proposal: io: CopyReader to the io package

Abstract

This proposal suggests the addition of the CopyReader function to the io package in the Go standard library.

Background

When processing HTTP requests, both middleware and handlers often need to read the request body. However, as the io.Reader interface, from which the body is read, is a forward-only stream, you run into an issue if you need to read it more than once. Once the body is read in the handler (endpoint), it has been consumed and cannot be read again. This characteristic of io.Reader causes difficulty because there's no capability for re-reading a stream that's already been read.

Proposal

• CopyReader is a function in io package.
• CopyReader is got 2 parameters src / dst that is io.Reader type.
• CopyReader return error

CopyReader example is below.

  var dst io.Reader
  src := strings.NewReader("hogehoge")
  err := io.CopyReader(src, dst) // dst has hogehoge

Rationale

Adding an io.CopyReader function to the io package would address a common pain point in working with io.Reader interfaces: the inability to re-read an already consumed stream. This is particularly troublesome when developing middleware for HTTP servers where both the middleware and the handler want to read the request body. Once the body is consumed by the io.Reader in the handler, it cannot be read again in the middleware, which limits the ability to process request data in middleware functionality such as logging or data validation.

It encourages developers to read from the io.Reader freely, without having to excessively plan ahead or manage temporary workaround solutions that is implemented by bytes/io.NopCloser. This gives more freedom to write more readable and maintainable code.