Several AsyncRead impls unnecessarily zero buffers in prepare_uninitialized_buffer

[Luexa]

Version

tokio 0.2.13

Platform

This is an issue for all platforms.

Subcrates

I haven't reviewed the full source, but I've reviewed process and can confirm that the ChildStdout and ChildStderr types are affected.

Description

ChildStdout and ChildStderr implement AsyncRead by deferring to an internal PollEvented of a platform specific type: UNIX uses the standard library's child streams with EventedFd and Windows uses NamedPipe from mio_named_pipes. PollEvented naturally cannot implement prepare_uninitialized_buffer in any way other than zeroing, due to std::io::Read::initializer being unstable, however ChildStdout and ChildStderr use well-behaved types internally and should be known not to read from the buffer. Thus, the buffer needs not be initialized and prepare_uninitialized_buffer should be overwritten for these types to be a no-op.

Currently this bug affects me because I cannot call prepare_uninitialized_buffer due to the unacceptable zeroing, despite this being part of the contract of AsyncRead when an uninitialized buffer is involved. I just have to assume that the tokio implementation will remain well-behaved.