\* \* %CopyrightBegin% \* \* Copyright Ericsson AB 2021. All Rights Reserved. \* \* Licensed under the Apache License, Version 2.0 (the "License"); \* you may not use this file except in compliance with the License. \* You may obtain a copy of the License at \* \* http://www.apache.org/licenses/LICENSE-2.0 \* \* Unless required by applicable law or agreed to in writing, software \* distributed under the License is distributed on an "AS IS" BASIS, \* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. \* See the License for the specific language governing permissions and \* limitations under the License. \* \* %CopyrightEnd% \* (* --- Model of the new link protocol introduced in Erlang/OTP 23.3 --- The protocol is documented in the ERTS User's Guide -> Distribution Protocol -> Protocol Between Connected Nodes -> Link Protocol -> New Link Protocol This model only models a link between two processes. This since a link between one pair of processes is completely independent of links between other pairs of processes. This model also assumes that the connection between the processes does not fail. In the real world a connection can of course fail. This is however taken care of by clearing the link information on both ends when a connection fails, and tracking which instantiation of a connection between the nodes signals arrive on and ignoring signals from old instantiations of connections. That is, connection loss is trivially taken care of since we just start over again from scratch if the connection is lost. The documentation of the protocol talks about "process local information about links". This information is stored in the process state record below. The 'other' field contains the identifier of the other process. The 'type' field acts as "active flag". The link is active when 'type' equals "linked" and not active when 'type' equals "unlinked". If the 'wait_unlink_ack' field contains a value larger than or equal to zero it is the "unlink id" of an unlink request we have issued and are waiting for to get acknowledged. If the 'wait_unlink_ack' field contains -1 we are not waiting for an acknowledgment. When 'type' equals "unlinked" and 'wait_unlink_ack' equals -1, we would in the documented protocol have removed the "process local information about the link". In this model we, however, keep the state, but in this state instead of removing it. Messages are tagged with a message number in order to model the signal order of Erlang. The message number of the unlink signal is also used as "unlink id". The model has been checked with the following parameters: Declared constants: PROCS <- {"a", "b"} Temporal formula: FairSpec Deadlock: disabled Invariants: TypeOK ValidState State Constraint: /\ \E proc \in PROCS : procState[proc].next_send =< 15 /\ Cardinality(msgs) =< 10 That is, we have checked all states where processes send up to 15 signals with at most 10 outstanding signals. Deadlock checking has been disabled since we intentionally stop when we have no outstanding signals (in 'Next') in order to avoid checking signal sequences equivalent to sequences we already have checked. *) ------------------------------ MODULE NewLinking ------------------------------ EXTENDS Integers, TLC, FiniteSets CONSTANTS PROCS \* PROCS should be a set of exactly two process names VARIABLES procState, \* Set of process states; procState[proc] is state of proc msgs \* Set of messages sent vars == <> \* Set of possible process states... procStateRec == [self : PROCS, other: PROCS, type : {"linked", "unlinked"}, wait_unlink_ack : Nat \cup {-1}, next_send : Nat, next_recv : Nat] \* Set of possible messages... Messages == [type : {"link", "unlink", "unlink_ack"}, from : PROCS, to : PROCS, msg_no : Nat, ack : Nat \cup {-1}] TypeOK == /\ procState \in [PROCS -> procStateRec] /\ msgs \subseteq Messages Init == /\ msgs = {} /\ procState = [p \in PROCS |-> [self |-> p, other |-> CHOOSE p2 \in PROCS : p2 /= p, type |-> "unlinked", next_send |-> 0, next_recv |-> 0, wait_unlink_ack |-> -1]] MkMsg(self, mtype, accnr) == [type |-> mtype, from |-> procState[self].self, to |-> procState[self].other, msg_no |-> procState[self].next_send, ack |-> accnr] Link(self) == /\ procState[self].type = "unlinked" /\ msgs' = msgs \cup {MkMsg(self, "link", -1)} /\ procState' = [procState EXCEPT ![self].type = "linked", ![self].next_send = @ + 1, ![self].wait_unlink_ack = -1] Unlink(self) == /\ procState[self].type = "linked" /\ msgs' = msgs \cup {MkMsg(self, "unlink", -1)} /\ procState' = [procState EXCEPT ![self].type = "unlinked", ![self].next_send = @ + 1, ![self].wait_unlink_ack = procState[self].next_send] RecvLink(self, msg) == LET type == IF procState[self].wait_unlink_ack /= -1 THEN "unlinked" ELSE "linked" IN /\ msgs' = msgs \ {msg} /\ procState' = [procState EXCEPT ![self].type = type, ![self].next_recv = @ + 1] RecvUnlink(self, msg) == /\ msgs' = (msgs \ {msg}) \cup {MkMsg(self, "unlink_ack", procState[self].next_recv)} /\ procState' = [procState EXCEPT ![self].type = "unlinked", ![self].next_recv = @ + 1, ![self].next_send = @ + 1] RecvUnlinkAck(self, msg) == LET wack == IF procState[self].wait_unlink_ack = msg.ack THEN -1 ELSE procState[self].wait_unlink_ack IN /\ msgs' = msgs \ {msg} /\ procState' = [procState EXCEPT ![self].next_recv = @ + 1, ![self].wait_unlink_ack = wack] Recv(self) == /\ \E m \in msgs : /\ m.to = self /\ m.msg_no = procState[self].next_recv /\ LET msg == CHOOSE m \in msgs : /\ m.to = self /\ m.msg_no = procState[self].next_recv IN CASE msg.type = "link" -> RecvLink(self, msg) [] msg.type = "unlink" -> RecvUnlink(self, msg) [] msg.type = "unlink_ack" -> RecvUnlinkAck(self, msg) (* If we have no outstanding messages; both processes should have the same view about whether they are linked or not... *) ValidState == IF msgs /= {} THEN TRUE ELSE \A p \in PROCS : \A p2 \in PROCS : procState[p].type = procState[p2].type Next == /\ (msgs /= {} \/ \A p \in PROCS : procState[p].next_send = 0) /\ \E p \in PROCS : \/ Recv(p) \/ Link(p) \/ Unlink(p) Spec == Init /\ [][Next]_vars FairSpec == Spec /\ WF_vars(Next) ============================================================================= \* Modification History \* Last modified Mon Jan 25 11:26:06 CET 2021 by rickard.green \* Created Wed Jan 20 13:11:46 CET 2021 by rickard.green