Avoid piping the same file multiple times in case saveRequestFile is called twice.

[arthurfiorette]

In cases where multiple hooks and the handler needs to use the saved request files, and they cannot share the first call's result of saveRequestFile, they would call it twice or more which would result into piping the same file to disk more than once. As we can check by the following code:

fastify.post('/', async (req, rep) => {
  await req.saveRequestFiles() // [{ filepath: '/tmp/0388009396720a01.txt' }]
  await req.saveRequestFiles() // [{ filepath: '/tmp/0388009396720a02.txt' }]
  await req.saveRequestFiles() // [{ filepath: '/tmp/0388009396720a03.txt' }]
})

Now, saveRequestFiles will use the same system file and do not write the disk more than needed.

fastify.post('/', async (req, rep) => {
  await req.saveRequestFiles() // [{ filepath: '/tmp/0388009396720a04.txt' }]
  await req.saveRequestFiles() // [{ filepath: '/tmp/0388009396720a04.txt' }]
  await req.saveRequestFiles() // [{ filepath: '/tmp/0388009396720a04.txt' }]
  //                                       (Same filepath) ⬆️

  // where they are saved
  req.savedRequestFiles // [{ filepath: '/tmp/0388009396720a04.txt' }]
})

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[arthurfiorette]

@climba03003, sorry for pinging, but are you willing to merge this pr?

[joao-cabral]

I need this merge 😓

[felipe-bergamaschi]

@mcollina We need this merge 😓

[climba03003]

LGTM.
Sorry for the delay, I really do not have time currently.
The implementation is good.

[mcollina]

lgtm