[erts] Fix immediate noproc scenarios of erlang:monitor/3 BIF

Since the BIF constructed and sent an ordinary 'DOWN' message when
detecting that the target could not be looked up, configured tag and/or
alias options were lost.

9 files changed, 370 insertions, 109 deletions

diff --git a/erts/emulator/beam/bif.c b/erts/emulator/beam/bif.c
index 34ace8c61b..3fc0267f99 100644
--- a/erts/emulator/beam/bif.c
+++ b/erts/emulator/beam/bif.c
@@ -394,6 +394,14 @@ demonitor(Process *c_p, Eterm ref, Eterm *multip)
        return am_true;
    }
 
+   case ERTS_MON_TYPE_DIST_PORT: {
+       ASSERT(is_external_port(mon->other.item));
+       ASSERT(external_pid_dist_entry(mon->other.item)
+              == erts_this_dist_entry);
+       erts_monitor_release(mon);
+       return am_true;
+   }
+
    case ERTS_MON_TYPE_PROC:
        erts_proc_sig_send_demonitor(mon);
        return am_true;
@@ -419,14 +427,12 @@ demonitor(Process *c_p, Eterm ref, Eterm *multip)
 
        if (is_external_pid(to))
            dep = external_pid_dist_entry(to);
-       else {
-           /* Monitoring a name at node to */
+       else /* Monitoring a name at node to */
            dep = erts_sysname_to_connected_dist_entry(to);
-           ASSERT(dep != erts_this_dist_entry);
-           if (!dep) {
-               erts_monitor_release(mon);
-               return am_false;
-           }
+
+       if (!dep || dep == erts_this_dist_entry) {
+           erts_monitor_release(mon);
+           return am_false;
        }
 
        code = erts_dsig_prepare(&ctx, dep, c_p, ERTS_PROC_LOCK_MAIN,
@@ -559,43 +565,6 @@ badarg:
     BIF_ERROR(BIF_P, BADARG);
 }
 
-/* Type must be atomic object! */
-void
-erts_queue_monitor_message(Process *p,
-			   ErtsProcLocks *p_locksp,
-			   Eterm ref,
-			   Eterm type,
-			   Eterm item,
-			   Eterm reason)
-{
-    Eterm tup;
-    Eterm* hp;
-    Eterm reason_copy, ref_copy, item_copy;
-    Uint reason_size, ref_size, item_size, heap_size;
-    ErlOffHeap *ohp;
-    ErtsMessage *msgp;
-
-    reason_size = IS_CONST(reason) ? 0 : size_object(reason);
-    item_size   = IS_CONST(item) ? 0 : size_object(item);
-    ref_size    = size_object(ref);
-
-    heap_size = 6+reason_size+ref_size+item_size;
-
-    msgp = erts_alloc_message_heap(p, p_locksp, heap_size,
-				   &hp, &ohp);
-
-    reason_copy = (IS_CONST(reason)
-		   ? reason
-		   : copy_struct(reason, reason_size, &hp, ohp));
-    item_copy   = (IS_CONST(item)
-		   ? item
-		   : copy_struct(item, item_size, &hp, ohp));
-    ref_copy    = copy_struct(ref, ref_size, &hp, ohp);
-
-    tup = TUPLE5(hp, am_DOWN, ref_copy, type, item_copy, reason_copy);
-    erts_queue_message(p, *p_locksp, msgp, tup, am_system);
-}
-
 Uint16
 erts_monitor_opts(Eterm opts, Eterm *tag)
 {
@@ -649,7 +618,6 @@ erts_monitor_opts(Eterm opts, Eterm *tag)
 static BIF_RETTYPE monitor(Process *c_p, Eterm type, Eterm target,
                            Uint16 add_oflags, Eterm tag) 
 {
-    Eterm tmp_heap[3];
     Eterm ref, id, name;
     ErtsMonitorData *mdp;
     BIF_RETTYPE ret_val;
@@ -677,9 +645,11 @@ static BIF_RETTYPE monitor(Process *c_p, Eterm type, Eterm target,
                 mdp->origin.flags |= add_oflags;
                 erts_monitor_tree_insert(&ERTS_P_MONITORS(c_p),
                                          &mdp->origin);
-                if (!erts_proc_sig_send_monitor(&mdp->u.target, id))
+                if (is_not_internal_pid(id)
+                    || !erts_proc_sig_send_monitor(&mdp->u.target, id)) {
                     erts_proc_sig_send_monitor_down(&mdp->u.target,
                                                     am_noproc);
+                }
             }
 
             goto done;
@@ -689,11 +659,7 @@ static BIF_RETTYPE monitor(Process *c_p, Eterm type, Eterm target,
         local_named_process:
             name = target;
             id = erts_whereis_name_to_id(c_p, target);
-            if (is_internal_pid(id))
-                goto local_process;
-            target = TUPLE2(&tmp_heap[0], name,
-                            erts_this_dist_entry->sysname);
-            goto noproc;
+            goto local_process;
         }
 
