Parse the Multiple Labels capability instead of panicking

The capability parser had a todo!() for capability code 8 (RFC 8277), and
the encoder two more. A peer advertising it took the session task down.
It is now parsed into its (AFI, SAFI, count) entries and round-trips.

Also carries clippy's mechanical fixes for this file.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

crates/packet/src/open.rs

@@ -171,11 +171,9 @@ impl OpenOption {
                 }
                 Ok((buf, Self::Capabilities(capabilities)))
             }
-            other => {
-                return Err(Failure(BgpParserError::ProtocolError(
+            other => Err(Failure(BgpParserError::ProtocolError(
                 ProtocolErrorValues::UnknownOpenOption(other),
-                )));
-            }
+            ))),
         }
     }
 
@@ -207,6 +205,15 @@ impl OpenOption {
     }
 }
 
+/// One address family in a Multiple Labels capability (RFC 8277 §2.1).
+#[derive(Debug, Clone, Copy, Serialize, Deserialize, PartialEq, Eq)]
+pub struct MultiLabelEntry {
+    pub afi: u16,
+    pub safi: u8,
+    /// Maximum number of labels in one NLRI of this family.
+    pub count: u8,
+}
+
 #[derive(Debug, Serialize, Deserialize, EnumDiscriminants, PartialEq, Eq)]
 #[repr(u8)]
 pub enum Capability {
@@ -235,8 +242,11 @@ pub enum Capability {
         header: BgpSecHeader,
         afi: AddressFamilyId,
     } = 7,
-    /// Multiple labels compatibility (RFC 8277).
-    MultiLabelCompat {} = 8,
+    /// Multiple labels (RFC 8277 §2.1): for each address family, how many
+    /// labels the speaker can handle in one NLRI.
+    MultiLabelCompat {
+        entries: Vec<MultiLabelEntry>,
+    } = 8,
     BgpRole {
         role: BgpRole,
     } = 9,
@@ -292,7 +302,7 @@ impl Capability {
             Capability::ExtendedNextHop { .. } => CapabilityDiscriminants::ExtendedNextHop as u8,
             Capability::ExtendedMessage => CapabilityDiscriminants::ExtendedMessage as u8,
             Capability::BgpSec { .. } => CapabilityDiscriminants::BgpSec as u8,
-            Capability::MultiLabelCompat {} => CapabilityDiscriminants::MultiLabelCompat as u8,
+            Capability::MultiLabelCompat { .. } => CapabilityDiscriminants::MultiLabelCompat as u8,
             Capability::BgpRole { .. } => CapabilityDiscriminants::BgpRole as u8,
             Capability::GracefulRestart { .. } => CapabilityDiscriminants::GracefulRestart as u8,
             Capability::FourByteAsn { .. } => CapabilityDiscriminants::FourByteAsn as u8,
@@ -414,11 +424,25 @@ impl Capability {
                 ))
             }
             v if v == CapabilityDiscriminants::MultiLabelCompat as u8 => {
-                todo!()
+                // The AFI and SAFI are kept as numbers: this capability is
+                // used with labelled families we have no named constant for.
+                let parse_entry = |buf: &'a [u8]| -> IResult<
+                    &'a [u8],
+                    MultiLabelEntry,
+                    BgpParserError<&'a [u8]>,
+                > {
+                    let (buf, afi) = be_u16(buf)?;
+                    let (buf, safi) = be_u8(buf)?;
+                    let (buf, count) = be_u8(buf)?;
+                    Ok((buf, MultiLabelEntry { afi, safi, count }))
+                };
+                let (cap_buf, entries) = nom::multi::many1(parse_entry).parse(cap_buf)?;
+                require_empty!(cap_buf, "MultiLabelCompat");
+                Ok((buf, Self::MultiLabelCompat { entries }))
             }
             v if v == CapabilityDiscriminants::BgpRole as u8 => {
                 let (cap_buf, role) = be_u8(cap_buf)?;
-                let role = BgpRole::try_from(role).map_err(|e| fail!("{}", e))?;
+                let role = BgpRole::from(role);
                 require_empty!(cap_buf, "BgpRole");
                 Ok((buf, Self::BgpRole { role }))
             }
@@ -555,7 +579,13 @@ impl Capability {
                 out.put_u8(header.0);
                 out.put_u16(*afi as u16);
             }
-            Capability::MultiLabelCompat {} => todo!(),
+            Capability::MultiLabelCompat { entries } => {
+                for entry in entries {
+                    out.put_u16(entry.afi);
+                    out.put_u8(entry.safi);
+                    out.put_u8(entry.count);
+                }
+            }
             Capability::BgpRole { role } => {
                 out.put_u8(role.into());
             }
@@ -600,7 +630,7 @@ impl Capability {
                 for llgr in llgrs {
                     out.put_u16(llgr.afi as u16);
                     out.put_u8(llgr.safi as u8);
-                    out.put_u8(llgr.flags as u8);
+                    out.put_u8(llgr.flags);
                     out.put(&llgr.stale_time.to_be_bytes()[1..]);
                 }
             }
@@ -621,7 +651,7 @@ impl Capability {
             Capability::ExtendedNextHop { entries } => 6 * entries.len() as u8,
             Capability::ExtendedMessage => 0,
             Capability::BgpSec { .. } => 3,
-            Capability::MultiLabelCompat {} => todo!(),
+            Capability::MultiLabelCompat { entries } => 4 * entries.len() as u8,
             Capability::BgpRole { .. } => 1,
             Capability::GracefulRestart { entries, .. } => 2 + 4 * entries.len() as u8,
             Capability::FourByteAsn { .. } => 4,
@@ -818,7 +848,9 @@ mod tests {
     use bytes::BytesMut;
     use eyre::Result;
 
