Clarify Read trait blocking behavior

Author: ivmarkov

embedded-io/src/lib.rs

@@ -300,16 +300,23 @@ impl<E: fmt::Debug> std::error::Error for WriteFmtError<E> {}
 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 blocks until at least one byte is available.
+    /// If no bytes are currently available to read:
+    /// - The method blocks until at least one byte becomes available;
+    /// - Once at least one (or more) bytes become available, a non-zero amount of those is read to the
+    ///   beginning of `buf`, and the amount is returned, *without waiting or blocking any further for
+    ///   more bytes to become 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 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 blocking and 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.
     ///
-    /// 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.
+    /// This blocking 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 `buf.len() == 0`, `read` returns without blocking, with either `Ok(0)` or an error.
     /// The `Ok(0)` doesn't indicate EOF, unlike when called with a non-empty buffer.