         if (is_external_pid(target)) {
@@ -701,8 +667,15 @@ static BIF_RETTYPE monitor(Process *c_p, Eterm type, Eterm target,
             int code;
 
             dep = external_pid_dist_entry(target);
-            if (dep == erts_this_dist_entry)
-                goto noproc;
+            if (dep == erts_this_dist_entry) {
+                mdp = erts_monitor_create(ERTS_MON_TYPE_DIST_PROC, ref,
+                                          c_p->common.id, target,
+                                          NIL, tag);
+                mdp->origin.flags |= add_oflags;
+                erts_monitor_tree_insert(&ERTS_P_MONITORS(c_p), &mdp->origin);
+                erts_proc_sig_send_monitor_down(&mdp->u.target, am_noproc);
+                goto done;
+            }
 
             id = target;
             name = NIL;
@@ -795,16 +768,19 @@ static BIF_RETTYPE monitor(Process *c_p, Eterm type, Eterm target,
         local_named_port:
             name = target;
             id = erts_whereis_name_to_id(c_p, target);
-            if (is_internal_port(id))
-                goto local_port;
-            target = TUPLE2(&tmp_heap[0], name,
-                            erts_this_dist_entry->sysname);
-            goto noproc;
+            goto local_port;
         }
 
         if (is_external_port(target)) {
-            if (erts_this_dist_entry == external_port_dist_entry(target))
-                goto noproc;
+            if (erts_this_dist_entry == external_port_dist_entry(target)) {
+                mdp = erts_monitor_create(ERTS_MON_TYPE_DIST_PORT, ref,
+                                          c_p->common.id, target,
+                                          NIL, tag);
+                mdp->origin.flags |= add_oflags;
+                erts_monitor_tree_insert(&ERTS_P_MONITORS(c_p), &mdp->origin);
+                erts_proc_sig_send_monitor_down(&mdp->u.target, am_noproc);
+                goto done;
+            }
             goto badarg;
         }
 
@@ -843,23 +819,6 @@ static BIF_RETTYPE monitor(Process *c_p, Eterm type, Eterm target,
 badarg:
 
     ERTS_BIF_PREP_ERROR(ret_val, c_p, BADARG);
-
-    if (0) {
-        ErtsProcLocks locks;
-noproc:
-        locks = ERTS_PROC_LOCK_MAIN;
-
-        erts_queue_monitor_message(c_p,
-                                   &locks,
-                                   ref,
-                                   type,
-                                   target,
-                                   am_noproc);
-        if (locks != ERTS_PROC_LOCK_MAIN)
-            erts_proc_unlock(c_p, locks & ~ERTS_PROC_LOCK_MAIN);
-
-        ERTS_BIF_PREP_RET(ret_val, ref);
-    }
     
 done:
 
diff --git a/erts/emulator/beam/break.c b/erts/emulator/beam/break.c
index fa2f77dc7c..b1b0ed6971 100644
--- a/erts/emulator/beam/break.c
+++ b/erts/emulator/beam/break.c
@@ -186,6 +186,7 @@ static int doit_print_monitor(ErtsMonitor *mon, void *vpcontext, Sint reds)
     case ERTS_MON_TYPE_PORT:
     case ERTS_MON_TYPE_TIME_OFFSET:
     case ERTS_MON_TYPE_DIST_PROC:
+    case ERTS_MON_TYPE_DIST_PORT:
     case ERTS_MON_TYPE_RESOURCE:
     case ERTS_MON_TYPE_NODE:
 
diff --git a/erts/emulator/beam/erl_bif_info.c b/erts/emulator/beam/erl_bif_info.c
index d5e598515c..17f55aaa81 100644
--- a/erts/emulator/beam/erl_bif_info.c
+++ b/erts/emulator/beam/erl_bif_info.c
@@ -291,6 +291,11 @@ static int do_make_one_mon_element(ErtsMonitor *mon, void * vpmlc, Sint reds)
         t = AM_dist_process;
         break;
     }
+    case ERTS_MON_TYPE_DIST_PORT: {
+        ERTS_DECL_AM(dist_port);
+        t = AM_dist_port;
+        break;
+    }
     case ERTS_MON_TYPE_RESOURCE: {
         ERTS_DECL_AM(resource);
         t = AM_resource;
@@ -578,6 +583,7 @@ static int collect_one_origin_monitor(ErtsMonitor *mon, void *vmicp, Sint reds)
         case ERTS_MON_TYPE_PROC:
         case ERTS_MON_TYPE_PORT:
         case ERTS_MON_TYPE_DIST_PROC:
+        case ERTS_MON_TYPE_DIST_PORT:
         case ERTS_MON_TYPE_TIME_OFFSET:
             if (mon->flags & ERTS_ML_FLG_SPAWN_PENDING)
                 break; /* Not an active monitor... */
@@ -630,6 +636,7 @@ static int collect_one_target_monitor(ErtsMonitor *mon, void *vmicp, Sint reds)
         case ERTS_MON_TYPE_PROC:
         case ERTS_MON_TYPE_PORT:
         case ERTS_MON_TYPE_DIST_PROC:
+        case ERTS_MON_TYPE_DIST_PORT:
 
