Add persistent_term:get(Key, DefaultValue)

https://bugs.erlang.org/browse/ERL-843

6 files changed, 45 insertions, 3 deletions

diff --git a/erts/doc/src/persistent_term.xml b/erts/doc/src/persistent_term.xml
index 1eda7f8d76..9d3c9afd80 100644
--- a/erts/doc/src/persistent_term.xml
+++ b/erts/doc/src/persistent_term.xml
@@ -256,6 +256,22 @@ will be slower as the number of persistent terms increases.</pre>
     </func>
 
     <func>
+      <name name="get" arity="2" since="OTP 21.3"/>
+      <fsummary>Get the value for a persistent term.</fsummary>
+      <desc>
+        <p>Retrieve the value for the persistent term associated with
+        the key <c><anno>Key</anno></c>. The lookup will be made in
+	constant time and the value will not be copied to the heap
+	of the calling process.</p>
+	<p>This function returns <c><anno>Default</anno></c> if no
+	term has been stored with the key <c><anno>Key</anno></c>.</p>
+	<p>If the calling process holds on to the value of the
+	persistent term and the persistent term is deleted in the future,
+	the term will be copied to the process.</p>
+      </desc>
+    </func>
+
+    <func>
       <name name="info" arity="0" since="OTP 21.2"/>
       <fsummary>Get information about persistent terms.</fsummary>
       <desc>
diff --git a/erts/emulator/beam/bif.tab b/erts/emulator/beam/bif.tab
index c96278b10c..8419244832 100644
--- a/erts/emulator/beam/bif.tab
+++ b/erts/emulator/beam/bif.tab
@@ -738,3 +738,4 @@ bif erts_internal:spawn_system_process/3
 
 bif erlang:integer_to_list/2
 bif erlang:integer_to_binary/2
+bif persistent_term:get/2
diff --git a/erts/emulator/beam/erl_bif_persistent.c b/erts/emulator/beam/erl_bif_persistent.c
index 9dca768a18..5a78a043ce 100644
--- a/erts/emulator/beam/erl_bif_persistent.c
+++ b/erts/emulator/beam/erl_bif_persistent.c
@@ -332,6 +332,23 @@ BIF_RETTYPE persistent_term_get_1(BIF_ALIST_1)
     BIF_ERROR(BIF_P, BADARG);
 }
 
+BIF_RETTYPE persistent_term_get_2(BIF_ALIST_2)
+{
+    Eterm key = BIF_ARG_1;
+    Eterm result = BIF_ARG_2;
+    HashTable* hash_table = (HashTable *) erts_atomic_read_nob(&the_hash_table);
+    Uint entry_index;
+    Eterm term;
+
+    entry_index = lookup(hash_table, key);
+    term = hash_table->term[entry_index];
+    if (is_boxed(term)) {
+        ASSERT(is_tuple_arity(term, 2));
+        result = tuple_val(term)[2];
+    }
+    BIF_RET(result);
+}
+
 BIF_RETTYPE persistent_term_erase_1(BIF_ALIST_1)
 {
     Eterm key = BIF_ARG_1;
diff --git a/erts/emulator/test/persistent_term_SUITE.erl b/erts/emulator/test/persistent_term_SUITE.erl
index 58038e24b7..93eb026ced 100644
--- a/erts/emulator/test/persistent_term_SUITE.erl
+++ b/erts/emulator/test/persistent_term_SUITE.erl
@@ -6,7 +6,7 @@
 %% Licensed under the Apache License, Version 2.0 (the "License");
 %% you may not use this file except in compliance with the License.
 %% You may obtain a copy of the License at
-%5
+%%
 %%     http://www.apache.org/licenses/LICENSE-2.0
 %%
 %% Unless required by applicable law or agreed to in writing, software
@@ -60,7 +60,8 @@ basic(_Config) ->
              Key = {?MODULE,{key,I}},
              true = persistent_term:erase(Key),
              false = persistent_term:erase(Key),
-             {'EXIT',{badarg,_}} = (catch persistent_term:get(Key))
+             {'EXIT',{badarg,_}} = (catch persistent_term:get(Key)),
+             {not_present,Key} = persistent_term:get(Key, {not_present,Key})
          end || I <- Seq],
     [] = [P || {{?MODULE,_},_}=P <- pget(Chk)],
     chk(Chk).
diff --git a/erts/preloaded/ebin/persistent_term.beam b/erts/preloaded/ebin/persistent_term.beam
index e94ef983be..c882e4fad4 100644
--- a/erts/preloaded/ebin/persistent_term.beam
+++ b/erts/preloaded/ebin/persistent_term.beam
diff --git a/erts/preloaded/src/persistent_term.erl b/erts/preloaded/src/persistent_term.erl
index 5d0c266127..ee7e49b6cb 100644
--- a/erts/preloaded/src/persistent_term.erl
+++ b/erts/preloaded/src/persistent_term.erl
@@ -19,7 +19,7 @@
 %%
 -module(persistent_term).
 
--export([erase/1,get/0,get/1,info/0,put/2]).
+-export([erase/1,get/0,get/1,get/2,info/0,put/2]).
 
 -type key() :: term().
 -type value() :: term().
@@ -41,6 +41,13 @@ get() ->
 get(_Key) ->
     erlang:nif_error(undef).
 
+-spec get(Key, Default) -> Value when
+      Key :: key(),
+      Default :: value(),
+      Value :: value().
+get(_Key, _Default) ->
+    erlang:nif_error(undef).
+
 -spec info() -> Info when
       Info :: #{'count':=Count,'memory':=Memory},
       Count :: non_neg_integer(),