feature: compare local pids (#611)

* rustler_sys: add 'nif_compare_pids'
* rustler: add (Partial)Eq/Ord for LocalPid
* rustler_tests: add tests for LocalPid cmp/eq
* sys: define 'enif_compare_pids' to behave like macro in C code
* tests: add unit test with equality check for local pids

rustler/src/types/local_pid.rs

@@ -1,5 +1,6 @@
 use crate::wrapper::{pid, ErlNifPid};
 use crate::{Decoder, Encoder, Env, Error, NifResult, Term};
+use std::cmp::Ordering;
 use std::mem::MaybeUninit;
 
 #[derive(Copy, Clone)]
@@ -36,6 +37,27 @@ impl Encoder for LocalPid {
     }
 }
 
+impl PartialEq for LocalPid {
+    fn eq(&self, other: &Self) -> bool {
+        unsafe { rustler_sys::enif_compare_pids(self.as_c_arg(), other.as_c_arg()) == 0 }
+    }
+}
+
+impl Eq for LocalPid {}
+
+impl PartialOrd for LocalPid {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for LocalPid {
+    fn cmp(&self, other: &Self) -> Ordering {
+        let cmp = unsafe { rustler_sys::enif_compare_pids(self.as_c_arg(), other.as_c_arg()) };
+        cmp.cmp(&0)
+    }
+}
+
 impl<'a> Env<'a> {
     /// Return the calling process's pid.
     ///

rustler_sys/src/rustler_sys_api.rs

@@ -203,6 +203,12 @@ pub unsafe fn enif_make_pid(_env: *mut ErlNifEnv, pid: ErlNifPid) -> ERL_NIF_TER
     pid.pid
 }
 
+/// See [enif_compare_pids](http://erlang.org/doc/man/erl_nif.html#enif_compare_pids) in the Erlang docs
+pub unsafe fn enif_compare_pids(pid1: *const ErlNifPid, pid2: *const ErlNifPid) -> c_int {
+    // Mimics the implementation of the enif_compare_pids macro
+    enif_compare((*pid1).pid, (*pid2).pid)
+}
+
 /// See [ErlNifSysInfo](http://www.erlang.org/doc/man/erl_nif.html#ErlNifSysInfo) in the Erlang docs.
 #[allow(missing_copy_implementations)]
 #[repr(C)]

rustler_tests/lib/rustler_test.ex

@@ -33,6 +33,9 @@ defmodule RustlerTest do
   def sum_list(_), do: err()
   def make_list(), do: err()
 
+  def compare_local_pids(_, _), do: err()
+  def are_equal_local_pids(_, _), do: err()
+
   def term_debug(_), do: err()
 
   def term_debug_and_reparse(term) do

rustler_tests/native/rustler_test/src/lib.rs

@@ -5,6 +5,7 @@ mod test_dirty;
 mod test_env;
 mod test_error;
 mod test_list;
+mod test_local_pid;
 mod test_map;
 mod test_nif_attrs;
 mod test_path;
@@ -28,6 +29,8 @@ rustler::init!(
         test_primitives::echo_i128,
         test_list::sum_list,
         test_list::make_list,
+        test_local_pid::compare_local_pids,
+        test_local_pid::are_equal_local_pids,
         test_term::term_debug,
         test_term::term_eq,
         test_term::term_cmp,

rustler_tests/native/rustler_test/src/test_local_pid.rs

@@ -0,0 +1,17 @@
+use std::cmp::Ordering;
+
+use rustler::LocalPid;
+
+#[rustler::nif]
+pub fn compare_local_pids(lhs: LocalPid, rhs: LocalPid) -> i32 {
+    match lhs.cmp(&rhs) {
+        Ordering::Less => -1,
+        Ordering::Equal => 0,
+        Ordering::Greater => 1,
+    }
+}
+
+#[rustler::nif]
+pub fn are_equal_local_pids(lhs: LocalPid, rhs: LocalPid) -> bool {
+    lhs == rhs
+}

rustler_tests/test/local_pid_test.exs

@@ -0,0 +1,35 @@
+defmodule RustlerTest.LocalPidTest do
+  use ExUnit.Case, async: true
+
+  def make_pid() do
+    {:ok, pid} = Task.start(fn -> :ok end)
+    pid
+  end
+
+  def compare(lhs, rhs) do
+    cond do
+      lhs < rhs -> -1
+      lhs == rhs -> 0
+      lhs > rhs -> 1
+    end
+  end
+
+  test "local pid comparison" do
+    # We make sure that the code we have in rust code matches the comparisons
+    # that are performed in the BEAM code.
+    pids = for _ <- 1..3, do: make_pid()
+
+    for lhs <- pids, rhs <- pids do
+      assert RustlerTest.compare_local_pids(lhs, rhs) == compare(lhs, rhs)
+    end
+  end
+
+  test "local pid equality" do
+    pids = for _ <- 1..3, do: make_pid()
+
+    for lhs <- pids, rhs <- pids do
+      expected = lhs == rhs
+      assert RustlerTest.are_equal_local_pids(lhs, rhs) == expected
+    end
+  end
+end