rpc: Unset partition before forwarding to remote datacenter (#11758)

.changelog/11758.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+rpc: unset partition before forwarding to remote datacenter
+```

agent/consul/rpc.go

@@ -89,6 +89,19 @@ const (
 
 var ErrChunkingResubmit = errors.New("please resubmit call for rechunking")
 
+// partitionUnsetter is used to describe requests values that can unset their
+// EnterpriseMeta.Partition value.
+type partitionUnsetter interface {
+	// UnsetPartition is used to strip a Partition value from the request before
+	// it is forwarded to a remote datacenter. By unsetting the value, the server
+	// that handles the request can decide which partition should be used (or do nothing).
+	// This ensures that servers that are Partition-enabled (pre-1.11, or non-Enterprise)
+	// don't inadvertently cause servers that are not Partition-enabled (<= 1.10 or non-Enterprise)
+	// to filter their responses by Partition. In other words, this ensures upgraded servers
+	// remain compatible with non-upgraded servers.
+	UnsetPartition()
+}
+
 func (s *Server) rpcLogger() hclog.Logger {
 	return s.loggers.Named(logging.RPC)
 }
@@ -655,6 +668,14 @@ func (s *Server) forwardRequestToOtherDatacenter(info structs.RPCInfo, forwardTo
 				}
 			}
 		}
+		// In order to interoperate with servers that can interpret Partition, but
+		// may not handle it correctly (eg. 1.10 servers), we need to unset the value.
+		// Unsetting the Partition ensures that the server that handles the request
+		// uses its Partition, or an empty value (aka doing nothing).
+		// For requests that are not Partition-aware, this is a no-op.
+		if v, ok := info.(partitionUnsetter); ok {
+			v.UnsetPartition()
+		}
 
 		return true, forwardToDC(dc)
 	}

agent/structs/structs_oss.go

@@ -47,6 +47,10 @@ func (m *EnterpriseMeta) WithWildcardNamespace() *EnterpriseMeta {
 	return &emptyEnterpriseMeta
 }
 
+func (m *EnterpriseMeta) UnsetPartition() {
+	// do nothing
+}
+
 // TODO(partition): stop using this
 func NewEnterpriseMetaInDefaultPartition(_ string) EnterpriseMeta {
 	return emptyEnterpriseMeta