Add constructor for Seg6Header

As the structure Seg6Header is tagged with `non_exhaustive`,
there is currently no way for an external library to create
a Seg6Header structure. By defining a constructor function `new`
for Seg6Header, this patch should allow to create Seg6Header
instances. In addition, this constructor checks if the segment
list is valid, i.e. if at least one segment is present,
preventing an underflow exception in `emit_value`.

Signed-off-by: Aperence <anthony.doeraene@hotmail.com>

Author: Aperence

src/route/mod.rs

@@ -36,6 +36,6 @@ pub use self::{
     next_hops::{RouteNextHop, RouteNextHopBuffer, RouteNextHopFlags},
     preference::RoutePreference,
     realm::RouteRealm,
-    seg6::{RouteSeg6IpTunnel, Seg6Header, Seg6Mode},
+    seg6::{RouteSeg6IpTunnel, Seg6Error, Seg6Header, Seg6Mode},
     via::{RouteVia, RouteViaBuffer},
 };

src/route/seg6.rs

@@ -103,6 +103,22 @@ buffer!(Seg6SegmentBuffer(SEG6_SEGMENT_LEN) {
     rest: (slice, SEG6_SEGMENT_LEN..)
 });
 
+#[derive(Debug)]
+#[non_exhaustive]
+pub enum Seg6Error {
+    InvalidSegmentList,
+}
+
+impl std::fmt::Display for Seg6Error {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self{
+            Seg6Error::InvalidSegmentList => write!(f, "Invalid segment list. A segment list should contain at least one segment."),
+        }
+    }
+}
+
+impl std::error::Error for Seg6Error {}
+
 /// Netlink attributes for `RTA_ENCAP` with `RTA_ENCAP_TYPE` set to
 /// `LWTUNNEL_ENCAP_SEG6`.
 #[derive(Debug, PartialEq, Eq, Clone)]
@@ -115,6 +131,17 @@ pub struct Seg6Header {
 }
 
 impl Seg6Header {
+    pub fn new(
+        mode: Seg6Mode,
+        segments: Vec<Ipv6Addr>,
+    ) -> Result<Seg6Header, Seg6Error> {
+        if !segments.is_empty() {
+            Ok(Seg6Header { mode, segments })
+        } else {
+            Err(Seg6Error::InvalidSegmentList)
+        }
+    }
+
     fn push_segments(buf: &mut [u8], mut segments: Vec<Ipv6Addr>) {
         if let Some(segment) = segments.pop() {
             let mut segment_buffer = Seg6SegmentBuffer::new(buf);