feature: compare local pids

[hengermax]

For a feature in a library we're writing we wanted to compare LocalPids without encoding/decoding them into Terms. For this I thought the process was simple:

• Add nif_compare_pids to rustler_sys (by adding a b.func under the appropriate version). See: enif_compare_pids
• Implement PartialEq/Eq/PartialOrd/Ord for LocalPid
• Write some tests

However, when I'm running all the test suites (specifically the rustler_tests one by calling mix test) I'm now running into:

{:error,
 {:load_failed,
  ~c"Failed to load NIF library: 'dlopen(/Users/maxh/Development/rustler/rustler_tests/_build/test/lib/rustler_test/priv/native/librustler_test.so, 0x0002): symbol not found in flat namespace '_enif_compare_pids''"}}

I know this implies it cannot find the symbol in the erlang libs, but I was unable to find the location where the libraries are included to figure out which specific symbol I should be including.

Hence two questions:

1. Would you want to include this feature for LocalPid?
2. If so: could somebody help to point me in the right direction to make sure I'm referencing the right symbol?

Additional information:

• I'm running OSX
• If I switch back to the master branch (and clear all /target, /_build folders) I can run the tests without issues.

[evnu]

I believe enif_compare_pids is not a function, but a macro. Thus, there isn't a symbol for it.

[hengermax]

Welp, that is a bit silly from me. Before I "found" that enif_compare_pids I hopped through a lot of C macros to figure out what enif_compare was doing under the hood. But at no point did I consider the possibility that my missing symbol would be missing because it was actually a macro... Would you still accept the PR if I would implement the comparisons?

I have two options. Note that Term::cmp calls enif_compare using Term::as_c_arg as the arguments. That c_arg, looking at LocalPid::encoder, is equal to pid::make_pid(env.as_c_arg(), self.c). Which is an alias for enif_make_pid, which is actually manually implemented to just return the ErlNifPid::pid field, without referring to the Env.

So, if you would allow this, we could:

1. drop the env argument to pid::make_pid and enif_make_pid. So we may call enif_compare directly (but it is a bit ugly, as we're no longer following the interface as specified in the erlang documentation).
2. Hardcode a enif_compare_pids, just like enif_make_pid, which would simply call enif_compare with the internal ErlNifPid::pid field. Just like the C macro does in the link you provided.
3. ???

I believe (2) is a reasonable solution. What do you think?

[filmor]

Yes, (2) would be an acceptable solution. I'd rather not have us change interfaces of wrapped functions, there is no guarantee that they won't actually require their parameters at some point.

[filmor]

Looks good to me. We'll have to follow this up with a minor version bump on rustler-sys and a corresponding bump of the dependency in rustler.

[hengermax]

Thanks for the quick replies and the nice library!

[hengermax]

Just to be sure: do you want me to bump the version or will somebody else?

[filmor]

It's a note for myself, I will do that after this one is merged :). Just waiting for another review from @evnu before merging.

[evnu]

LGTM, thanks!