Fix decoding max length (#2816)

## Issue Addressed

N/A

## Proposed Changes

Fix encoder max length to the correct value (`MAX_RPC_SIZE`).

beacon_node/lighthouse_network/src/rpc/codec/ssz_snappy.rs

@@ -1112,4 +1112,43 @@ mod tests {
             RPCError::InvalidData
         );
     }
+
+    /// Test sending a message with encoded length prefix > MAX_RPC_SIZE.
+    #[test]
+    fn test_decode_invalid_length() {
+        // 10 byte snappy stream identifier
+        let stream_identifier: &'static [u8] = b"\xFF\x06\x00\x00sNaPpY";
+
+        assert_eq!(stream_identifier.len(), 10);
+
+        // Status message is 84 bytes uncompressed. `max_compressed_len` is 32 + 84 + 84/6 = 130.
+        let status_message_bytes = StatusMessage {
+            fork_digest: [0; 4],
+            finalized_root: Hash256::from_low_u64_be(0),
+            finalized_epoch: Epoch::new(1),
+            head_root: Hash256::from_low_u64_be(0),
+            head_slot: Slot::new(1),
+        }
+        .as_ssz_bytes();
+
+        let mut uvi_codec: Uvi<usize> = Uvi::default();
+        let mut dst = BytesMut::with_capacity(1024);
+
+        // Insert length-prefix
+        uvi_codec.encode(MAX_RPC_SIZE + 1, &mut dst).unwrap();
+
+        // Insert snappy stream identifier
+        dst.extend_from_slice(stream_identifier);
+
+        // Insert payload
+        let mut writer = FrameEncoder::new(Vec::new());
+        writer.write_all(&status_message_bytes).unwrap();
+        writer.flush().unwrap();
+        dst.extend_from_slice(writer.get_ref());
+
+        assert_eq!(
+            decode(Protocol::Status, Version::V1, &mut dst).unwrap_err(),
+            RPCError::InvalidData
+        );
+    }
 }

beacon_node/lighthouse_network/src/rpc/outbound.rs

@@ -2,7 +2,7 @@ use std::marker::PhantomData;
 
 use super::methods::*;
 use super::protocol::Protocol;
-use super::protocol::ProtocolId;
+use super::protocol::{ProtocolId, MAX_RPC_SIZE};
 use super::RPCError;
 use crate::rpc::protocol::Encoding;
 use crate::rpc::protocol::Version;
@@ -150,7 +150,7 @@ where
             Encoding::SSZSnappy => {
                 let ssz_snappy_codec = BaseOutboundCodec::new(SSZSnappyOutboundCodec::new(
                     protocol,
-                    usize::max_value(),
+                    MAX_RPC_SIZE,
                     self.fork_context.clone(),
                 ));
                 OutboundCodec::SSZSnappy(ssz_snappy_codec)

beacon_node/lighthouse_network/src/rpc/protocol.rs

@@ -81,7 +81,7 @@ lazy_static! {
 }
 
 /// The maximum bytes that can be sent across the RPC.
-const MAX_RPC_SIZE: usize = 1_048_576; // 1M
+pub const MAX_RPC_SIZE: usize = 1_048_576; // 1M
 /// The protocol prefix the RPC protocol id.
 const PROTOCOL_PREFIX: &str = "/eth2/beacon_chain/req";
 /// Time allowed for the first byte of a request to arrive before we time out (Time To First Byte).