             micp->mi[micp->mi_i].entity.term = mon->other.item;
             micp->mi[micp->mi_i].node = NIL;
diff --git a/erts/emulator/beam/erl_monitor_link.c b/erts/emulator/beam/erl_monitor_link.c
index 23fd3dd566..5746cd3e96 100644
--- a/erts/emulator/beam/erl_monitor_link.c
+++ b/erts/emulator/beam/erl_monitor_link.c
@@ -51,6 +51,7 @@ ml_get_key(ErtsMonLnkNode *mln)
     case ERTS_MON_TYPE_PROC:
     case ERTS_MON_TYPE_PORT:
     case ERTS_MON_TYPE_DIST_PROC:
+    case ERTS_MON_TYPE_DIST_PORT:
     case ERTS_MON_TYPE_TIME_OFFSET:
     case ERTS_MON_TYPE_RESOURCE: {
         ErtsMonitorData *mdp = erts_monitor_to_data(mln);
@@ -891,6 +892,7 @@ erts_monitor_create(Uint16 type, Eterm ref, Eterm orgn, Eterm trgt, Eterm name,
             break;
         }
     case ERTS_MON_TYPE_DIST_PROC:
+    case ERTS_MON_TYPE_DIST_PORT:
     case ERTS_MON_TYPE_RESOURCE:
     case ERTS_MON_TYPE_NODE:
     case ERTS_MON_TYPE_NODES: {
@@ -1203,7 +1205,8 @@ erts_monitor_size(ErtsMonitor *mon)
             && mon->type != ERTS_MON_TYPE_NODES) {
             if (!is_immed(mdep->md.ref))
                 hsz += NC_HEAP_SIZE(mdep->md.ref);
-            if (mon->type == ERTS_MON_TYPE_DIST_PROC) {
+            if (mon->type == ERTS_MON_TYPE_DIST_PROC
+                || mon->type == ERTS_MON_TYPE_DIST_PORT) {
                 if (!is_immed(mdep->md.origin.other.item))
                     hsz += NC_HEAP_SIZE(mdep->md.origin.other.item);
                 if (!is_immed(mdep->md.u.target.other.item))
diff --git a/erts/emulator/beam/erl_monitor_link.h b/erts/emulator/beam/erl_monitor_link.h
index 4c0a47963c..d2cd7ead4d 100644
--- a/erts/emulator/beam/erl_monitor_link.h
+++ b/erts/emulator/beam/erl_monitor_link.h
@@ -93,7 +93,8 @@
  *                      Key:            Reference
  *                      Name:           Name (atom) if by name
  *
- *              Valid keys are only ordinary internal references.
+ *              Valid keys are only ordinary internal references or internal
+ *              pid-reference.
  *
  *              Origin part of the monitor is stored in the monitor tree of
  *              origin process and target part of the monitor is stored in
@@ -112,7 +113,8 @@
  *                      Key:            Reference
  *                      Name:           Name (atom) if by name
  *
- *              Valid keys are only ordinary internal references.
+ *              Valid keys are only ordinary internal references or internal
+ *              pid-reference.
  *
  *              Origin part of the monitor is stored in the monitor tree of
  *              origin process/port and target part of the monitor is stored
@@ -130,7 +132,8 @@
  *              Shared:
  *                      Key:            Reference
  *
- *              Valid keys are only ordinary internal references.
+ *              Valid keys are only ordinary internal references or internal
+ *              pid-reference.
  *
  *              Origin part of the monitor is stored in the monitor tree of
  *              origin process and target part of the monitor is stored in
@@ -153,7 +156,8 @@
  *                      Name:           Name (atom) if by name
  *                      Dist:           Pointer to dist structure
  *
- *              Valid keys are only ordinary internal references.
+ *              Valid keys are only ordinary internal references or internal
+ *              pid-reference.
  *
  *              Origin part of the monitor is stored in the monitor tree of
  *              origin process and target part of the monitor is stored in
@@ -182,6 +186,30 @@
  *              process.
  *
  *
+ *              --- ERTS_MON_TYPE_DIST_PORT -----------------------------------
+ *
+ *              A local process (origin) monitors a port (target) on an old
+ *              incarnation of the local node. Note that it is currently only
+ *              for this since operations against remote ports is not
+ *              supported.
+ *
+ *              Origin:
+ *                      Other Item:     Monitored port identifier
+ *              Target:
+ *                      Other Item:     Local process identifier
+ *              Shared:
+ *                      Key:            Reference
+ *                      Dist:           NULL
+ *
+ *              Valid keys are only ordinary internal references or internal
+ *              pid-reference.
+ *
+ *              Origin part of the monitor is stored in the monitor tree of
+ *              origin process and target part is currently only used when
+ *              passing monitor down signal (monitors against old incarnations
+ *              will always immediately trigger monitor down noproc since the
+ *              process wont be alive).
+ *
  *              --- ERTS_MON_TYPE_RESOURCE ------------------------------------
  *
  *              A NIF resource (origin) monitors a process (target).
@@ -425,16 +453,17 @@
 #define ERTS_ML_STATE_ALIAS_DEMONITOR   (((Uint16) 2) << ERTS_ML_STATE_ALIAS_SHIFT)
 #define ERTS_ML_STATE_ALIAS_ONCE        (((Uint16) 3) << ERTS_ML_STATE_ALIAS_SHIFT)
 
