VSR: Fix liveness issue for misdirected reply messages

We now also ignore unexpected commands in the Replica's
on_message() handler as we were doing for the Client,
and we use an exhaustive switch, which would also have
been better in the first place.

Reported-by: @ThreeFx
Refs: #9

src/vsr/client.zig

@@ -136,11 +136,11 @@ pub fn Client(comptime StateMachine: type, comptime MessageBus: type) type {
                 .reply => self.on_reply(message),
                 .eviction => self.on_eviction(message),
                 else => {
-                    // This could be because of a misdirected packet.
-                    log.warn(
-                        "{}: on_message: unexpected command {}",
-                        .{ self.id, message.header.command },
-                    );
+                    log.warn("{}: on_message: ignoring misdirected {s} message", .{
+                        self.id,
+                        @tagName(message.header.command),
+                    });
+                    return;
                 },
             }
         }

src/vsr/replica.zig

@@ -456,6 +456,9 @@ pub fn Replica(
                 return;
             }
 
+            // No client or replica should ever send a .reserved message.
+            assert(message.header.command != .reserved);
+
             if (message.header.cluster != self.cluster) {
                 log.warn("{}: on_message: wrong cluster (cluster must be {} not {})", .{
                     self.replica,
@@ -485,12 +488,21 @@ pub fn Replica(
                 .do_view_change => self.on_do_view_change(message),
                 .start_view => self.on_start_view(message),
                 .recovery => self.on_recovery(message),
+                .recovery_response => return, // TODO
                 .request_start_view => self.on_request_start_view(message),
                 .request_prepare => self.on_request_prepare(message),
                 .request_headers => self.on_request_headers(message),
                 .headers => self.on_headers(message),
                 .nack_prepare => self.on_nack_prepare(message),
-                else => unreachable,
+                // A replica should never handle misdirected messages intended for a client:
+                .eviction, .reply => {
+                    log.warn("{}: on_message: ignoring misdirected {s} message", .{
+                        self.replica,
+                        @tagName(message.header.command),
+                    });
+                    return;
+                },
+                .reserved => unreachable,
             }
 
             if (self.loopback_queue) |loopback_message| {