Next.js expands request.body in memory

Link to the code that reproduces this issue

https://github.com/mtsmfm/nextjs-memory-issue

To Reproduce

1. clone https://github.com/mtsmfm/nextjs-memory-issue
2. npm install
3. npm run dev
4. fallocate -l 100M 100mbfile
5. curl localhost:3000/debug -X POST -F 'file=@./100mbfile'
6. fallocate -l 1G 1gbfile
7. curl localhost:3000/debug -X POST -F 'file=@./1gbfile'

Current vs. Expected behavior

I expect request.body behaves like a stream and it consumes fixed amount of memory.
But currently Next.js consumes 300MB+ for 100MB file and 1200MB+ for 1GB file.

Actually with plain http.createServer, it only consumes ~40MB on my machine.
https://github.com/mtsmfm/nextjs-memory-issue/blob/4d1a347f2e59c4f3ae0b3f775f47327088bbc1b0/plain.mjs#L63-L79

I guess one of the cause is conversion from http.IncomingMessage to Request class.

https://github.com/vercel/next.js/blob/v14.0.4/packages/next/src/server/web/spec-extension/adapters/next-request.ts#L99
https://github.com/mtsmfm/nextjs-memory-issue/blob/4d1a347f2e59c4f3ae0b3f775f47327088bbc1b0/plain.mjs#L24
https://github.com/mtsmfm/nextjs-memory-issue/blob/4d1a347f2e59c4f3ae0b3f775f47327088bbc1b0/plain.mjs#L81-L97

Verify canary release

• I verified that the issue exists in the latest Next.js canary release

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP Thu Oct 5 21:02:42 UTC 2023
Binaries:
  Node: 20.9.0
  npm: 10.1.0
  Yarn: 1.22.19
  pnpm: N/A
Relevant Packages:
  next: 14.0.5-canary.7
  eslint-config-next: N/A
  react: 18.2.0
  react-dom: 18.2.0
  typescript: 5.3.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Not sure

Additional context

No response

Start with the linked reproducer and its plain.mjs comparison, following the listed npm and curl steps to measure memory use for 100MB and 1GB uploads. Then read packages/next/src/server/web/spec-extension/adapters/next-request.ts around line 99 and compare the IncomingMessage-to-Request conversion. Done means the reproduction no longer expands request.body proportionally beyond the expected streaming behavior.