-#define ERTS_MON_TYPE_MAX               ((Uint16) 8)
+#define ERTS_MON_TYPE_MAX               ((Uint16) 9)
 
 #define ERTS_MON_TYPE_PROC              ((Uint16) 0)
 #define ERTS_MON_TYPE_PORT              ((Uint16) 1)
 #define ERTS_MON_TYPE_TIME_OFFSET       ((Uint16) 2)
 #define ERTS_MON_TYPE_DIST_PROC         ((Uint16) 3)
-#define ERTS_MON_TYPE_RESOURCE          ((Uint16) 4)
-#define ERTS_MON_TYPE_NODE              ((Uint16) 5)
-#define ERTS_MON_TYPE_NODES             ((Uint16) 6)
-#define ERTS_MON_TYPE_SUSPEND           ((Uint16) 7)
+#define ERTS_MON_TYPE_DIST_PORT         ((Uint16) 4)
+#define ERTS_MON_TYPE_RESOURCE          ((Uint16) 5)
+#define ERTS_MON_TYPE_NODE              ((Uint16) 6)
+#define ERTS_MON_TYPE_NODES             ((Uint16) 7)
+#define ERTS_MON_TYPE_SUSPEND           ((Uint16) 8)
 #define ERTS_MON_TYPE_ALIAS             ERTS_MON_TYPE_MAX
 
 #define ERTS_MON_LNK_TYPE_MAX           (ERTS_MON_TYPE_MAX + ((Uint16) 3))
@@ -1166,12 +1195,13 @@ int erts_monitor_list_foreach_delete_yielding(ErtsMonitor **list,
  * Can create all types of monitors
  *
  * When the function is called it is assumed that:
- * - 'ref' is an internal ordinary reference if type is ERTS_MON_TYPE_PROC,
- *   ERTS_MON_TYPE_PORT, ERTS_MON_TYPE_TIME_OFFSET, or ERTS_MON_TYPE_RESOURCE
+ * - 'ref' is an ordinary internal reference or internal pid-reference if type
+ *   is ERTS_MON_TYPE_PROC, ERTS_MON_TYPE_PORT, ERTS_MON_TYPE_DIST_PORT,
+ *   ERTS_MON_TYPE_TIME_OFFSET, or ERTS_MON_TYPE_RESOURCE
  * - 'ref' is NIL if type is ERTS_MON_TYPE_NODE, ERTS_MON_TYPE_NODES, or
  *   ERTS_MON_TYPE_SUSPEND
- * - 'ref' is and ordinary internal reference or an external reference if
- *   type is ERTS_MON_TYPE_DIST_PROC
+ * - 'ref' is and ordinary internal reference, internal pid-reference or an
+ *   external reference if type is ERTS_MON_TYPE_DIST_PROC
  * - 'name' is an atom or NIL if type is ERTS_MON_TYPE_PROC,
  *   ERTS_MON_TYPE_PORT, or ERTS_MON_TYPE_DIST_PROC
  * - 'name is NIL if type is ERTS_MON_TYPE_TIME_OFFSET, ERTS_MON_TYPE_RESOURCE,
@@ -1180,8 +1210,9 @@ int erts_monitor_list_foreach_delete_yielding(ErtsMonitor **list,
  *
  * @param[in]     type          ERTS_MON_TYPE_PROC, ERTS_MON_TYPE_PORT,
  *                              ERTS_MON_TYPE_TIME_OFFSET, ERTS_MON_TYPE_DIST_PROC,
- *                              ERTS_MON_TYPE_RESOURCE, ERTS_MON_TYPE_NODE,
- *                              ERTS_MON_TYPE_NODES, or ERTS_MON_TYPE_SUSPEND
+ *                              ERTS_MON_TYPE_DIST_PORT, ERTS_MON_TYPE_RESOURCE,
+ *                              ERTS_MON_TYPE_NODE, ERTS_MON_TYPE_NODES, or
+ *                              ERTS_MON_TYPE_SUSPEND
  *
  * @param[in]     ref           A reference or NIL depending on type
  *
diff --git a/erts/emulator/beam/erl_proc_sig_queue.c b/erts/emulator/beam/erl_proc_sig_queue.c
index 83b6991809..bfd6d08e14 100644
--- a/erts/emulator/beam/erl_proc_sig_queue.c
+++ b/erts/emulator/beam/erl_proc_sig_queue.c
@@ -2086,15 +2086,17 @@ erts_proc_sig_send_demonitor(ErtsMonitor *mon)
     Uint16 type = mon->type;
     Eterm to = mon->other.item;
 
