Revert match conditional, and use cfg_attr to avoid struct duplication

Tested on:
- Docker - Linux 5.10.104-linuxkit aarch64
- Pop-OS! 22.04 amd64

Author: Steve McFarlin

ch04/a-epoll/src/ffi.rs

@@ -1,3 +1,13 @@
+//! # FIXES:
+//! The number is identical to the number in the GitHub issue tracker
+//!
+//! ## FIX ISSUE #5
+//! See: https://github.com/PacktPublishing/Asynchronous-Programming-in-Rust/issues/5
+//! Readers reported wrong results when running the example on ARM64 instruction set
+//! (aarch64). The reason turned out to be that the `Event` struct is only `repr(packed)`
+//! on `x86-64` systems due to backwards compatibility. Fixed by conditionally
+//! compiling the #[repr(packed)] attribute.
+
 pub const EPOLL_CTL_ADD: i32 = 1;
 pub const EPOLLIN: i32 = 0x1;
 pub const EPOLLET: i32 = 1 << 31;
@@ -11,17 +21,9 @@ extern "C" {
 }
 
 #[derive(Debug)]
-#[cfg(not(target_arch = "aarch64"))]
-#[repr(C, packed)]
-pub struct Event {
-    pub(crate) events: u32,
-    // Token to identify event
-    pub(crate) epoll_data: usize,
-}
-
-#[derive(Debug)]
-#[cfg(target_arch = "aarch64")]
 #[repr(C)]
+// FIX #5
+#[cfg_attr(target_arch = "x86_64", repr(packed))]
 pub struct Event {
     pub(crate) events: u32,
     // Token to identify event

ch04/a-epoll/src/main.rs

@@ -49,7 +49,7 @@ fn handle_events(
 
         loop {
             match streams[index].read(&mut data) {
-                Ok(0) => {
+                Ok(n) if n == 0 => {
                     // FIX #4
                     // `insert` returns false if the value already existed in the set. We
                     // handle it here since we must be sure that the TcpStream is fully