process: Wake up read and write on `EPOLLERR` (#2218)

## Motivation

#2174

On epoll platforms, the read end of a pipe closing is signaled to the write end
through the `EPOLLERR` event [[1](http://man7.org/linux/man-pages/man2/epoll_ctl.2.html)]. If readiness is not registered for this
event, it will silently pass through `epoll_wait` calls.

Additionally, this specific case that `EPOLLERR` is triggered leaves the write
end of the pipe (parent process) waiting for a wakeup that never occurs.

## Solution

Similar to the `HUP` event on Unix platforms, errors are now always masked
through registrations so that both read and write ends of a connection are made
aware of errors.

In cases where pipes are used and the read end closes, write ends that are
waiting for a wakeup are properly notified and try to write again. This allows
a client to observe `BrokenPipe` and go through the proper cleanup and/or
restablishment of connection.

Closes #2174

Signed-off-by: Kevin Leimkuhler <kevin@kleimkuhler.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/tokio/src/io/driver/mod.rs b/tokio/src/io/driver/mod.rs
index e707d3a5..d8535d9a 100644
--- a/tokio/src/io/driver/mod.rs
+++ b/tokio/src/io/driver/mod.rs
@@ -146,7 +146,7 @@ impl Driver {
             return;
         }
 
-        if ready.is_writable() || platform::is_hup(ready) {
+        if ready.is_writable() || platform::is_hup(ready) || platform::is_error(ready) {
             wr = io.writer.take_waker();
         }
 
@@ -293,7 +293,7 @@ impl Direction {
                 // Everything except writable is signaled through read.
                 mio::Ready::all() - mio::Ready::writable()
             }
-            Direction::Write => mio::Ready::writable() | platform::hup(),
+            Direction::Write => mio::Ready::writable() | platform::hup() | platform::error(),
         }
     }
 }