gAdded a magic number to the message serializing. (#141)

The point here is to make sure we ignore message coming
from a different program, or coming from a version of chitchat
that was not properly versioned.

In that case, the enum discriminant could coincide with our
version number.

chitchat/src/message.rs

@@ -6,6 +6,8 @@ use crate::delta::Delta;
 use crate::digest::Digest;
 use crate::serialize::{Deserializable, Serializable};
 
+const MAGIC_NUMBER: u16 = 45_139;
+
 /// Chitchat message.
 ///
 /// Each variant represents a step of the gossip "handshake"
@@ -72,6 +74,7 @@ impl MessageType {
 
 impl Serializable for ChitchatMessage {
     fn serialize(&self, buf: &mut Vec<u8>) {
+        buf.extend(MAGIC_NUMBER.to_le_bytes());
         ProtocolVersion::V0.to_code().serialize(buf);
 
         match self {
@@ -96,34 +99,39 @@ impl Serializable for ChitchatMessage {
     }
 
     fn serialized_len(&self) -> usize {
-        1 + match self {
-            ChitchatMessage::Syn { cluster_id, digest } => {
-                1 + cluster_id.serialized_len() + digest.serialized_len()
+        2 + 1
+            + match self {
+                ChitchatMessage::Syn { cluster_id, digest } => {
+                    1 + cluster_id.serialized_len() + digest.serialized_len()
+                }
+                ChitchatMessage::SynAck { digest, delta } => {
+                    1 + digest.serialized_len() + delta.serialized_len()
+                }
+                ChitchatMessage::Ack { delta } => 1 + delta.serialized_len(),
+                ChitchatMessage::BadCluster => 1,
             }
-            ChitchatMessage::SynAck { digest, delta } => {
-                1 + digest.serialized_len() + delta.serialized_len()
-            }
-            ChitchatMessage::Ack { delta } => 1 + delta.serialized_len(),
-            ChitchatMessage::BadCluster => 1,
-        }
     }
 }
 
 impl Deserializable for ChitchatMessage {
     fn deserialize(buf: &mut &[u8]) -> anyhow::Result<Self> {
-        let protocol_version = buf
-            .first()
-            .copied()
-            .and_then(ProtocolVersion::from_code)
-            .context("invalid protocol version")?;
+        if buf.len() < 3 {
+            bail!("buffer too small to store the magic number and the protocol version");
+        }
+        let magic_number = u16::from_le_bytes(buf[0..2].try_into().unwrap());
+        if magic_number != MAGIC_NUMBER {
+            bail!("invalid chitchat magic number");
+        }
+        let protocol_version =
+            ProtocolVersion::from_code(buf[2]).context("invalid protocol version")?;
 
         if protocol_version != ProtocolVersion::V0 {
             bail!(
                 "unsupported protocol version `{}`",
                 protocol_version.to_code()
             )
         }
-        buf.consume(1);
+        buf.consume(3);
 
         let message_type = buf
             .first()
@@ -164,7 +172,7 @@ mod tests {
                 cluster_id: "cluster-a".to_string(),
                 digest: Digest::default(),
             };
-            test_serdeser_aux(&syn, 15);
+            test_serdeser_aux(&syn, 17);
         }
         {
             let mut digest = Digest::default();
@@ -175,7 +183,7 @@ mod tests {
                 cluster_id: "cluster-a".to_string(),
                 digest,
             };
-            test_serdeser_aux(&syn, 66);
+            test_serdeser_aux(&syn, 68);
         }
     }
 
@@ -186,8 +194,9 @@ mod tests {
                 digest: Digest::default(),
                 delta: Delta::default(),
             };
-            // 1 (protocol version) + 1 (message tag) + 2 (digest len) + 1 (delta end op)
-            test_serdeser_aux(&syn_ack, 5);
+            // 2 (magic number) + 1 (protocol version) + 1 (message tag) + 2 (digest len) + 1 (delta
+            // end op)
+            test_serdeser_aux(&syn_ack, 7);
         }
         {
             // 2 bytes.
@@ -212,7 +221,7 @@ mod tests {
             let syn_ack = ChitchatMessage::SynAck { digest, delta };
             // 1 byte (protocol version) + 1 byte (message tag) + 53 bytes (digest) + 60 bytes
             // (delta).
-            test_serdeser_aux(&syn_ack, 1 + 1 + 53 + 60);
+            test_serdeser_aux(&syn_ack, 2 + 1 + 1 + 53 + 60);
         }
     }
 
@@ -221,7 +230,7 @@ mod tests {
         {
             let delta = Delta::default();
             let ack = ChitchatMessage::Ack { delta };
-            test_serdeser_aux(&ack, 3);
+            test_serdeser_aux(&ack, 5);
         }
         {
             // 4 bytes.
@@ -233,12 +242,12 @@ mod tests {
             delta.add_kv(&node, "key", "value", 0, true);
             delta.set_serialized_len(60);
             let ack = ChitchatMessage::Ack { delta };
-            test_serdeser_aux(&ack, 1 + 1 + 60);
+            test_serdeser_aux(&ack, 2 + 1 + 1 + 60);
         }
     }
 
     #[test]
     fn test_bad_cluster() {
-        test_serdeser_aux(&ChitchatMessage::BadCluster, 2);
+        test_serdeser_aux(&ChitchatMessage::BadCluster, 4);
     }
 }