Add 'Find' command to virtio net test util

For finding thet network interface corresponding to a MAC address.

Change-Id: I3bafd4979c47a5299b279bfaf16ec8ffaaaf90d6

src/bin/virtio_input_test_util/main.rs

diff --git a/src/bin/virtio_input_test_util/main.rs b/src/bin/virtio_input_test_util/main.rs
index 0637d24..d0ac6c6 100644
--- a/src/bin/virtio_input_test_util/main.rs
+++ b/src/bin/virtio_input_test_util/main.rs
@@ -1,6 +1,6 @@
 mod events;
 
-use events::{EventReader, EventType, InputEvent, KeyboardCode, KeyboardValue};
+use crate::events::{EventReader, EventType, InputEvent, KeyboardCode, KeyboardValue};
 use std::path::Path;
 use std::sync::mpsc;
 

src/bin/virtio_net_test_util.rs

diff --git a/src/bin/virtio_net_test_util.rs b/src/bin/virtio_net_test_util.rs
index 37c6bfe..cef8fb7 100644
--- a/src/bin/virtio_net_test_util.rs
+++ b/src/bin/virtio_net_test_util.rs
@@ -4,19 +4,57 @@
 
 #![deny(warnings)]
 
+use std::fs;
+use std::io::{Error, ErrorKind};
 use std::net::UdpSocket;
+use std::path::Path;
 use structopt::StructOpt;
 
+const NET_DIR: &str = "/sys/class/net";
+
 #[derive(StructOpt, Debug)]
-struct Transfer {
-    send_byte: u8,
-    receive_byte: u8,
-    length: usize,
+enum Command {
+    /// Find the name of network interface with the given MAC address.
+    Find {
+        mac_address: String,
+    },
+    /// Transfer a buffer of <length> containing <send_byte> and wait for a response containing
+    /// <receive_byte>.
+    Transfer {
+        send_byte: u8,
+        receive_byte: u8,
+        length: usize,
+    },
+}
+
+fn get_interface(mac_address: String) -> std::io::Result<String> {
+    let dir = Path::new(NET_DIR);
+    if !dir.is_dir() {
+        return Err(Error::new(
+            ErrorKind::Other,
+            format!("{} is not a directory", NET_DIR),
+        ));
+    }
+    for entry in fs::read_dir(dir)?.collect::<std::io::Result<Vec<_>>>()? {
+        let path = entry.path().join("address");
+        let address = fs::read_to_string(path)?;
+        if address.trim().to_lowercase() == mac_address.trim().to_lowercase() {
+            return entry.file_name().into_string().or(Err(Error::new(
+                ErrorKind::Other,
+                "Failed to read device name",
+            )));
+        }
+    }
+    Err(Error::new(ErrorKind::NotFound, "Could not find interface"))
 }
 
 fn main() -> std::io::Result<()> {
-    match Transfer::from_args() {
-        Transfer {
+    match Command::from_args() {
+        Command::Find { mac_address } => {
+            let interface = get_interface(mac_address)?;
+            println!("{:}", interface);
+        }
+        Command::Transfer {
             send_byte,
             receive_byte,
             length,