-    ASSERT(is_internal_pid(to));
+    ASSERT(is_internal_pid(to) || to == am_undefined);
     ASSERT(erts_monitor_is_origin(mon));
     ASSERT(!erts_monitor_is_in_table(mon));
 
     sig->common.tag = ERTS_PROC_SIG_MAKE_TAG(ERTS_SIG_Q_OP_DEMONITOR,
                                              type, 0);
     
-    if (!proc_queue_signal(NULL, to, sig, ERTS_SIG_Q_OP_DEMONITOR))
+    if (is_not_internal_pid(to)
+        || !proc_queue_signal(NULL, to, sig, ERTS_SIG_Q_OP_DEMONITOR)) {
         erts_monitor_release(mon);
+    }
 }
 
 int
@@ -3660,7 +3662,7 @@ convert_to_down_message(Process *c_p,
             hsz += 3;  /* reg name 2-tuple */
         else {
             ASSERT(is_pid(mdp->origin.other.item)
-                   || is_internal_port(mdp->origin.other.item));
+                   || is_port(mdp->origin.other.item));
             hsz += NC_HEAP_SIZE(mdp->origin.other.item);
         }
 
@@ -3705,6 +3707,22 @@ convert_to_down_message(Process *c_p,
                 ERL_MESSAGE_FROM(mp) = mdp->origin.other.item;
             }
             break;
+        case ERTS_MON_TYPE_DIST_PORT: {
+#ifdef DEBUG
+            ErtsMonitorDataExtended *mdep = (ErtsMonitorDataExtended *) mdp;
+#endif
+            ASSERT(mdp->origin.flags & ERTS_ML_FLG_EXTENDED);
+            type = am_port;
+            ASSERT(node == am_undefined);
+            ASSERT(!mdep->dist);
+            ASSERT(is_external_port(from)
+                   && (external_port_dist_entry(from)
+                       == erts_this_dist_entry));
+            node = erts_this_dist_entry->sysname;
+            ASSERT(is_atom(node) && node != am_undefined);
+            ERL_MESSAGE_FROM(mp) = node;
+            break;
+        }
         case ERTS_MON_TYPE_PROC:
             type = am_process;
             if (mdp->origin.other.item == am_undefined) {
@@ -3722,8 +3740,14 @@ convert_to_down_message(Process *c_p,
                 ErtsMonitorDataExtended *mdep;
                 ASSERT(mdp->origin.flags & ERTS_ML_FLG_EXTENDED);
                 mdep = (ErtsMonitorDataExtended *) mdp;
-                ASSERT(mdep->dist);
-                node = mdep->dist->nodename;
+                if (mdep->dist)
+                    node = mdep->dist->nodename;
+                else {
+                    ASSERT(is_external_pid(from));
+                    ASSERT(external_pid_dist_entry(from)
+                           == erts_this_dist_entry);
+                    node = erts_this_dist_entry->sysname;
+                }
             }
             ASSERT(is_atom(node) && node != am_undefined);
             ERL_MESSAGE_FROM(mp) = node;
@@ -4861,6 +4885,7 @@ handle_alias_message(Process *c_p, ErtsMessage *sig, ErtsMessage ***next_nm_sig)
         erts_pid_ref_delete(alias);
 
         switch (mon->type) {
+        case ERTS_MON_TYPE_DIST_PORT:
         case ERTS_MON_TYPE_ALIAS:
             erts_monitor_release(mon);
             break;
@@ -5071,6 +5096,7 @@ erts_proc_sig_handle_incoming(Process *c_p, erts_aint32_t *statep,
 
