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, 261 insertions, 1 deletions

diff --git a/erts/emulator/test/nif_SUITE_data/nif_SUITE.c b/erts/emulator/test/nif_SUITE_data/nif_SUITE.c
index 3747291e7e..003a0f2929 100644
--- a/erts/emulator/test/nif_SUITE_data/nif_SUITE.c
+++ b/erts/emulator/test/nif_SUITE_data/nif_SUITE.c
@@ -52,6 +52,15 @@ static ErlNifMutex* dbg_trace_lock;
 #define DBG_TRACE4(FMT, A, B, C, D)
 #endif
 
+/*
+ * Hack to get around this function missing from the NIF API.
+ * TODO: Add this function/macro in the appropriate place, probably with
+ *       enif_make_pid() in erl_nif_api_funcs.h
+ */
+#ifndef enif_make_port
+#define enif_make_port(ENV, PORT) ((void)(ENV),(const ERL_NIF_TERM)((PORT)->port_id))
+#endif
+
 static int static_cntA; /* zero by default */
 static int static_cntB = NIF_SUITE_LIB_VER * 100;
 
@@ -76,6 +85,11 @@ static ERL_NIF_TERM atom_stats;
 static ERL_NIF_TERM atom_done;
 static ERL_NIF_TERM atom_stop;
 static ERL_NIF_TERM atom_null;
+static ERL_NIF_TERM atom_pid;
+static ERL_NIF_TERM atom_port;
+static ERL_NIF_TERM atom_send;
+static ERL_NIF_TERM atom_lookup;
+static ERL_NIF_TERM atom_badarg;
 
 typedef struct
 {
@@ -170,6 +184,9 @@ static ErlNifResourceTypeInit frenzy_rt_init = {
     frenzy_resource_down
 };
 
+static ErlNifResourceType* whereis_resource_type;
+static void whereis_thread_resource_dtor(ErlNifEnv* env, void* obj);
+
 static int get_pointer(ErlNifEnv* env, ERL_NIF_TERM term, void** pp)
 {
     ErlNifBinary bin;
@@ -223,6 +240,9 @@ static int load(ErlNifEnv* env, void** priv_data, ERL_NIF_TERM load_info)
 						      &frenzy_rt_init,
 						      ERL_NIF_RT_CREATE, NULL);
 
+    whereis_resource_type = enif_open_resource_type(env, NULL, "nif_SUITE.whereis",
+                            whereis_thread_resource_dtor, ERL_NIF_RT_CREATE, NULL);
+
     atom_false = enif_make_atom(env,"false");
     atom_true = enif_make_atom(env,"true");
     atom_self = enif_make_atom(env,"self");
@@ -244,6 +264,11 @@ static int load(ErlNifEnv* env, void** priv_data, ERL_NIF_TERM load_info)
     atom_done = enif_make_atom(env,"done");
     atom_stop = enif_make_atom(env,"stop");
     atom_null = enif_make_atom(env,"null");
+    atom_pid = enif_make_atom(env, "pid");
+    atom_port = enif_make_atom(env, "port");
+    atom_send = enif_make_atom(env, "send");
+    atom_lookup = enif_make_atom(env, "lookup");
+    atom_badarg = enif_make_atom(env, "badarg");
 
     *priv_data = data;
     return 0;
@@ -1137,6 +1162,237 @@ static void fill(void* dst, unsigned bytes, int seed)
     }
 }
 
