net_util: code readability improvements

Small cleanup to improve code readability.
Specifically, refactoring a huge loop body into
a function call.

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>
On-behalf-of: SAP philipp.schuster@sap.com

net_util/src/open_tap.rs

@@ -58,6 +58,54 @@ fn check_mq_support(if_name: &Option<&str>, queue_pairs: usize) -> Result<()> {
     Ok(())
 }
 
+/// Opens a Tap device and configures it.
+///
+/// Afterward, further RX queues can be opened with a common config.
+fn open_tap_rx_q_0(
+    if_name: Option<&str>,
+    ip_addr: Option<IpAddr>,
+    netmask: Option<IpAddr>,
+    host_mac: &mut Option<MacAddr>,
+    mtu: Option<u16>,
+    num_rx_q: usize,
+    flags: Option<i32>,
+) -> Result<Tap> {
+    // Check if the given interface exists before we create it.
+    let tap_exists = if_name.is_some_and(|n| Path::new(&format!("/sys/class/net/{n}")).exists());
+
+    let tap = match if_name {
+        Some(name) => Tap::open_named(name, num_rx_q, flags).map_err(Error::TapOpen)?,
+        // Create a new Tap device in Linux, if none was specified.
+        None => Tap::new(num_rx_q).map_err(Error::TapOpen)?,
+    };
+    // Don't overwrite ip configuration of existing interfaces:
+    if !tap_exists {
+        if let Some(ip) = ip_addr {
+            tap.set_ip_addr(ip, netmask)
+                .map_err(Error::TapSetIpNetmask)?;
+        }
+    } else {
+        warn!(
+            "Tap {} already exists. IP configuration will not be overwritten.",
+            if_name.unwrap_or_default()
+        );
+    }
+    if let Some(mac) = host_mac {
+        tap.set_mac_addr(*mac).map_err(Error::TapSetMac)?
+    } else {
+        *host_mac = Some(tap.get_mac_addr().map_err(Error::TapGetMac)?)
+    }
+    if let Some(mtu) = mtu {
+        tap.set_mtu(mtu as i32).map_err(Error::TapSetMtu)?;
+    }
+    tap.enable().map_err(Error::TapEnable)?;
+
+    tap.set_vnet_hdr_size(vnet_hdr_len() as i32)
+        .map_err(Error::TapSetVnetHdrSize)?;
+
+    Ok(tap)
+}
+
 /// Create a new virtio network device with the given IP address and
 /// netmask.
 pub fn open_tap(
@@ -71,9 +119,6 @@ pub fn open_tap(
 ) -> Result<Vec<Tap>> {
     let mut taps: Vec<Tap> = Vec::new();
     let mut ifname: String = String::new();
-    let vnet_hdr_size = vnet_hdr_len() as i32;
-    // Check if the given interface exists before we create it.
-    let tap_existed = if_name.is_some_and(|n| Path::new(&format!("/sys/class/net/{n}")).exists());
 
     // In case the tap interface already exists, check if the number of
     // queues is appropriate. The tap might not support multiqueue while
@@ -85,40 +130,16 @@ pub fn open_tap(
     for i in 0..num_rx_q {
         let tap: Tap;
         if i == 0 {
-            tap = match if_name {
-                Some(name) => Tap::open_named(name, num_rx_q, flags).map_err(Error::TapOpen)?,
-                None => Tap::new(num_rx_q).map_err(Error::TapOpen)?,
-            };
-            // Don't overwrite ip configuration of existing interfaces:
-            if !tap_existed {
-                if let Some(ip) = ip_addr {
-                    tap.set_ip_addr(ip, netmask)
-                        .map_err(Error::TapSetIpNetmask)?;
-                }
-            } else {
-                warn!(
-                    "Tap {} already exists. IP configuration will not be overwritten.",
-                    if_name.unwrap_or_default()
-                );
-            }
-            if let Some(mac) = host_mac {
-                tap.set_mac_addr(*mac).map_err(Error::TapSetMac)?
-            } else {
-                *host_mac = Some(tap.get_mac_addr().map_err(Error::TapGetMac)?)
-            }
-            if let Some(mtu) = mtu {
-                tap.set_mtu(mtu as i32).map_err(Error::TapSetMtu)?;
-            }
-            tap.enable().map_err(Error::TapEnable)?;
-
-            tap.set_vnet_hdr_size(vnet_hdr_size)
-                .map_err(Error::TapSetVnetHdrSize)?;
-
+            // Special handling is required for the first RX queue, such as
+            // configuring the device. Subsequent iterations will then use the
+            // same device.
+            tap = open_tap_rx_q_0(if_name, ip_addr, netmask, host_mac, mtu, num_rx_q, flags)?;
+            // Set the name of the tap device we open in subsequent iterations.
             ifname = String::from_utf8(tap.get_if_name()).unwrap();
         } else {
             tap = Tap::open_named(ifname.as_str(), num_rx_q, flags).map_err(Error::TapOpen)?;
 
-            tap.set_vnet_hdr_size(vnet_hdr_size)
+            tap.set_vnet_hdr_size(vnet_hdr_len() as i32)
                 .map_err(Error::TapSetVnetHdrSize)?;
         }
         taps.push(tap);