             switch (type) {
             case ERTS_MON_TYPE_DIST_PROC:
+            case ERTS_MON_TYPE_DIST_PORT:
             case ERTS_MON_TYPE_PROC:
             case ERTS_MON_TYPE_PORT:
                 tmon = (ErtsMonitor *) sig;
@@ -5890,6 +5916,7 @@ erts_proc_sig_handle_exit(Process *c_p, Sint *redsp,
             case ERTS_MON_TYPE_PORT:
             case ERTS_MON_TYPE_PROC:
             case ERTS_MON_TYPE_DIST_PROC:
+            case ERTS_MON_TYPE_DIST_PORT:
             case ERTS_MON_TYPE_NODE:
             case ERTS_MON_TYPE_NODES:
             case ERTS_MON_TYPE_SUSPEND:
@@ -6064,6 +6091,7 @@ clear_seq_trace_token(ErtsMessage *sig)
             case ERTS_MON_TYPE_PORT:
             case ERTS_MON_TYPE_PROC:
             case ERTS_MON_TYPE_DIST_PROC:
+            case ERTS_MON_TYPE_DIST_PORT:
             case ERTS_MON_TYPE_NODE:
             case ERTS_MON_TYPE_NODES:
             case ERTS_MON_TYPE_SUSPEND:
@@ -6143,6 +6171,7 @@ erts_proc_sig_signal_size(ErtsSignal *sig)
         case ERTS_MON_TYPE_PORT:
         case ERTS_MON_TYPE_PROC:
         case ERTS_MON_TYPE_DIST_PROC:
+        case ERTS_MON_TYPE_DIST_PORT:
         case ERTS_MON_TYPE_NODE:
         case ERTS_MON_TYPE_SUSPEND:
             size = erts_monitor_size((ErtsMonitor *) sig);
@@ -7457,6 +7486,7 @@ erts_proc_sig_debug_foreach_sig(Process *c_p,
                     case ERTS_MON_TYPE_PORT:
                     case ERTS_MON_TYPE_PROC:
                     case ERTS_MON_TYPE_DIST_PROC:
+                    case ERTS_MON_TYPE_DIST_PORT:
                     case ERTS_MON_TYPE_NODE:
                         mon_func((ErtsMonitor *) sig, arg, -1);
                         break;
diff --git a/erts/emulator/beam/erl_process.c b/erts/emulator/beam/erl_process.c
index 776094dfbf..e2eb445184 100644
--- a/erts/emulator/beam/erl_process.c
+++ b/erts/emulator/beam/erl_process.c
@@ -13315,6 +13315,10 @@ erts_proc_exit_handle_monitor(ErtsMonitor *mon, void *vctxt, Sint reds)
             erts_fire_nif_monitor(mon);
             mon = NULL;
             break;
+        case ERTS_MON_TYPE_DIST_PORT:
+            /* Target of a dist port monitor is never inserted... */
+            ERTS_INTERNAL_ERROR("Unexpected dist port monitor");
+            break;
         case ERTS_MON_TYPE_DIST_PROC: {
             ErtsMonLnkDist *dist;
             DistEntry *dep;
@@ -13390,7 +13394,8 @@ erts_proc_exit_handle_monitor(ErtsMonitor *mon, void *vctxt, Sint reds)
             break;
         case ERTS_MON_TYPE_PORT: {
             Port *prt;
-            ASSERT(is_internal_port(mon->other.item));
+            ASSERT(is_internal_port(mon->other.item)
+                   || mon->other.item == am_undefined);
             prt = erts_port_lookup_raw(mon->other.item);
             if (prt) {
                 if (erts_port_demonitor(c_p, prt, mon) != ERTS_PORT_OP_DROPPED)
@@ -13398,6 +13403,9 @@ erts_proc_exit_handle_monitor(ErtsMonitor *mon, void *vctxt, Sint reds)
             }
             break;
         }
+        case ERTS_MON_TYPE_DIST_PORT:
+            /* just release it... */
+            break;
         case ERTS_MON_TYPE_DIST_PROC: {
             ErtsMonLnkDist *dist;
             DistEntry *dep;
@@ -13412,7 +13420,14 @@ erts_proc_exit_handle_monitor(ErtsMonitor *mon, void *vctxt, Sint reds)
             
             mdp = erts_monitor_to_data(mon);
             dist = ((ErtsMonitorDataExtended *) mdp)->dist;
-            ASSERT(dist);
+            if (!dist) {
+                ASSERT(is_external_pid(mon->other.item));
+		ASSERT(external_pid_dist_entry(mon->other.item)
+                       == erts_this_dist_entry);
+                /* Target part is in our signal queue... */
+                mdp = NULL;
+                break;
+            }
             if (mon->flags & ERTS_ML_FLG_NAME) {
                 watched = ((ErtsMonitorDataExtended *) mdp)->u.name;
                 ASSERT(is_atom(watched));
diff --git a/erts/emulator/beam/global.h b/erts/emulator/beam/global.h
index feab1ba53e..232d3df452 100644
--- a/erts/emulator/beam/global.h
+++ b/erts/emulator/beam/global.h
@@ -931,13 +931,6 @@ Eterm erts_bld_bin_list(Uint **hpp, Uint *szp, ErlOffHeap* oh, Eterm tail);
 void erts_bif_info_init(void);
 
 /* bif.c */
-
-void erts_queue_monitor_message(Process *,
-				ErtsProcLocks*,
-				Eterm,
-				Eterm,
-				Eterm,
-				Eterm);
 void erts_init_trap_export(Export* ep, Eterm m, Eterm f, Uint a,
 			   Eterm (*bif)(Process*, Eterm*, ErtsCodePtr));
 void erts_init_bif(void);
diff --git a/erts/emulator/test/monitor_SUITE.erl b/erts/emulator/test/monitor_SUITE.erl
index 2b3c3b9477..e99b413c2a 100644
--- a/erts/emulator/test/monitor_SUITE.erl
+++ b/erts/emulator/test/monitor_SUITE.erl
@@ -30,7 +30,7 @@
          large_exit/1, list_cleanup/1, mixer/1, named_down/1, otp_5827/1,
          monitor_time_offset/1, monitor_tag_storage/1,
          unexpected_alias_at_demonitor_gh5310/1,
-         down_on_alias_gh5310/1]).
+         down_on_alias_gh5310/1, monitor_3_noproc_gh6185/1]).
 