+/* enif_whereis_... tests */
+
+enum {
+    /* results */
+    WHEREIS_SUCCESS,
+    WHEREIS_ERROR_TYPE,
+    WHEREIS_ERROR_LOOKUP,
+    WHEREIS_ERROR_SEND,
+    /* types */
+    WHEREIS_LOOKUP_PID,     /* enif_whereis_pid() */
+    WHEREIS_LOOKUP_PORT     /* enif_whereis_port() */
+};
+
+typedef union {
+    ErlNifPid   pid;
+    ErlNifPort  port;
+} whereis_term_data_t;
+
+/* single use, no cross-thread access/serialization */
+typedef struct {
+    ErlNifEnv* env;
+    ERL_NIF_TERM name;
+    whereis_term_data_t res;
+    ErlNifTid tid;
+    int type;
+} whereis_thread_resource_t;
+
+static whereis_thread_resource_t* whereis_thread_resource_create(void)
+{
+    whereis_thread_resource_t* rp = (whereis_thread_resource_t*)
+        enif_alloc_resource(whereis_resource_type, sizeof(*rp));
+    memset(rp, 0, sizeof(*rp));
+    rp->env = enif_alloc_env();
+
+    return rp;
+}
+
+static void whereis_thread_resource_dtor(ErlNifEnv* env, void* obj)
+{
+    whereis_thread_resource_t* rp = (whereis_thread_resource_t*) obj;
+    enif_free_env(rp->env);
+}
+
+static int whereis_type(ERL_NIF_TERM type)
+{
+    if (enif_is_identical(type, atom_pid))
+        return WHEREIS_LOOKUP_PID;
+
+    if (enif_is_identical(type, atom_port))
+        return WHEREIS_LOOKUP_PORT;
+
+    return WHEREIS_ERROR_TYPE;
+}
+
+static int whereis_lookup_internal(
+    ErlNifEnv* env, int type, ERL_NIF_TERM name, whereis_term_data_t* out)
+{
+    if (type == WHEREIS_LOOKUP_PID)
+        return enif_whereis_pid(env, name, & out->pid)
+            ? WHEREIS_SUCCESS : WHEREIS_ERROR_LOOKUP;
+
+    if (type == WHEREIS_LOOKUP_PORT)
+        return enif_whereis_port(env, name, & out->port)
+            ? WHEREIS_SUCCESS : WHEREIS_ERROR_LOOKUP;
+
+    return WHEREIS_ERROR_TYPE;
+}
+
+static int whereis_send_internal(
+    ErlNifEnv* env, int type, whereis_term_data_t* to, ERL_NIF_TERM msg)
+{
+    if (type == WHEREIS_LOOKUP_PID)
+        return enif_send(env, & to->pid, NULL, msg)
+            ? WHEREIS_SUCCESS : WHEREIS_ERROR_SEND;
+
+    if (type == WHEREIS_LOOKUP_PORT)
+        return enif_port_command(env, & to->port, NULL, msg)
+            ? WHEREIS_SUCCESS : WHEREIS_ERROR_SEND;
+
+    return WHEREIS_ERROR_TYPE;
+}
+
+static int whereis_resolved_term(
+    ErlNifEnv* env, int type, whereis_term_data_t* res, ERL_NIF_TERM* out)
+{
+    switch (type) {
+        case WHEREIS_LOOKUP_PID:
+            *out = enif_make_pid(env, & res->pid);
+            break;
+        case WHEREIS_LOOKUP_PORT:
+            *out = enif_make_port(env, & res->port);
+            break;
+        default:
+            return WHEREIS_ERROR_TYPE;
+    }
+    return WHEREIS_SUCCESS;
+}
+
+static ERL_NIF_TERM whereis_result_term(ErlNifEnv* env, int result)
+{
+    ERL_NIF_TERM err;
+    switch (result)
+    {
+        case WHEREIS_SUCCESS:
+            return atom_ok;
+        case WHEREIS_ERROR_LOOKUP:
+            err = atom_lookup;
+            break;
+        case WHEREIS_ERROR_SEND:
+            err = atom_send;
+            break;
+        case WHEREIS_ERROR_TYPE:
+            err = atom_badarg;
+            break;
+        default:
+            err = enif_make_int(env, -result);
+            break;
+    }
+    return enif_make_tuple2(env, atom_error, err);
+}
+
+static void* whereis_lookup_thread(void* arg)
+{
+    whereis_thread_resource_t* rp = (whereis_thread_resource_t*) arg;
+    int rc;
+
+    /* enif_whereis_xxx should work with allocated or null env */
+    rc = whereis_lookup_internal(
+        ((rp->type == WHEREIS_LOOKUP_PID) ? NULL : rp->env),
+        rp->type, rp->name, & rp->res);
+
+    return (((char*) NULL) + rc);
+}
+
+/* whereis_term(Type, Name) -> pid() | port() | false */
