Add identical changes to the async Read variant

Author: ivmarkov

embedded-io-async/src/lib.rs

@@ -19,28 +19,30 @@ pub use embedded_io::{
 pub trait Read: ErrorType {
     /// Read some bytes from this source into the specified buffer, returning how many bytes were read.
     ///
-    /// If no bytes are currently available to read, this function waits until at least one byte is available.
-    ///
-    /// If bytes are available, a non-zero amount of bytes is read to the beginning of `buf`, and the amount
-    /// is returned. It is not guaranteed that *all* available bytes are returned, it is possible for the
-    /// implementation to read an amount of bytes less than `buf.len()` while there are more bytes immediately
-    /// available.
+    /// If no bytes are currently available to read:
+    /// - The method waits until at least one byte becomes available;
+    /// - Once at least one (or more) bytes become available, a non-zero amount of those is copied to the
+    ///   beginning of `buf`, and the amount is returned, *without waiting any further for more bytes to
+    ///   become available*.
+    ///
+    /// If bytes are available to read:
+    /// - A non-zero amount of bytes is read to the beginning of `buf`, and the amount is returned immediately,
+    ///   *without waiting for more bytes to become available*;
+    /// - It is not guaranteed that *all* available bytes are returned, it is possible for the implementation to
+    ///   read an amount of bytes less than `buf.len()` while there are more bytes immediately available.
+    ///
+    /// This waiting behavior is important for the cases where `Read` represents the "read" leg of a pipe-like
+    /// protocol (a socket, a pipe, a serial line etc.). The semantics is that the caller - by passing a non-empty
+    /// buffer - does expect _some_ data (one or more bytes) - but _not necessarily `buf.len()` or more bytes_ -
+    /// to become available, before the peer represented by `Read` would stop sending bytes due to
+    /// application-specific reasons (as in the peer waiting for a response to the data it had sent so far).
     ///
     /// If the reader is at end-of-file (EOF), `Ok(0)` is returned. There is no guarantee that a reader at EOF
     /// will always be so in the future, for example a reader can stop being at EOF if another process appends
     /// more bytes to the underlying file.
     ///
     /// If `buf.len() == 0`, `read` returns without waiting, with either `Ok(0)` or an error.
     /// The `Ok(0)` doesn't indicate EOF, unlike when called with a non-empty buffer.
-    ///
-    /// Implementations are encouraged to make this function side-effect-free on cancel (AKA "cancel-safe"), i.e.
-    /// guarantee that if you cancel (drop) a `read()` future that hasn't completed yet, the stream's
-    /// state hasn't changed (no bytes have been read).
-    ///
-    /// This is not a requirement to allow implementations that read into the user's buffer straight from
-    /// the hardware with e.g. DMA.
-    ///
-    /// Implementations should document whether they're actually side-effect-free on cancel or not.
     async fn read(&mut self, buf: &mut [u8]) -> Result<usize, Self::Error>;
 
     /// Read the exact number of bytes required to fill `buf`.