[erlang-bugs] Wrong dialyzer spec for erlang:process_info/2

[Kostis Sagonas]

On 10/19/2012 05:04 PM, Michał Ptaszek wrote:
> Hey,
>
> Let's define a function which is supposed to return us a list of links
> for a given process:
>
> -spec links(pid()) ->  [port() | pid()].
> links(Pid) ->
>      {links, Links} = process_info(Pid, links),
>      Links.
>
> Now let's run the dialyzer on the beam file:
>
> dialyzer -Wno_return -Wunderspecs --fullpath --plt .plt pi.beam
>    Checking whether the PLT .plt is up-to-date... yes
>    Proceeding with analysis...
> pi.erl:5: Type specification pi:links(pid()) ->  [port() | pid()] is a
> supertype of the success typing: pi:links(pid()) ->  [pid()]
>   done in 0m1.45s
> done (warnings were emitted)
>
> However it's clearly not right. For instance:
> (mynode@REDACTED)1>  erlang:process_info(whereis(erl_prim_loader), links).
> {links,[#Port<0.1>,<0.0.0>]}

Ah, the hard-coded type information again. The main problem is that this 
information was obtained by faithfully reading (decrypting?) the 
Erlang/OTP documentation. Sometimes, this documentation specifies only 
half the truth... (Also sometimes, this is done on purpose.)

For example, the published documentation of process_info/2 reads that 
the function returns:
...
   {links, Pids}
     Pids is a list of pids, with processes to which the process has a link.

(See http://erlang.org/doc/man/erlang.html#process_info-2)

So, it's not so clear whether the fact that it occasionally also returns 
ports as well as pids is to be relied upon in applications or not, 
though most probably it is.

*Aside*: I would suggest to the OTP folks to also correct such 
incomplete inaccurate documentation of the BIFs, not only the specs.

> I wanted to fix it by myself, however could not find the right place
> to apply the patch. Where can I find specs for ERTS BIFs?

In Erlang/OTP R15B02 (and previous) or in 'maint' the hard-coded type 
information is in lib/hipe/cerl/erl_bif_types.erl.  Contrary to what 
Patrik suggested, in most cases it's actually pretty easy to understand 
what's going on.  For example, the relevant line (in 'maint') is 1270. 
It currently reads:

     ['links'] -> t_tuple([InfoItem, t_list(t_pid())]);

Change it to:

     ['links'] -> t_tuple([InfoItem, t_list(t_sup(t_pid(), t_port()))]);

Then on the top-level just issue 'make hipe'.  The next time you try to 
run dialyzer it will automatically find out that the PLT is out of date 
and will rebuild it.

Kostis