 -export([y2/1, g/1, g0/0, g1/0, large_exit_sub/1]).
 
@@ -45,7 +45,7 @@ all() ->
      list_cleanup, mixer, named_down, otp_5827,
      monitor_time_offset, monitor_tag_storage,
      unexpected_alias_at_demonitor_gh5310,
-     down_on_alias_gh5310].
+     down_on_alias_gh5310, monitor_3_noproc_gh6185].
 
 groups() -> 
     [{remove_monitor, [],
@@ -1038,6 +1038,228 @@ down_on_alias_gh5310_test(ImmedExitReason, DeMonSched, TermSched) ->
     DeMonPid ! Go,
     receive Done -> ok end.
 
+
+monitor_3_noproc_gh6185(Config) when is_list(Config) ->
+    monitor_3_noproc_gh6185_test(false, false),
+    monitor_3_noproc_gh6185_test(true, false),
+    monitor_3_noproc_gh6185_test(false, true),
+    monitor_3_noproc_gh6185_test(true, true),
+    monitor_3_noproc_gh6185_exit_test(false, false),
+    monitor_3_noproc_gh6185_exit_test(true, false),
+    monitor_3_noproc_gh6185_exit_test(false, true),
+    monitor_3_noproc_gh6185_exit_test(true, true).
+
+monitor_3_noproc_gh6185_test(AliasTest, TagTest) ->
+    NodeName = node(),
+    UN = undefined_name_gh6185,
+    UNN = {UN, NodeName},
+    undefined = whereis(UN),
+
+    {AliasOpt, CheckAlias}
+        = case AliasTest of
+              false ->
+                  {[], fun (_NotAnAlias) -> ok end};
+              true ->
+                  {[{alias, explicit_unalias}],
+                   fun (Alias) ->
+                           AMsg1 = make_ref(),
+                           OMsg1 = make_ref(),
+                           Alias ! AMsg1,
+                           self() ! OMsg1,
+                           receive OMsg1 -> ok end,
+                           receive AMsg1 -> ok
+                           after 0 -> ct:fail(missing_alias_message)
+                           end,
+                           unalias(Alias),
+                           AMsg2 = make_ref(),
+                           OMsg2 = make_ref(),
+                           Alias ! AMsg2,
+                           self() ! OMsg2,
+                           receive OMsg2 -> ok end,
+                           receive AMsg2 -> ct:fail(unexpected_alias_message)
+                           after 0 -> ok
+                           end
+                   end}
+          end,
+
+    TagFun = case TagTest of
+                 false ->
+                     fun () ->
+                             {'DOWN', []}
+                     end;
+                 true ->
+                     fun () ->
+                             Tag = make_ref(),
+                             {Tag, [{tag, Tag}]}
+                     end
+             end,
+
+    %% not registerd process...
+    {Tag1, TagOpt1} = TagFun(),
+    M1 = erlang:monitor(process, UN, AliasOpt ++ TagOpt1),
+    receive
+        {Tag1, M1, process, UNN, noproc} ->
+            ok;
+        ID1 when element(2, ID1) == M1 ->
+            ct:fail({invalid_down, ID1})
+    after 100 ->
+            ct:fail(missing_down)
+    end,
+    CheckAlias(M1),
+
+    {Tag2, TagOpt2} = TagFun(),
+    M2 = erlang:monitor(process, UNN, AliasOpt ++ TagOpt2),
+    receive
+        {Tag2, M2, process, UNN, noproc} ->
+            ok;
+        ID2 when element(2, ID2) == M2 ->
+            ct:fail({invalid_down, ID2})
+    after 100 ->
+            ct:fail(missing_down)
+    end,
+    CheckAlias(M2),
+
+    %% Not registered port...
+    {Tag3, TagOpt3} = TagFun(),
+    M3 = erlang:monitor(port, UN, AliasOpt ++ TagOpt3),
+    receive
+        {Tag3, M3, port, UNN, noproc} ->
+            ok;
+        ID3 when element(2, ID3) == M3 ->
+            ct:fail({invalid_down, ID3})
+    after 100 ->
+            ct:fail(missing_down)
+    end,
+    CheckAlias(M3),
+
+    {Tag4, TagOpt4} = TagFun(),
+    M4 = erlang:monitor(port, UNN, AliasOpt ++ TagOpt4),
+    receive
+        {Tag4, M4, port, UNN, noproc} ->
+            ok;
+        ID4 when element(2, ID4) == M4 ->
+            ct:fail({invalid_down, ID4})
+    after 100 ->
+            ct:fail(missing_down)
+    end,
+    CheckAlias(M4),
+
+
+    OldCreation = case erlang:system_info(creation) of
+                      Creation when Creation =< 4 -> 16#ffffffff;
+                      Creation -> Creation - 1
+                  end,
+
+    %% Process of old incarnation...
+    Pid = erts_test_utils:mk_ext_pid({NodeName, OldCreation}, 4711, 17),
