@@ -297,7 +300,12 @@ function fastifyMultipart (fastify, options, done) {
       process.nextTick(() => cleanup(err))
     })
 
-    request.pipe(bb)
+    const stream = transformRequest(request)
+    if (!stream || typeof stream.pipe !== 'function') throw new InvalidTransformRequestError()
+
+    pipeline(stream, bb, (err) => {
+      cleanup(err)
+    })
 
     function onField (name, fieldValue, fieldnameTruncated, valueTruncated, encoding, contentType) {
       // don't overwrite prototypes
@@ -435,8 +443,6 @@ function fastifyMultipart (fastify, options, done) {
     }
 
     function cleanup (err) {
-      request.unpipe(bb)
-
       if ((err || request.aborted) && currentFile) {
         currentFile.destroy()
         currentFile = null