Add enif_whereis_...() functions

#### Why do we need this new feature?

There are cases when a NIF needs to send a message, using `enif_send()`, to a long-lived process with a registered name.

A common use-case is logging, where asynchronous fire-and-forget messages are the norm.

There can also be cases where a yielding or dirty NIF or background thread may request a callback from a service with additional information it needs to complete its operation, yielding or waiting (with suitable timeouts, etc) until its state has been updated through the NIF module's API.

NIFs can only send messages to pids, and the lack of name resolution leaves a complicated dance between separate monitoring processes and the NIF as the only way to keep a NIF informed of the whereabouts of such long-lived processes.

Providing a reliable, built-in facility for NIFs to resolve process (or port) names simplifies these use cases considerably.

#### Risks or uncertain artifacts?

Testing has not exposed any significant risk.

The implementation behaves as expected on regular and dirty scheduler threads as well as non-scheduler threads.

By constraining the `enif_whereis_...()` functions to their minimal scopes and using patterns consistent with related functions, the implementation, testing, and maintenance burden is low.

The API and behavior of existing functions is unchanged.

#### How did you solve it?

While extending `enif_send()` to operate on a pid or an atom (as `erlang:send/2` does) was attractive, it would have entailed changing the type of its `to_pid` parameter and thereby breaking backward compatibility.

The same consideration applies to `enif_port_command()`.

That leaves a choice between 1, 2, or 3 new functions:

 1. `enif_whereis()`
 2. `enif_whereis_pid()` and `enif_whereis_port()`
 3. All of the above.

While option (1), directly mimicking the behavior of `erlang:whereis/1`, is appealing, it poses potential problems if `pid()` or `port()` are subsequently implemented as non-integral types that must be bound to an owning `ErlNifEnv` instance.

Therefore, option (2) has been chosen to use `ErlNifPid`/`ErlNifPort` structures in the API to maintain proper term ownership semantics.

1 files changed, 58 insertions, 0 deletions

diff --git a/erts/emulator/beam/erl_nif.c b/erts/emulator/beam/erl_nif.c
index ea835d1b64..4815e5e7bb 100644
--- a/erts/emulator/beam/erl_nif.c
+++ b/erts/emulator/beam/erl_nif.c
@@ -879,6 +879,64 @@ enif_port_command(ErlNifEnv *env, const ErlNifPort* to_port,
     return res;
 }
 
+/*
+ *  env must be the caller's environment in a scheduler or NULL in a
+ *      non-scheduler thread.
+ *  name must be an atom - anything else will just waste time.
+ */
+static Eterm call_whereis(ErlNifEnv *env, Eterm name)
+{
+    Process *c_p;
+    Eterm res;
+    int scheduler;
+    int unlock;
+
+    execution_state(env, &c_p, &scheduler);
+    ASSERT((c_p && scheduler) || (!c_p && !scheduler));
+
+    unlock = 0;
+    if (scheduler < 0) {
+        /* dirty scheduler */
+        if (ERTS_PROC_IS_EXITING(c_p))
+            return 0;
+
+        if (env->proc->static_flags & ERTS_STC_FLG_SHADOW_PROC) {
+            erts_smp_proc_lock(c_p, ERTS_PROC_LOCK_MAIN);
+            unlock = 1;
+        }
+    }
+    res = erts_whereis_name_to_id(c_p, name);
+
+    if (unlock)
+        erts_smp_proc_unlock(c_p, ERTS_PROC_LOCK_MAIN);
+
+    return res;
+}
+
+int enif_whereis_pid(ErlNifEnv *env, ERL_NIF_TERM name, ErlNifPid *pid)
+{
+    Eterm res;
+
+    if (is_not_atom(name))
+        return 0;
+
+    res = call_whereis(env, name);
+    /* enif_get_local_ functions check the type */
+    return enif_get_local_pid(env, res, pid);
+}
+
+int enif_whereis_port(ErlNifEnv *env, ERL_NIF_TERM name, ErlNifPort *port)
+{
+    Eterm res;
+
+    if (is_not_atom(name))
+        return 0;
+
+    res = call_whereis(env, name);
+    /* enif_get_local_ functions check the type */
+    return enif_get_local_port(env, res, port);
+}
+
 ERL_NIF_TERM enif_make_copy(ErlNifEnv* dst_env, ERL_NIF_TERM src_term)
 {
     Uint sz;