+static ERL_NIF_TERM
+whereis_term(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
+{
+    whereis_term_data_t res;
+    ERL_NIF_TERM ret;
+    int type, rc;
+
+    if (argc != 2)  /* allow non-atom name for testing */
+        return enif_make_badarg(env);
+
+    if ((type = whereis_type(argv[0])) == WHEREIS_ERROR_TYPE)
+        return enif_make_badarg(env);
+
+    rc = whereis_lookup_internal(env, type, argv[1], & res);
+    if (rc == WHEREIS_SUCCESS) {
+        rc = whereis_resolved_term(env, type, & res, & ret);
+    }
+    return (rc == WHEREIS_SUCCESS) ? ret : atom_false;
+}
+
+/* whereis_send(Type, Name, Message) -> ok | {error, Reason} */
+static ERL_NIF_TERM
+whereis_send(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
+{
+    whereis_term_data_t to;
+    int type, rc;
+
+    if (argc != 3 || !enif_is_atom(env, argv[1]))
+        return enif_make_badarg(env);
+
+    if ((type = whereis_type(argv[0])) == WHEREIS_ERROR_TYPE)
+        return enif_make_badarg(env);
+
+    rc = whereis_lookup_internal(env, type, argv[1], & to);
+    if (rc == WHEREIS_SUCCESS)
+        rc = whereis_send_internal(env, type, & to, argv[2]);
+
+    return whereis_result_term(env, rc);
+}
+
+/* whereis_thd_lookup(Type, Name) -> {ok, Resource} | {error, SysErrno} */
+static ERL_NIF_TERM
+whereis_thd_lookup(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
+{
+    whereis_thread_resource_t* rp;
+    int type, rc;
+
+    if (argc != 2 || !enif_is_atom(env, argv[1]))
+        return enif_make_badarg(env);
+
+    if ((type = whereis_type(argv[0])) == WHEREIS_ERROR_TYPE)
+        return enif_make_badarg(env);
+
+    rp = whereis_thread_resource_create();
+    rp->type = type;
+    rp->name = enif_make_copy(rp->env, argv[1]);
+
+    rc = enif_thread_create(
+        "nif_SUITE:whereis_thd", & rp->tid, whereis_lookup_thread, rp, NULL);
+
+    if (rc == 0) {
+        return enif_make_tuple2(env, atom_ok, enif_make_resource(env, rp));
+    }
+    else {
+        enif_release_resource(rp);
+        return enif_make_tuple2(env, atom_error, enif_make_int(env, rc));
+    }
+}
+
+/* whereis_thd_result(Resource) -> {ok, pid() | port()} | {error, ErrNum} */
+static ERL_NIF_TERM
+whereis_thd_result(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
+{
+    whereis_thread_resource_t* rp;
+    ERL_NIF_TERM ret;
+    char* thdret; /* so we can keep compilers happy converting to int */
+    int rc;
+
+    if (argc != 1
+    || !enif_get_resource(env, argv[0], whereis_resource_type, (void**) & rp))
+        return enif_make_badarg(env);
+
+    if ((rc = enif_thread_join(rp->tid, (void**) & thdret)) != 0)
+        return enif_make_tuple2(env, atom_error, enif_make_int(env, rc));
+    
+    rc = (int)(thdret - ((char*) NULL));
+    if (rc == WHEREIS_SUCCESS) {
+        rc = whereis_resolved_term(env, rp->type, & rp->res, & ret);
+    }
+    ret = (rc == WHEREIS_SUCCESS)
+        ? enif_make_tuple2(env, atom_ok, ret) : whereis_result_term(env, rc);
+    
+    enif_release_resource(rp);
+    return ret;
+}
+
 #define MAKE_TERM_REUSE_LEN 16
 struct make_term_info
 {
@@ -2968,7 +3224,11 @@ static ErlNifFunc nif_funcs[] =
     {"monitor_process_nif", 4, monitor_process_nif},
     {"demonitor_process_nif", 2, demonitor_process_nif},
     {"compare_monitors_nif", 2, compare_monitors_nif},
-    {"monitor_frenzy_nif", 4, monitor_frenzy_nif}
+    {"monitor_frenzy_nif", 4, monitor_frenzy_nif},
+    {"whereis_send", 3, whereis_send},
+    {"whereis_term", 2, whereis_term},
+    {"whereis_thd_lookup", 2, whereis_thd_lookup},
+    {"whereis_thd_result", 1, whereis_thd_result}
 };
 
 ERL_NIF_INIT(nif_SUITE,nif_funcs,load,NULL,upgrade,unload)