!swim disallow addMember of non UID peers

Sources/SWIM/SWIMInstance.swift

@@ -393,17 +393,30 @@ extension SWIM {
             }
         )
 
-        /// Take care about peers with UID and without (!), some shells may not be quite good about this.
+        /// Note that peers without UID (in their `Node`) will NOT be added to the membership.
+        ///
+        /// This is because a cluster member must be a _specific_ peer instance, and not some arbitrary "some peer on that host/port",
+        /// which a Node without UID represents. The only reason we allow for peers and nodes without UID, is to simplify making
+        /// initial contact with a node - i.e. one can construct a peer to "there should be a peer on this host/port" to send an initial ping,
+        /// however in reply a peer in gossip must ALWAYS include it's unique identifier in the node - such that we know it from
+        /// any new instance of a process on the same host/port pair.
         internal func addMember(_ peer: SWIMPeer, status: SWIM.Status) -> [AddMemberDirective] {
             var directives: [AddMemberDirective] = []
 
+            // Guard 1) protect against adding already known dead members
             if self.hasTombstone(peer.node) {
                 // We saw this member already and even confirmed it dead, it shall never be added again
                 self.log.debug("Attempt to re-add already confirmed dead peer \(peer), ignoring it.")
                 directives.append(.memberAlreadyKnownDead(Member(peer: peer, status: .dead, protocolPeriod: 0)))
                 return directives
             }
 
+            // Guard 2) protect against adding non UID members
+            guard peer.node.uid != nil else {
+                self.log.warning("Ignoring attempt to add peer representing node without UID: \(peer)")
+                return directives
+            }
+
             let maybeExistingMember = self.member(for: peer)
             if let existingMember = maybeExistingMember, existingMember.status.supersedes(status) {
                 // we already have a newer state for this member

Tests/SWIMTests/SWIMInstanceTests.swift

@@ -906,6 +906,16 @@ final class SWIMInstanceTests: XCTestCase {
         XCTAssertNil(swim.nextPeerToPing())
     }
 
+    func test_addMember_shouldNotAddPeerWithoutUID() {
+        let swim = SWIM.Instance(settings: .init(), myself: self.myself)
+
+        let other = TestPeer(node: .init(protocol: "test", host: "127.0.0.1", port: 111, uid: nil))
+        let directives = swim.addMember(other, status: .alive(incarnation: 0))
+        XCTAssertEqual(directives.count, 0)
+        XCTAssertFalse(swim.isMember(other))
+        XCTAssertNil(swim.nextPeerToPing())
+    }
+
     func test_addMember_shouldReplaceMemberIfDifferentUID() {
         let swim = SWIM.Instance(settings: .init(), myself: self.myself)
         _ = swim.addMember(self.second, status: .alive(incarnation: 0))