1\* 2\* %CopyrightBegin% 3\* 4\* Copyright Ericsson AB 2021. All Rights Reserved. 5\* 6\* Licensed under the Apache License, Version 2.0 (the "License"); 7\* you may not use this file except in compliance with the License. 8\* You may obtain a copy of the License at 9\* 10\* http://www.apache.org/licenses/LICENSE-2.0 11\* 12\* Unless required by applicable law or agreed to in writing, software 13\* distributed under the License is distributed on an "AS IS" BASIS, 14\* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 15\* See the License for the specific language governing permissions and 16\* limitations under the License. 17\* 18\* %CopyrightEnd% 19\* 20 21(* 22 --- Model of the new link protocol introduced in Erlang/OTP 23.3 --- 23 24 The protocol is documented in the ERTS User's Guide -> 25 Distribution Protocol -> Protocol Between Connected Nodes -> 26 Link Protocol -> New Link Protocol 27 28 This model only models a link between two processes. This since a link 29 between one pair of processes is completely independent of links between 30 other pairs of processes. This model also assumes that the connection 31 between the processes does not fail. In the real world a connection can 32 of course fail. This is however taken care of by clearing the link 33 information on both ends when a connection fails, and tracking which 34 instantiation of a connection between the nodes signals arrive on and 35 ignoring signals from old instantiations of connections. That is, 36 connection loss is trivially taken care of since we just start over 37 again from scratch if the connection is lost. 38 39 The documentation of the protocol talks about "process local information 40 about links". This information is stored in the process state record below. 41 The 'other' field contains the identifier of the other process. The 'type' 42 field acts as "active flag". The link is active when 'type' equals 43 "linked" and not active when 'type' equals "unlinked". If the 44 'wait_unlink_ack' field contains a value larger than or equal to zero 45 it is the "unlink id" of an unlink request we have issued and are waiting 46 for to get acknowledged. If the 'wait_unlink_ack' field contains -1 we 47 are not waiting for an acknowledgment. When 'type' equals "unlinked" and 48 'wait_unlink_ack' equals -1, we would in the documented protocol have 49 removed the "process local information about the link". In this model we, 50 however, keep the state, but in this state instead of removing it. 51 Messages are tagged with a message number in order to model the signal 52 order of Erlang. The message number of the unlink signal is also used as 53 "unlink id". 54 55 The model has been checked with the following parameters: 56 57 Declared constants: 58 PROCS <- {"a", "b"} 59 60 Temporal formula: 61 FairSpec 62 63 Deadlock: 64 disabled 65 66 Invariants: 67 TypeOK 68 ValidState 69 70 State Constraint: 71 /\ \E proc \in PROCS : procState[proc].next_send =< 15 72 /\ Cardinality(msgs) =< 10 73 74 That is, we have checked all states where processes send up to 15 signals 75 with at most 10 outstanding signals. 76 77 Deadlock checking has been disabled since we intentionally stop when 78 we have no outstanding signals (in 'Next') in order to avoid checking 79 signal sequences equivalent to sequences we already have checked. 80 81*) 82------------------------------ MODULE NewLinking ------------------------------ 83EXTENDS Integers, TLC, FiniteSets 84CONSTANTS PROCS \* PROCS should be a set of exactly two process names 85 86VARIABLES 87 procState, \* Set of process states; procState[proc] is state of proc 88 msgs \* Set of messages sent 89 90vars == <<procState, msgs>> 91 92\* Set of possible process states... 93procStateRec == 94 [self : PROCS, 95 other: PROCS, 96 type : {"linked", "unlinked"}, 97 wait_unlink_ack : Nat \cup {-1}, 98 next_send : Nat, 99 next_recv : Nat] 100 101\* Set of possible messages... 102Messages == 103 [type : {"link", "unlink", "unlink_ack"}, from : PROCS, to : PROCS, msg_no : Nat, ack : Nat \cup {-1}] 104 105TypeOK == 106 /\ procState \in [PROCS -> procStateRec] 107 /\ msgs \subseteq Messages 108 109Init == 110 /\ msgs = {} 111 /\ procState = [p \in PROCS |-> [self |-> p, 112 other |-> CHOOSE p2 \in PROCS : p2 /= p, 113 type |-> "unlinked", 114 next_send |-> 0, 115 next_recv |-> 0, 116 wait_unlink_ack |-> -1]] 117 118MkMsg(self, mtype, accnr) == 119 [type |-> mtype, 120 from |-> procState[self].self, 121 to |-> procState[self].other, 122 msg_no |-> procState[self].next_send, 123 ack |-> accnr] 124 125Link(self) == 126 /\ procState[self].type = "unlinked" 127 /\ msgs' = msgs \cup {MkMsg(self, "link", -1)} 128 /\ procState' = [procState EXCEPT ![self].type = "linked", 129 ![self].next_send = @ + 1, 130 ![self].wait_unlink_ack = -1] 131 132Unlink(self) == 133 /\ procState[self].type = "linked" 134 /\ msgs' = msgs \cup {MkMsg(self, "unlink", -1)} 135 /\ procState' = [procState EXCEPT ![self].type = "unlinked", 136 ![self].next_send = @ + 1, 137 ![self].wait_unlink_ack = procState[self].next_send] 138 139RecvLink(self, msg) == 140 LET type == IF procState[self].wait_unlink_ack /= -1 141 THEN "unlinked" 142 ELSE "linked" 143 IN /\ msgs' = msgs \ {msg} 144 /\ procState' = [procState EXCEPT ![self].type = type, 145 ![self].next_recv = @ + 1] 146 147RecvUnlink(self, msg) == 148 /\ msgs' = (msgs \ {msg}) \cup {MkMsg(self, 149 "unlink_ack", 150 procState[self].next_recv)} 151 /\ procState' = [procState EXCEPT ![self].type = "unlinked", 152 ![self].next_recv = @ + 1, 153 ![self].next_send = @ + 1] 154 155RecvUnlinkAck(self, msg) == 156 LET wack == IF procState[self].wait_unlink_ack = msg.ack 157 THEN -1 158 ELSE procState[self].wait_unlink_ack 159 IN /\ msgs' = msgs \ {msg} 160 /\ procState' = [procState EXCEPT ![self].next_recv = @ + 1, 161 ![self].wait_unlink_ack = wack] 162 163Recv(self) == 164 /\ \E m \in msgs : /\ m.to = self 165 /\ m.msg_no = procState[self].next_recv 166 /\ LET msg == CHOOSE m \in msgs : /\ m.to = self 167 /\ m.msg_no = procState[self].next_recv 168 IN CASE msg.type = "link" -> RecvLink(self, msg) 169 [] msg.type = "unlink" -> RecvUnlink(self, msg) 170 [] msg.type = "unlink_ack" -> RecvUnlinkAck(self, msg) 171 172(* 173 If we have no outstanding messages; both processes should 174 have the same view about whether they are linked or not... 175*) 176ValidState == 177 IF msgs /= {} 178 THEN TRUE 179 ELSE \A p \in PROCS : \A p2 \in PROCS : procState[p].type = procState[p2].type 180 181Next == 182 /\ (msgs /= {} \/ \A p \in PROCS : procState[p].next_send = 0) 183 /\ \E p \in PROCS : \/ Recv(p) 184 \/ Link(p) 185 \/ Unlink(p) 186 187Spec == Init /\ [][Next]_vars 188 189FairSpec == Spec /\ WF_vars(Next) 190 191============================================================================= 192\* Modification History 193\* Last modified Mon Jan 25 11:26:06 CET 2021 by rickard.green 194\* Created Wed Jan 20 13:11:46 CET 2021 by rickard.green 195