[erlang-questions] 19.x <-> 18.x wire protocol incompatibility ?

[Alexandre Snarskii]

On Wed, Oct 12, 2016 at 07:54:53PM +0200, Sverker Eriksson wrote:
> This is a bug. ei_connect_xinit() should do that masking for you.

Thanks for clarification. 
https://github.com/erlang/otp/pull/1202

> The idea was to introduce larger creation values in a stepwise
> protocol compatible way.  OTP 19 should understand and forward
> ERL_NEW_PID_EXT's but never create them. And then in some future
> major release start using large creation values encoded
> with ERL_NEW_PID_EXT.
> 
>  From the release notes of 19.0:
> 
> OTP-13488    Application(s): erl_interface, erts, jinterface
> 
>                 Handle terms (pids,ports and refs) from nodes with a
>                 'creation' value larger than 3. This is a preparation
>                 of the distribution protocol to allow OTP 19 nodes to
>                 correctly communicate with future nodes (20 or higher).
>                 The 'creation' value differentiates different
>                 incarnations of the same node (name).
> 
> 
> /Sverker, Erlang/OTP
> 
> 
> On 10/12/2016 07:11 PM, Alexandre Snarskii wrote:
> > Hi!
> >
> > Got into interesting 'does not work' situation while connecting newly
> > deployed c-node (compiled with otp 19.0 libraries) with older 18.3
> > erlang node: nodes were unable to communicate with messages
> >
> > 2016-10-12 18:43:28.404 [warning] emulator '<erlang node name>' got a
> >    corrupted external term from '<c-node name>' on distribution channel ...
> >
> > logged on erlang side.
> >
> > After some research, root cause was isolated: C-node was initialized using
> > ei_connect_xinit with random() % 16 for its creation id. In 18.x, Pid of
> > C-node was encoded as ERL_PID_EXT and used only two lower bits of creation id.
> > However, in 19.x there is a new Pid wire presentation, ERL_NEW_PID_EXT, which
> > encodes all 32 bits of creation id. Worse yet, this presentation is
> > automatically selected for encoding any Pid with creation id > 3, and,
> > as this presentation is not known by older 18.x nodes, this leads to
> > connection drop.
> >
> > Solution: if you expect your C-nodes to communicate with pre-19.x erlang
> > nodes, you may use only values 0..3 for creation id. In my case I just
> > replaced random() % 16 with random() % 4.
> >   
> > Question: may be it makes sense to document this wire protocol incompatibility
> > somewhere ?
> >
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> >
> 
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