Add enif_ioq_peek_head

This introduces a way to retrieve erlang terms from NIF IO queues
without having to resort to copying.

OTP-14797

2 files changed, 32 insertions, 0 deletions

diff --git a/erts/emulator/test/nif_SUITE.erl b/erts/emulator/test/nif_SUITE.erl
index bec2291867..2afeb7e4cf 100644
--- a/erts/emulator/test/nif_SUITE.erl
+++ b/erts/emulator/test/nif_SUITE.erl
@@ -3037,14 +3037,17 @@ nif_ioq(Config) ->
          {enqbraw,a},
          {enqbraw,a, 5},
          {peek,   a},
+         {peek_head,  a},
          {deq,    a, 42},
 
          %% Test enqv
          {enqv,   a, 2, 100},
+         {peek_head,  a},
          {deq,    a, all},
 
          %% This skips all elements but one in the iolist
          {enqv,   a, 5, iolist_size(nif_ioq_payload(5)) - 1},
+         {peek_head,  a},
          {peek,   a},
 
          %% Test to enqueue a bunch of refc binaries
@@ -3074,9 +3077,13 @@ nif_ioq(Config) ->
 
     Q = ioq_nif(create),
 
+    false = ioq_nif(peek_head, Q),
+
     {'EXIT', {badarg, _}} = (catch ioq_nif(deq, Q, 1)),
     {'EXIT', {badarg, _}} = (catch ioq_nif(enqv, Q, 1, 1234)),
 
+    false = ioq_nif(peek_head, Q),
+
     {'EXIT', {badarg, _}} = (catch ioq_nif(enqv, Q, [atom_in_list], 0)),
     {'EXIT', {badarg, _}} = (catch ioq_nif(enqv, Q, [make_ref()], 0)),
     {'EXIT', {badarg, _}} = (catch ioq_nif(enqv, Q, [256], 0)),
@@ -3085,6 +3092,8 @@ nif_ioq(Config) ->
     {'EXIT', {badarg, _}} = (catch ioq_nif(enqv, Q, [1 bsl 64], 0)),
     {'EXIT', {badarg, _}} = (catch ioq_nif(enqv, Q, [{tuple}], 0)),
 
+    false = ioq_nif(peek_head, Q),
+
     {'EXIT', {badarg, _}} = (catch ioq_nif(inspect,  [atom_in_list], use_stack)),
     {'EXIT', {badarg, _}} = (catch ioq_nif(inspect,  [make_ref()], no_stack)),
     {'EXIT', {badarg, _}} = (catch ioq_nif(inspect,  [256], use_stack)),
@@ -3145,6 +3154,20 @@ nif_ioq_run([{peek, Name} = H|T], State) ->
 
     true = iolist_to_binary(B) == iolist_to_binary(Data),
     nif_ioq_run(T, State);
+nif_ioq_run([{peek_head, Name} = H|T], State) ->
+    #{ q := IOQ, b := B } = maps:get(Name, State),
+    RefData = iolist_to_binary(B),
+
+    ct:log("~p", [H]),
+
+    {true, QueueHead} = ioq_nif(peek_head, IOQ),
+    true = byte_size(QueueHead) > 0,
+
+    {RefHead, _Tail} = split_binary(RefData, byte_size(QueueHead)),
+
+    true = QueueHead =:= RefHead,
+
+    nif_ioq_run(T, State);
 nif_ioq_run([{deq, Name, all}|T], State) ->
     #{ q := IOQ, b := B } = maps:get(Name, State),
     Size = ioq_nif(size, IOQ),
diff --git a/erts/emulator/test/nif_SUITE_data/nif_SUITE.c b/erts/emulator/test/nif_SUITE_data/nif_SUITE.c
index 79560a38aa..fa9ae1015c 100644
--- a/erts/emulator/test/nif_SUITE_data/nif_SUITE.c
+++ b/erts/emulator/test/nif_SUITE_data/nif_SUITE.c
@@ -3403,6 +3403,15 @@ static ERL_NIF_TERM ioq(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
 		return enif_make_badarg(env);
 	    else
 		return enif_make_atom(env, "true");
+        } else if (enif_is_identical(argv[0], enif_make_atom(env, "peek_head"))) {
+            ERL_NIF_TERM head_term;
+
+            if(enif_ioq_peek_head(env, ioq->q, NULL, &head_term)) {
+                return enif_make_tuple2(env,
+                    enif_make_atom(env, "true"), head_term);
+            }
+
+            return enif_make_atom(env, "false");
         } else if (enif_is_identical(argv[0], enif_make_atom(env, "peek"))) {
             int iovlen, num, i, off = 0;
             SysIOVec *iov = enif_ioq_peek(ioq->q, &iovlen);