-    use crate::open::{Capability, ExtendedNextHop, GracefulRestartEntry, OpenMessage, OpenOption};
+    use crate::open::{
+        Capability, ExtendedNextHop, GracefulRestartEntry, MultiLabelEntry, OpenMessage, OpenOption,
+    };
 
     use bgp_primitives::{
         constants::{AddressFamilyId, SubsequentAfi},
@@ -986,6 +1018,40 @@ mod tests {
     /// Returning the (empty) per-capability buffer instead makes the list
     /// parser stop early and silently lose every later capability, e.g. the
     /// 4-octet ASN that routers list after graceful restart.
+    /// A peer advertising Multiple Labels used to hit a `todo!()` and take
+    /// the session task down with it.
+    #[test]
+    fn multiple_labels_capability_round_trips() {
+        let ctx = ParserContext::default();
+        // code 8, length 8: (IPv4, labelled unicast, 2) and (IPv6, labelled unicast, 3)
+        let wire = [0x08, 0x08, 0x00, 0x01, 0x04, 0x02, 0x00, 0x02, 0x04, 0x03];
+        let (rest, cap) = Capability::from_wire(&ctx, &wire).unwrap();
+        assert!(rest.is_empty());
+        assert_eq!(
+            cap,
+            Capability::MultiLabelCompat {
+                entries: vec![
+                    MultiLabelEntry {
+                        afi: 1,
+                        safi: 4,
+                        count: 2
+                    },
+                    MultiLabelEntry {
+                        afi: 2,
+                        safi: 4,
+                        count: 3
+                    },
+                ]
+            }
+        );
+        let mut out = BytesMut::new();
+        cap.to_wire(&ctx, &mut out).unwrap();
+        assert_eq!(&out[..], &wire[..]);
+
+        // A truncated entry is a parse error, not a panic.
+        assert!(Capability::from_wire(&ctx, &[0x08, 0x03, 0x00, 0x01, 0x04]).is_err());
+    }
+
     #[test]
     fn capability_parsers_leave_the_following_bytes() {
         let ctx = ParserContext::default();