Tokio's `AsyncWrite` trait once again has support for vectored writes in
Tokio 0.3.4 (see tokio-rs/tokio#3149).
This branch re-enables vectored writes in Hyper for HTTP/1. Using
vectored writes in HTTP/2 will require an upstream change in the `h2`
crate as well.
I've removed the adaptive write buffer implementation
that attempts to detect whether vectored IO is or is not available,
since the Tokio 0.3.4 `AsyncWrite` trait exposes this directly via the
`is_write_vectored` method. Now, we just ask the IO whether or not it
supports vectored writes, and configure the buffer accordingly. This
makes the implementation somewhat simpler.
This also removes `http1_writev()` methods from the builders. These are
no longer necessary, as Hyper can now determine whether or not
to use vectored writes based on `is_write_vectored`, rather than trying
to auto-detect it.
Closes#2320
BREAKING CHANGE: Removed `http1_writev` methods from `client::Builder`,
`client::conn::Builder`, `server::Builder`, and `server::conn::Builder`.
Vectored writes are now enabled based on whether the `AsyncWrite`
implementation in use supports them, rather than though adaptive
detection. To explicitly disable vectored writes, users may wrap the IO
in a newtype that implements `AsyncRead` and `AsyncWrite` and returns
`false` from its `AsyncWrite::is_write_vectored` method.
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