+    {Tag5, TagOpt5} = TagFun(),
+    M5 = erlang:monitor(process, Pid, AliasOpt ++ TagOpt5),
+    receive
+        {Tag5, M5, process, Pid, noproc} ->
+            ok;
+        ID5 when element(2, ID5) == M5 ->
+            ct:fail({invalid_down, ID5})
+    after 100 ->
+            ct:fail(missing_down)
+    end,
+    CheckAlias(M5),
+
+    %% Port of old incarnation...
+    Prt = erts_test_utils:mk_ext_port({NodeName, OldCreation}, 4711),
+    {Tag6, TagOpt6} = TagFun(),
+    M6 = erlang:monitor(port, Prt, AliasOpt ++ TagOpt6),
+    receive
+        {Tag6, M6, port, Prt, noproc} ->
+            ok;
+        ID6 when element(2, ID6) == M6 ->
+            ct:fail({invalid_down, ID6})
+    after 100 ->
+            ct:fail(missing_down)
+    end,
+    CheckAlias(M6),
+
+    ok.
+
+monitor_3_noproc_gh6185_exit_test(AliasTest, TagTest) ->
+    %%
+    %% Testing that we handle these quite unusual monitors correct
+    %% in case the monotoring process dies right after setting up
+    %% the monitor. We cannot check any results, but we might hit
+    %% asserts, crashes, or memory leaks if any bugs exist...
+    %%
+
+    NodeName = node(),
+    UN = undefined_name_gh6185,
+    UNN = {UN, NodeName},
+    undefined = whereis(UN),
+
+    AliasOpt = case AliasTest of
+                   false -> [];
+                   true -> [{alias, explicit_unalias}]
+               end,
+
+    TagOpt = case TagTest of
+                 false -> [];
+                 true -> [{tag, make_ref()}]
+             end,
+
+    %% not registerd process...
+    {P1, M1} = spawn_monitor(fun () ->
+                                     erlang:yield(),
+                                     _ = erlang:monitor(process, UN, AliasOpt ++ TagOpt),
+                                     exit(bang)
+                             end),
+    receive {'DOWN', M1, process, P1, bang} -> ok end,
+    {P2, M2} = spawn_monitor(fun () ->
+                                     erlang:yield(),
+                                     _ = erlang:monitor(process, UNN, AliasOpt ++ TagOpt),
+                                     exit(bang)
+                             end),
+    receive {'DOWN', M2, process, P2, bang} -> ok end,
+
+    %% Not registered port...
+    {P3, M3} = spawn_monitor(fun () ->
+                                     erlang:yield(),
+                                     _ = erlang:monitor(port, UN, AliasOpt ++ TagOpt),
+                                     exit(bang)
+                             end),
+    receive {'DOWN', M3, process, P3, bang} -> ok end,
+    {P4, M4} = spawn_monitor(fun () ->
+                                     erlang:yield(),
+                                     _ = erlang:monitor(port, UNN, AliasOpt ++ TagOpt),
+                                     exit(bang)
+                             end),
+    receive {'DOWN', M4, process, P4, bang} -> ok end,
+
+
+    OldCreation = case erlang:system_info(creation) of
+                      Creation when Creation =< 4 -> 16#ffffffff;
+                      Creation -> Creation - 1
+                  end,
+
+    %% Process of old incarnation...
+    {P5, M5} = spawn_monitor(fun () ->
+                                     Pid = erts_test_utils:mk_ext_pid({NodeName,
+                                                                       OldCreation},
+                                                                      4711, 17),
+                                     erlang:yield(),
+                                     _ = erlang:monitor(process, Pid, AliasOpt ++ TagOpt),
+                                     exit(bang)
+                             end),
+    receive {'DOWN', M5, process, P5, bang} -> ok end,
+
+    %% Port of old incarnation...
+    {P6, M6} = spawn_monitor(fun () ->
+                                     Prt = erts_test_utils:mk_ext_port({NodeName,
+                                                                        OldCreation},
+                                                                       4711),
+                                     erlang:yield(),
+                                     _ = erlang:monitor(port, Prt, AliasOpt ++ TagOpt),
+                                     exit(bang)
+                             end),
+    receive {'DOWN', M6, process, P6, bang} -> ok end,
+    ok.
+
 %%
 %% ...
 %%